U.S. patent application number 12/337376 was filed with the patent office on 2009-06-25 for multiplexer for selection of an mri compatible band stop filter or switch placed in series with a particular therapy electrode of an active implantable medical device.
This patent application is currently assigned to Greatbatch Ltd.. Invention is credited to Gabe Kustra, Robert A. Stevenson.
Application Number | 20090163981 12/337376 |
Document ID | / |
Family ID | 41665095 |
Filed Date | 2009-06-25 |
United States Patent
Application |
20090163981 |
Kind Code |
A1 |
Stevenson; Robert A. ; et
al. |
June 25, 2009 |
MULTIPLEXER FOR SELECTION OF AN MRI COMPATIBLE BAND STOP FILTER OR
SWITCH PLACED IN SERIES WITH A PARTICULAR THERAPY ELECTRODE OF AN
ACTIVE IMPLANTABLE MEDICAL DEVICE
Abstract
An MRI-compatible electronic medical therapy system includes an
active medical device connected to a plurality of electrodes. A
multiplexer circuit includes at least one circuit protection device
in electrical series with the electrodes and the medical device.
The circuit protection device can be in the form of a switch or a
band stop filter. The circuit protection device is adapted to
permit current flow therethrough during normal medical device
related therapy, but substantially prevent current flow
therethrough in the presence of an induced electromagnetic
field.
Inventors: |
Stevenson; Robert A.;
(Canyon Country, CA) ; Kustra; Gabe; (Cheektowaga,
NY) |
Correspondence
Address: |
KELLY LOWRY & KELLEY, LLP
6320 CANOGA AVENUE, SUITE 1650
WOODLAND HILLS
CA
91367
US
|
Assignee: |
Greatbatch Ltd.
Clarence
NY
|
Family ID: |
41665095 |
Appl. No.: |
12/337376 |
Filed: |
December 17, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12337170 |
Dec 17, 2008 |
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12337376 |
|
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61016364 |
Dec 21, 2007 |
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61079693 |
Jul 10, 2008 |
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Current U.S.
Class: |
607/63 |
Current CPC
Class: |
A61N 1/3718 20130101;
A61N 1/05 20130101; A61N 1/025 20130101; A61N 1/3605 20130101; A61N
1/3754 20130101; H03H 2007/013 20130101; A61N 1/086 20170801; H03H
1/0007 20130101 |
Class at
Publication: |
607/63 |
International
Class: |
A61N 1/00 20060101
A61N001/00 |
Claims
1. An MRI-compatible electronic medical therapy system, comprising:
an active medical device (AMD) having an electronic circuit and a
power supply; a plurality of electrodes electrically connected to
the electronic circuit of the AMD and adapted for insertion into or
connection with biological tissue; and a multiplexer circuit,
including at least one, but fewer than the plurality of electrodes,
circuit protection device in electrical series with the plurality
of electrodes and the AMD electronic circuit, the at least one
circuit protection device adapted to permit current flow
therethrough during normal AMD-related therapy, but substantially
prevent current flow therethrough in the presence of an induced
electromagnetic field.
2. The system of claim 1, wherein the electromagnetic field
comprises a radio frequency, magnetic or static field.
3. The system of claim 1, wherein the electromagnetic field
comprises an MRI field.
4. The system of claim 1, wherein the circuit protection device
comprises a switch.
5. The system of claim 4, wherein the switch is closed during
normal AMD-related therapy, but opens in the presence of the
electromagnetic field.
6. The system of claim 5, wherein the switch comprises a
Micro-Electromechanical System (MEMS) switch.
7. The system of claim 6, wherein the MEMS switch includes a
micro-static relay activated in the presence of an electrostatic
field.
8. The system of claim 6, wherein the MEMS switch includes a
micro-magnetic relay activated in the presence of an magnetic
field.
9. The system of claim 1, wherein the switch comprises a switch
assembly having multiple switches or switch relays.
10. The system of claim 1, wherein the circuit protection device
comprises a band stop filter.
11. The system of claim 10, wherein the band stop filter comprises
a capacitor in parallel with a resistor, the levels of capacitance
and resistance selected for a predetermined frequency range of the
electromagnetic field.
12. An MRI-compatible electronic medical therapy system,
comprising: an active medical device (AMD) having an electronic
circuit and a power supply; a plurality of electrodes electrically
connected to the electronic circuit of the AMD and adapted for
insertion into or connection with biological tissue; and a
multiplexer circuit, including at least one, but fewer than the
plurality of electrodes, switch in electrical series with the
plurality of electrodes and the AMD electronic circuit, the at
least one switch normally closed to permit current flow
therethrough during normal AMD-related therapy, but open to prevent
current flow therethrough in the presence of an induced radio
frequency, magnetic or static electromagnetic field.
13. The system of claim 12, wherein the electromagnetic field
comprises an MRI field.
14. The system of claim 12, wherein the switch comprises a
Micro-Electromechanical System (MEMS) switch.
15. The system of claim 14, wherein the MEMS switch includes a
micro-static relay activated in the presence of an electrostatic
field.
16. The system of claim 14, wherein the MEMS switch includes a
micro-magnetic relay activated in the presence of an magnetic
field.
17. The system of claim 12, wherein the switch comprises a switch
assembly having multiple switches or switch relays.
18. An MRI-compatible electronic medical therapy system,
comprising: an active medical device (AMD) having an electronic
circuit and a power supply; a plurality of electrodes electrically
connected to the electronic circuit of the AMD and adapted for
insertion into or connection with biological tissue; and a
multiplexer circuit, including at least one, but fewer than the
plurality of electrodes, band stop filter in electrical series with
the plurality of electrodes and the AMD electronic circuit, the at
least one band stop filter adapted to permit current flow
therethrough during normal AMD-related therapy, but substantially
prevent current flow therethrough in the presence of an induced
radio frequency, magnetic or static electromagnetic field.
19. The system of claim 18, wherein the electromagnetic field
comprises an MRI field.
20. The system of claim 18, wherein the at least one band stop
filter comprises a capacitor in parallel with a resistor, the
levels of capacitance and resistance selected for a predetermined
frequency range of the electromagnetic field.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates generally to electronic switches and
switch assemblies adapted for use in active implantable medical
devices (AIMDs) such as cardiac pacemakers, cardioverter
defibrillators, neurostimulators and the like. The normally closed
electronic switch is designed to be selectively open just prior to
and during exposure of the medical device to diagnostic, therapy,
electrocautery surgical procedures, or imaging such as magnetic
resonance imaging (MRI). Disconnecting a distal tip electrode(s),
by opening an electronic switch eliminates the possibility that
undesirable RF currents could overheat said distal electrode and
undesirably flow into body tissue thereby creating the potential
for tissue damage (necrosis). For MRI imaging, opening the
electronic switch eliminates problems associated with low frequency
gradient fields as well as high frequency pulsed RF fields. The
present invention is also applicable to a wide range of external
medical devices, including externally worn drug pumps, EKG/ECG
electrodes, neurostimulators, ventricular assist devices and the
like, as well as a wide range of probes, catheters, monitoring lead
wires and the like, that may be temporarily inserted into or onto a
patient or that a patient may be wearing or connected to during
medical diagnostic procedures such as MRI.
[0002] Compatibility of cardiac pacemakers, implantable
defibrillators and other types of active implantable medical
devices with magnetic resonance imaging (MRI) and other types of
hospital diagnostic equipment has become a major issue. If one goes
to the websites of major cardiac pacemaker manufacturers in the
United States, one will see that the use of MRI is generally
contra-indicated with pacemakers and implantable defibrillators. A
similar contra-indication is found in the manuals of MRI equipment
manufacturers such. See also "Safety Aspects of Cardiac Pacemakers
in Magnetic Resonance Imaging", a dissertation submitted to the
Swiss Federal Institute of Technology Zurich presented by Roger
Christoph Luchinger. Dielectric Properties of Biological Tissues:
I. Literature Survey", by C. Gabriel, S. Gabriel and E. Cortout;
"Dielectric Properties of Biological Tissues: II. Measurements and
the Frequency Range 0 Hz to 20 GHz", by S. Gabriel, R. W. Lau and
C. Gabriel; "Dielectric Properties of Biological Tissues: Ill.
Parametric Models for the Dielectric Spectrum of Tissues", by S.
Gabriel, R. W. Lau and C. Gabriel; and "Advanced Engineering
Electromagnetics, C. A. Balanis, Wiley, 1989, all of which are
incorporated herein by reference.
[0003] However, an extensive review of the literature indicates
that MRI is indeed often used with pacemaker patients in spite of
the contra indications. The safety and feasibility of MRI in
patients with cardiac pacemakers is an issue of increasing
significance. The effects of MRI on patients' pacemaker systems
have only been analyzed retrospectively in some case reports. There
are a number of papers that indicate that MRI on new generation
pacemakers can be conducted up to 0.5 Tesla (T). Other papers go up
to 1.5 T for non-pacemaker dependent patients under highly
controlled conditions.
[0004] MRI is one of medicine's most valuable diagnostic tools. MRI
is, of course, extensively used for imaging, but is also
increasingly used for real-time procedures such as interventional
medicine (surgery). In addition, MRI is used in real time to guide
ablation catheters, neurostimulator tips, deep brain probes and the
like. An absolute contra-indication for pacemaker patients means
that pacemaker and ICD wearers are excluded from MRI. This is
particularly true of scans of the thorax and abdominal areas.
However, because of MRI's incredible value as a diagnostic tool for
imaging organs and other body tissues, many physicians simply take
the risk and go ahead and perform MRI on a pacemaker patient. The
literature indicates a number of precautions that physicians should
take in this case, including limiting the applied power of the MRI
in terms of the specific absorption rate (SAR), programming the
pacemaker to fixed or asynchronous pacing mode, having emergency
personnel and resuscitation equipment standing by (known as "Level
II" protocol), and careful reprogramming and evaluation of the
pacemaker and patient after the procedure is complete. There have
been reports of latent problems with cardiac pacemakers after an
MRI procedure occurring many days later (such as increase in or
loss of pacing pulse capture).
[0005] There are three types of electromagnetic fields used in an
MRI unit. The first type is the main static magnetic field
designated B.sub.0 which is used to align protons in body tissue.
The field strength varies from 0.5 to 3.0 Tesla in most of the
currently available MRI units in clinical use. Some of the newer
MRI system fields can go as high as 4 to 6 Tesla. At the recent
International Society for Magnetic Resonance in Medicine (ISMRIM)
conference, which was held on 5 and 6 Nov. 2005, it was reported
that certain research systems are going up as high as 11.7 Tesla. A
1.5 T MRI system is over 100,000 times the magnetic field strength
of the earth. A static magnetic field of this magnitude can induce
powerful magnetomechanical forces on any magnetic materials
implanted within the patient, including certain components within
the cardiac pacemaker and/or lead wire systems themselves. It is
unlikely that the static MRI magnetic field can induce currents
(dB/dt) into the pacemaker lead wire system and hence into the
pacemaker itself. It is a basic principle of physics that a
magnetic field must either be time-varying as it cuts across the
conductor (dB/dt), or the conductor itself must move within the
magnetic field for currents to be induced (dB/dx).
[0006] The second type of field produced by magnetic resonance
imaging equipment is the pulsed RF field which is generated by the
body coil or head coil, also referred to as B.sub.1. This is used
to change the energy state of the protons and illicit MRI signals
from tissue. The RF field is homogeneous in the central region and
has two main components: (1) the magnetic field is circularly
polarized in the actual plane; and (2) the electric field is
related to the magnetic field by Maxwell's equations. In general,
the RF field is switched on and off during measurements and usually
has a frequency of 21 MHz to 64 MHz to 128 MHz depending upon the
static magnetic field strength. The frequency of the RF pulse
varies with the field strength of the main static field, as
expressed in the Lamour Equation::RF PULSED FREQUENCY (in
MHz)=(42.56) (STATIC FIELD STRENGTH (T); where 42.56 MHz per Tesla
is the Lamour constant for H+ protons.
[0007] The third type of electromagnetic field is the time-varying
magnetic gradient field designated Gx, Gy, Gz which is used for
spatial localization. The gradient field changes its strength along
different orientations and operating frequencies on the order of 1
to 2.2 kHz. The vectors of the magnetic field gradients in the X, Y
and Z directions are produced by three sets of orthogonally
positioned coils and are switched on only during the measurements.
In some cases, the gradient field has been shown to elevate natural
heart rhythms (heart beat). This is not completely understood, but
it is a repeatable phenomenon. There have been some reports of
gradient field induced ventricular arrhythmias which could be life
threatening. The Gz gradient is used to distort the B.sub.0 field
in the z direction, thereby creating body `slices` of specific
thickness. The Gx and Gy fields are used to introduce phase and
frequency `markers` to specific protons, allowing for an x-y image
to be generated.
[0008] The gradient fields operate at roughly 1 to 2.2 kHz, and are
generated by three distinct, orthogonally oriented coils. These
fields are only active during image generation protocols, and have
been shown to have adverse effects on human physiology. These
effects are largely due to the induced voltages that are generated
by the application of a moving magnetic field on a large area.
Faraday's Law of Induction is expressed as:
V = A B t , ##EQU00001##
where A is the area of the loop, and dB/dt is change in magnetic
flux with respect to time. It has been shown that the induced
voltages generated by the gradient fields, if high enough, can
induce peripheral nerve stimulation (PNS). This has been reported
in literature as a sensation of pain or other discomfort while
running relatively high MRI gradients. In more extreme animal
testing, cardiac stimulation has been detected, although this has
taken roughly 80 times more energy to achieve than that of PNS. To
prevent PNS or cardiac stimulation from occurring, industry
standards have limited dB/dt to roughly 20 T/sec. Placing an
electronic switch in accordance with the present invention at or
near the distal tip electrode eliminates any chance that gradient
currents will be able to stimulate or capture body tissues.
[0009] Of interest is the effect of the gradient fields on AIMDs,
which typically have implanted lead systems. In the case of AIMDs
with unipolar lead systems, a circuit loop is formed between the
AIMD housing or can, the lead system, the distal TIP, and body
tissue (as the return path). An average area created by such a loop
is around 225 cm.sup.2 with the higher limit about 350 cm.sup.2.
When considering this with the 20 T/sec maximum, it can be seen
that the maximum induced voltage in the loop is 0.700V. When one
looks at the induced voltage at the pacing tip, it is typically an
order of magnitude lower than the induced voltage in the loop (due
to relatively high lead system and device impedances). This is much
lower than the typical pacing threshold required for an AIMD to
stimulate heart tissue.
[0010] It is instructive to note how voltages and EMI are induced
into an implanted or external lead wire system. At very low
frequency (VLF), voltages are induced at the input to the cardiac
pacemaker as currents circulate throughout the patient's body and
create differential voltage drops. In a unipolar system, because of
the vector displacement between the pacemaker housing and, for
example, the TIP electrode, voltage drop across body tissues may be
sensed due to Ohms Law and the circulating RF signal. At higher
frequencies, the implanted lead wire systems actually act as
antennas where currents are induced along their length. These
antennas are not very efficient due to the damping effects of body
tissue; however, this can often be offset by extremely high power
fields and/or body resonances. At very high frequencies (such as
cellular telephone frequencies), electromagnetic interference (EMI)
signals are induced only into the first area of the lead wire
system (for example, at the header block of a cardiac pacemaker).
This has to do with the wavelength of the signals involved and
where they couple efficiently into the system. Placing an
electronic switch in accordance with the present invention inside
or near the housing of the AIMD eliminates any possibility that the
EMI from Gradient fields may disrupt or interfere with AIMD
electronic circuits. An added benefit is that MRI RF currents are
also eliminated in the area near the AIMD which, for example, in a
pacemaker application eliminates the risk of esophageal ablation
due to overheating of adjacent lead wires.
[0011] Magnetic field coupling into an implanted lead wire system
is based on loop areas. For example, in a cardiac pacemaker, there
is a loop formed by the lead wire as it comes from the cardiac
pacemaker housing to its distal TIP located in the right ventricle.
The return path is through body fluid and tissue generally straight
from the TIP electrode in the right ventricle back up to the
pacemaker casing or housing. This forms an enclosed area which can
be measured from patient X-rays in square centimeters. The average
loop area is 200 to 225 cm. This is an average and is subject to
great statistical variation. For example, in a large adult patient
with an abdominal implant, the implanted loop area is much larger
(greater than 450 cm.sup.2).
[0012] Relating now to the specific case of MRI, the magnetic
gradient fields would be induced through enclosed loop areas.
However, the pulsed RF fields, which are generated by the body
coil, would be primarily induced into the lead wire system by
antenna action. By careful placement of the novel electronic switch
of the present invention, both MRI gradient and RF currents are
eliminated.
[0013] There are a number of other potential problems with MRI,
including: [0014] (1) Closure of the pacemaker reed switch. A
pacemaker reed switch, which can also be a Hall Effect device, is
designed to detect a permanent magnet held close to the patient's
chest. This magnet placement allows a physician or even the patient
to put the implantable medical device into what is known as the
"magnet mode response." The "magnet mode response" varies from one
manufacturer to another, however, in general, this puts the
pacemaker into a fixed rate or asynchronous pacing mode. This is
normally done for short times and is very useful for diagnostic
purposes. However, when a pacemaker is brought close to the MRI
scanner, the MRI static field can make the pacemaker's internal
reed switch close, which puts the pacemaker into a fixed rate or
asynchronous pacing mode. Worse yet, the reed switch may bounce or
oscillate. Asynchronous pacing may compete with the patient's
underlying cardiac rhythm. This is one reason why pacemaker/ICD
patients have generally been advised not to undergo MRI. Fixed rate
or asynchronous pacing for most patients is not an issue. However,
in patients with unstable conditions, such as myocardial ischemia,
there is a substantial risk for life threatening ventricular
fibrillation during asynchronous pacing. In most modern pacemakers
the magnetic reed switch (or Hall Effect device) function is
programmable. If the magnetic reed switch response is switched off,
then synchronous pacing is still possible even in strong magnetic
fields. The possibility to open and re-close the reed switch in the
main magnetic field by the gradient field cannot be excluded.
However, it is generally felt that the reed switch will remain
closed due to the powerful static magnetic field. It is
theoretically possible for certain reed switch orientations at the
gradient field to be capable of repeatedly closing and re-opening
the reed switch. Careful placement of the novel electronic switches
of the present invention disconnect AIMD therapy delivery.
Accordingly, it will not matter what the AIMD reed or Hall effect
switch does during MRI scans as the intermittent
synchronous/asynchronous pacing effects will not be able to reach
body tissue. This is another important advantage of the present
invention. [0015] (2) Reed switch damage. Direct damage to the reed
switch is theoretically possible, but has not been reported in any
of the known literature. In an article written by Roger Christoph
Luchinger of Zurich, he reports on testing in which reed switches
were exposed to the static magnetic field of MRI equipment. After
extended exposure to these static magnetic fields, the reed
switches functioned normally at close to the same field strength as
before the test. However, it is still important for the physician
to check the proper operation of the reed switch and the AIMD after
MRI scans are complete. [0016] (3) Pacemaker displacement. Some
parts of pacemakers, such as the batteries and reed switches,
contain ferrous magnetic materials and are thus subject to
mechanical forces during MRI (testing is done to ASTM Standards).
Pacemaker displacement may occur in response to magnetic force or
magnetic torque (newer pacemakers and ICDs have less ferrous
materials and are less susceptible to this). With the much smaller
sizes of modern AIMDS, most experts now report that force and
torques due to MRI are now negligible. [0017] (4) Radio frequency
field. At the pulsed RF frequencies of interest in MRI, RF energy
can be absorbed and converted to heat. The power deposited by RF
pulses during MRI is complex and is dependent upon the power,
duration and shape of the RF pulse, the relative long term time
averages of the pulses, the transmitted frequency, the number of RF
pulses applied per unit time, and the type of configuration of the
RF transmitter coil used. Specific absorption rate (SAR) is a
measure of how much energy is induced into body tissues. The amount
of heating also depends upon the volume of the various tissue (i.e.
muscle, fat, etc.) imaged, the electrical resistivity of tissue and
the configuration of the anatomical region imaged. There are also a
number of other variables that depend on the placement in the human
body of the AIMD and its associated lead wire(s). For example, it
will make a difference how much current is induced into a pacemaker
lead wire system as to whether it is a left or right pectoral
implant. In addition, the routing of the lead and the lead length
are also very critical as to the amount of induced current and
heating that would occur. Also, distal TIP design is very important
as the distal TIP itself can act as its own antenna. Location
within the MRI bore is also important since the electric fields
required to generate the RF increase exponentially as the patient
is moved away from MRI bore center-line (ISO center). The cause of
heating in an MRI environment is two fold: (a) RF field coupling to
the lead can occur which induces significant local heating; and (b)
currents induced during the RF transmission can flow into body
tissue and cause local Ohm's Law heating next to the distal TIP
electrode of the implanted lead. The RF field in an MRI scanner can
produce enough energy to induce lead wire currents sufficient to
destroy some of the adjacent myocardial tissue. Tissue ablation has
also been observed. The effects of this heating are not readily
detectable during the MRI. Indications that heating has occurred
would include an increase in pacing threshold, venous ablation,
Larynx ablation, myocardial perforation and lead penetration, or
even arrhythmias caused by scar tissue. Such long term heating
effects of MRI have not been well studied yet. Placing an
electronic switch in accordance with the present invention at or
near the distal electrode stops the flow of MRI RF currents and
therefore minimizes or altogether eliminates the above concerns.
[0018] (5) Alterations of pacing rate due to the applied radio
frequency field. It has been observed that the RF field may induce
undesirable fast cardiac pacing (QRS complex) rates. There are
various mechanisms which have been proposed to explain rapid
pacing: direct tissue stimulation, interference with pacemaker
electronics or pacemaker reprogramming (or reset). In all of these
cases, placing an electronic switch in accordance with the present
invention at or near the distal electrode and at or near the AIMD
stops the flow of MRI RF currents and therefore eliminates all of
the above concerns. Placing electronic switches in accordance with
present invention in series with the leads at the points of lead
wire ingress and egress into the AIMD housing (at the device
feedthrough EMI filter) prevents MRI RF fields from entering the
AIMD housing and therefore provides a very high degree of
protection to AIMD electronics. When used in combination with the
AIMD EMI filters, this will make alterations in pacemaker pacing
rate and/or pacemaker reprogramming much more unlikely. [0019] (6)
Time-varying magnetic gradient fields. The contribution of the
time-varying gradient to the total strength of the MRI magnetic
field is negligible; however, pacemaker systems could be affected
because these fields are rapidly applied and removed. The time rate
of change of the magnetic field is directly related to how much
electromagnetic force (EMF) and hence current can be induced into a
lead wire system. Luchinger reports that even using today's
gradient systems with a time-varying field up to 60 Tesla per
second, the induced currents are likely to stay below the
biological thresholds for cardiac fibrillation. A theoretical upper
limit for the induced voltage by the time-varying magnetic gradient
field is 20 volts. Such a voltage during more than 0.1 milliseconds
could be enough energy to directly pace the heart. The placement of
an electronic switch in accordance with the present invention at or
near the distal electrode eliminates such concerns. [0020] (7)
Heating. Currents induced by time-varying magnetic gradient fields
may lead to local heating. Researchers feel that the calculated
heating effect of the gradient field is much less as compared to
that caused by the RF field and therefore may be neglected.
[0021] There are additional problems possible with implantable
cardioverter defibrillators (ICDs), another type of AIMD. ICDs use
different and larger batteries which could cause higher magnetic
forces. The programmable sensitivity in ICDs is normally much
higher than it is for pacemakers; therefore, ICDs may falsely
detect a ventricular tacchyarrhythmia and inappropriately deliver
therapy. In this case, therapy might include anti-tachycardia
pacing, cardioversion or defibrillation (high voltage shock)
therapies. MRI magnetic fields may prevent detection of a dangerous
ventricular arrhythmia or fibrillation. There can also be heating
problems of ICD leads which are expected to be comparable to those
of pacemaker leads. Ablation of vascular walls is another concern.
There have also been reports of older model ICDs being severely
effected by the MRI pulsed RF field. In these cases, there have
been multiple microprocessor resets and even cases of permanent
damage where the ICD failed to function after the MRI procedure. In
addition, ICDs have exhibited a different type of problem when
exposed to MRI fields. That is, during an MRI exposure, the ICD
might inappropriately sense the MRI RF field or gradient fields as
a dangerous ventricular arrhythmia. In this case, the ICD will
attempt to charge its high energy storage capacitor and deliver a
high voltage shock to the heart. However, within this charging
circuit, there is a transformer that is necessary to function in
order to fully charge up the high energy storage capacitor. In the
presence of the main static field (B.sub.0) field, the ferrite core
of this transformer tends to saturate thereby reducing its
efficiency. This means the high energy storage capacitor cannot
fully charge. Reports of repeated low voltage shocks are in the
literature. These repeated shocks and this inefficient attempt to
charge the battery can cause premature battery depletion of the
ICD. Shortening of battery life is, of course, a highly undesirable
condition. Placing electronic switches in accordance with the
present invention at or near the distal electrodes and in series
with the lead wires at the ICD housing turns off both sensing and
therapy delivery which eliminates all of the above concerns.
[0022] In summary, there are a number of studies that have shown
that MRI patients with active implantable medical devices, such as
cardiac pacemakers, can be at risk for potential hazardous effects.
However, there are a number of anecdotal reports that MRI can be
safe for extremity imaging of pacemaker patients (i.e. the AIMD is
outside the bore). These anecdotal reports are of interest;
however, they are certainly not scientifically convincing that all
MRI can be safe. As previously mentioned, just variations in the
pacemaker lead wire length can significantly affect how much heat
is generated.
[0023] From the layman's point of view, this can be easily
explained by observing the typical length of the antenna on a
cellular telephone compared to the vertical rod antenna more common
on older automobiles. The relatively short antenna on the cell
phone is designed to efficiently couple with the very high
frequency wavelengths (approximately 950 MHz) of cellular telephone
signals. In a typical AM and FM radio in an automobile, these
wavelength signals would not efficiently couple to the relatively
short antenna of a cell phone. This is why the antenna on the
automobile is relatively longer.
[0024] An analogous situation exists on the MRI system. If one
assumes, for example, a 3.0 Tesla MRI system, which would have an
RF pulsed frequency of 128 MHz, there are certain exact implanted
lead lengths that would couple efficiently as fractions of the 128
MHz wavelength. Ignoring the effects of body tissue, as an example,
the basic wavelength equation (in air) in meters is 300 divided by
the frequency in MHz. Accordingly, for a 3.0 Tesla MRI system, the
wavelength is 2.34 meters or 234 centimeters. An exact wavelength
antenna then would be of this which is 58.59 centimeters. Both wave
and wave antennas are very efficient energy couplers. When the
shorter wavelengths in body tissue are accounted for, this falls
right into the range for the length of certain pacemaker lead wire
implants.
[0025] It is typical that a hospital will maintain an inventory of
various leads and that the implanting physician will make a
selection depending on the size of the patient, implant location
and other factors. Accordingly, the implanted or effective lead
wire length can vary considerably.
[0026] Another variable has to do with excess lead wire. It is
typical that the physician, after doing a pacemaker lead wire
insertion, will wrap up any excess lead wire in the pectoral
pocket. This can form one, two or even three turns of excess lead.
This forms a loop in that specific area, however, the resulting
longer length of wire that goes down into the right ventricle, is
what would then couple efficiently with the MRI RF pulsed
frequency. As one can see, the amount of unwound up lead length is
considerably variable depending upon patient geometry.
[0027] There are certain implanted lead wire lengths that just do
not couple efficiently with the MRI frequency and there are others
that would couple very efficiently and thereby produce the worst
case for heating. The actual situation for an implanted lead wire
is far more complex due to the varying permittivity and dielectric
properties of body tissues, and the accompanying shifts in
wavelengths.
[0028] The effect of an MRI system on the function of pacemakers,
ICDs, neurostimulators and other AMDs depends on various factors,
including the strength of the static magnetic field, the pulse
sequence (gradient and RF field used), the anatomic region being
imaged, and many other factors. Further complicating this is the
fact that each manufacturer's pacemaker and ICD designs behave
differently. Most experts still conclude that MRI for the pacemaker
patient should not be considered safe. Paradoxically, this also
does not mean that the patient should not receive MRI. The
physician must make an evaluation given the pacemaker patient's
condition and weigh the potential risks of MRI against the benefits
of this powerful diagnostic tool. As MRI technology progresses,
including higher field gradient changes over time applied to
thinner tissue slices at more rapid imagery, the situation will
continue to evolve and become more complex. An example of this
paradox is a pacemaker patient who is suspected to have a cancer of
the lung. RF ablation treatment of such a tumor may require
stereotactic imaging only made possible through real time fine
focus MRI. With the patient's life literally at risk, and with
informed patient consent, the physician may make the decision to
perform MRI in spite of all of the previously described attendant
risks to the pacemaker system.
[0029] Insulin drug pump systems do not seem to be of a major
current concern due to the fact that they have no significant
antenna components (such as implanted lead wires). However,
implantable pumps presently work on magneto-peristaltic systems,
and must be deactivated prior to MRI. There are newer (unreleased)
systems that would be based on solenoid systems which will have
similar problems.
[0030] It is clear that MRI will continue to be used in patients
with an active implantable medical device. There are a number of
other hospital procedures, including electrocautery surgery,
lithotripsy, etc., to which a pacemaker patient may also be
exposed. Accordingly, there is a need for circuit protection
devices which will improve the immunity of active implantable
medical device systems to diagnostic procedures such as MRI.
[0031] As one can see, many of the undesirable effects in an
implanted lead wire system from MRI and other medical diagnostic
procedures are related to undesirable induced currents in the lead
wire system. This can lead to overheating either in the lead wire
or at the tissue interface at the distal TIP. At the 2006 SMIT
Conference, the United States Food and Drug Administration (FDA)
reported on a neurostimulator patient whose implanted leads were
sufficiently heated that severe burns occurred resulting in the
need for multiple amputations. In pacemaker patients, these
currents can also directly stimulate the heart into sometimes
dangerous arrhythmias. The above descriptions of problems that a
pacemaker, ICD or neurostimulator patient may encounter during MRI
or similar medical diagnostic procedures are only examples of a
general need. A patient wearing external devices, such as an
external drug pump, an external neurostimulator, EKG leads, (skin
patches) or ventricular assist devices, may also encounter problems
during an MRI procedure. All of the above descriptions regarding
overheating of lead wires, overheating of distal tips or
electromagnetic interference are of concern. The novel electronic
switch of the present invention is applicable to all of these other
devices. It is also applicable to probes and catheters that are
used during certain real time medical imaging procedures such as
MRI. The present invention is applicable to a wide range of both
implanted and external medical device systems.
[0032] Accordingly, there is a need for a circuit protection device
that protects a patient undergoing high RF power medical diagnostic
procedures, such as an electronic switch placed either at the
device, in the lead wire system or at or near the distal tip
electrodes. Various methodologies are also needed wherein the
electronic switch can be preferentially opened and closed either by
a medical doctor or the radiologist just prior to medical
procedures such as magnetic resonance imaging. The present
invention fulfills these needs and provides other related
advantages.
SUMMARY OF THE INVENTION
[0033] The present invention comprises novel electronic switches to
be placed at the distal tip and/or various locations along the
medical device, lead wires or circuits. These switches prevent
current from circulating that could be induced by a medical
therapeutic diagnostic device.
[0034] For example, for an MRI system operating at 1.5 Tesla, the
pulsed RF frequency is 64 MHz, as described by the Lamour Equation.
The novel electronic switches of the present invention, when placed
at the distal tip of a pacemaker lead wire, will stop RF currents
from flowing through the distal tip (or ring electrode) and then
into body tissue. The electronic switch of the present invention
stops RF currents from flowing across the entire frequency
spectrum. This means that the RF fields and gradient fields that
could be induced from MRI imaging equipment cannot flow, for
example, into myocardial or other human tissues. This stops the
problems associated with overheating of lead wires, tissue damage
and excessive currents or temperatures, and also the potential for
gradient field capture of the heart which can lead to dangerous
arrhythmias.
[0035] The novel electronic switch can also reduce EMI or RF
currents or electromagnetic interference from flowing in the lead
wire system into the input of an active implantable medical device
(AIMD) such as a pacemaker. This provides added protection to
sensitive electronic circuits of the AIMD. It will be obvious to
those skilled in the art that all of the embodiments described
herein are equally applicable to a wide range of other implantable
and external medical devices, including deep brain stimulators,
spinal cord stimulators, drug pumps, probes, catheters and the
like.
[0036] Thus, in accordance with the present invention, an
MRI-compatible electronic medical therapy system comprises an
active medical device (AMD), having an electronic circuit and a
power supply. A plurality of electrodes are electrically connected
to the electronic circuit of the AMD and adapted for insertion into
or connection with biological tissue. A multiplexer circuit,
including at least one, but fewer than a plurality of electrodes,
circuit protection devices are in electrical series with the
plurality of electrodes and the AMD electronic circuit. The at
least one circuit protection device is adapted to permit current
flow therethrough during normal AMD-related therapy, but
substantially prevent current flow therethrough in the presence of
an induced electromagnetic field.
[0037] The medical device (AMD) may comprise a cochlear implant, a
piezoelectric sound bridge transducer, a neurostimulator, a brain
stimulator, a cardiac pacemaker, a ventricular assist device, an
artificial heart, a drug pump, a bond growth stimulator, a bone
growth stimulator, a bone fusion stimulator, a urinary incontinence
device, a pain relief spinal cord stimulator, an anti-tremor
stimulator, a gastric stimulator, an implantable cardioverter
defibrillator, a pH probe, a congestive heart failure device, a
pill camera, a neuromodulator, a cardiovascular stent, an
orthopedic implant, an external insulin pump, an external drug
pump, an external neurostimulator, a Holter monitor, an external
probe, or a catheter.
[0038] The induced electromagnetic field comprises a radio
frequency, magnetic or static field. The switch comprises a
normally closed switching element creating a closed circuit between
the electrode and the AMD, and which is automatically moved to an
open position to create an open circuit in the presence of the
induced radio frequency, electrostatic or magnetic field. The
switching element automatically closes on removal of the induced
radio frequency, electrostatic or magnetic field.
[0039] In a particularly preferred embodiment, the switch comprises
a micro-electromechanical system (MEMS) switch. The MEMS switch may
include a micro-electrostatic relay activated in the presence of an
electrostatic field, a micro-magnetic relay activated in the
presence of a magnetic field, or may be of a resistive-type or
capacitive-type switch.
[0040] The switch may comprise a switch assembly having multiple
switches or switch relays.
[0041] In another embodiment, the circuit protection device
comprises at least one band stop filter. Each band stop filter
comprises a capacitor in parallel with a resistor. The levels of
capacitance and resistance are selected for a predetermined
frequency range of the electromagnetic field, such as an MRI
field.
[0042] Other features and advantages of the present invention will
become apparent from the following more detailed description, taken
in conjunction with the accompanying drawings, which illustrate, by
way of example, the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0043] The accompanying drawings illustrate the invention. In such
drawings:
[0044] FIG. 1 is a wire-form diagram of a generic human body
showing a number of active medical devices (AMD) and associated
internal and external lead wires;
[0045] FIG. 2 is a perspective and somewhat schematic view of a
prior art active implantable medical device (AIMD) including a lead
wire directed to the heart of a patient;
[0046] FIG. 3 is a diagram of a unipolar active implantable medical
device (AIMD);
[0047] FIG. 4 is a diagram similar to FIG. 3, illustrating a
bipolar AIMD system;
[0048] FIG. 5 is a diagram similar to FIGS. 3 and 4, illustrating a
bipolar lead wire system with a distal TIP and RING electrode,
typically used in a cardiac pacemaker;
[0049] FIG. 6 is a line drawing showing an electronically
controlled switch module in series with a unipolar lead wire
system;
[0050] FIG. 7 is the schematic of the electronically controlled
switch of FIG. 6;
[0051] FIG. 8 is similar to FIG. 6 except that the switch module
has been replaced by a micro-electromechanical system (MEMS)
technology switch;
[0052] FIG. 9 is a tracing of an exemplary patient X-ray showing an
implanted cardiac pacemaker and cardioverter defibrillator and
corresponding lead wire system;
[0053] FIG. 10 is a line drawing of an exemplary patient cardiac
X-ray of a bi-ventricular lead wire system;
[0054] FIG. 11 illustrates a bipolar cardiac pacemaker lead wire
showing the distal TIP and distal RING electrodes;
[0055] FIG. 12 is an enlarged, fragmented schematic illustration of
the area illustrated by the line 12-12 in FIG. 11 with the
electronic switches of the present invention shown in series with
both the distal TIP and the RING electrodes;
[0056] FIG. 13 is a schematic diagram showing the distributed
electrical resistance and reactance components which make up the
impedance of a pacemaker bipolar lead wire system with switch
placement locations;
[0057] FIG. 14 is very similar to FIG. 2 and is a perspective and
somewhat schematic view of an AIMD including electronic switches of
the present invention, and optional RFID chips for identification
of therapeutic imaging compatible AMDs and/or associated lead wire
systems;
[0058] FIG. 15 is a line drawing of an AIMD showing electronic
switches of the present invention;
[0059] FIG. 16 is a side perspective view of a prior art
neurostimulator known in the art as a bion having a lead
incorporating a switch in accordance with the invention;
[0060] FIG. 17 is very similar to FIG. 16 with the addition of an
electronic switch of the present invention into the bion
itself;
[0061] FIG. 18 is a sectional view illustrating an electronic
switch module which has been hermetically sealed for protection
against body fluids;
[0062] FIG. 18A is the schematic diagram of the circuit from FIG.
18;
[0063] FIG. 19 is a sectional view which is very similar to FIG.
18, illustrating an alternative embodiment for an electronic switch
which can be optionally associated with radio frequency (RFID)
activation;
[0064] FIG. 20 is a schematic diagram of the hermetically sealed
electronic switch of FIG. 19;
[0065] FIG. 21 is a sectional and partially exploded view of yet
another hermetically sealed package containing the novel electronic
switch which is RFID activated in accordance with the present
invention;
[0066] FIG. 22 is a perspective view of a distal electrode pad
applicable to a wide variety of neurostimulator and neuromodulator
applications;
[0067] FIG. 23 is a sectional view taken generally along the line
23-23 of FIG. 22 showing the embedded switch in the open
position
[0068] FIG. 23A is a view of the switch of FIG. 23 now shown in the
closed position after application of electrostatic or external
magnetic force.
[0069] FIG. 24 is a perspective view of a novel electronic switch
which has been embedded on a substrate with a hermetic seal or
encapsulant for protection against body fluids;
[0070] FIG. 25 is an alternate cylindrical encapsulation of an
electronic switch which is very similar in principle to FIG.
24;
[0071] FIG. 26 is a cross-section of an active fixation cardiac
electrode tip incorporating an electronic switch of the present
invention;
[0072] FIG. 27 is a sectional view of a cardiac active fixation tip
electrode embodying the novel electronic switch which has been gold
brazed to provide torsional strength;
[0073] FIG. 28 is a fragmented sectional view of a prior art
neurostimulation electrode probe;
[0074] FIG. 29 is an enlarged sectional view of the area 29 in FIG.
28 illustrating the addition of electronic switches in accordance
with the present invention;
[0075] FIG. 30 is a fragmented sectional view of a prior art
broadband low pass filter capacitor illustrating how an electronic
switch in accordance with the present invention is incorporated
therein;
[0076] FIG. 31 is a diagrammatic representation of the human heart,
showing a left ventricular (LV) endocardial lead system embodying
the present invention;
[0077] FIG. 32 is a table showing the relationship between the
medical French gauge converted to diameter in millimeters and
inches;
[0078] FIG. 33 is an enlarged perspective view of the lead system
of FIG. 31;
[0079] FIG. 34 is a diagrammatic representation of the human heart,
showing epicardial lead wire attachment to the outside of the left
ventricle in accordance with the present invention;
[0080] FIG. 35 is a cross-sectional view of an epicardial lead
incorporating the electronic switch of the present invention taken
generally along the line 35-35 in FIG. 34;
[0081] FIG. 36 illustrates a split cylinder cuff electrode designed
to wrap around a nerve which incorporates electronic switches of
the present invention;
[0082] FIG. 37 illustrates a self-sizing helical cuff coil
incorporating one or more electronic switches, in accordance with
the present invention;
[0083] FIG. 38 is a sectional view taken generally along the line
38-38 in FIG. 37;
[0084] FIG. 39 illustrates a nerve cuff employing a multiplicity of
electrodes and electronic switches for a large nerve trunk, in
accordance with the present invention;
[0085] FIG. 40 illustrates one methodology of putting multiple
electronic switches in series with each of the lead wires of the
multiple cuff electrode of FIG. 39;
[0086] FIG. 41 illustrates a multi-conductor lead body connected to
a multiple electronic switch array that has multiple electrodes, in
accordance with the present invention;
[0087] FIG. 42 is a diagrammatic, side cross-sectional view of a
human head showing the placement of a deep brain probe and
electrode incorporating electronic switches in accordance with the
present invention;
[0088] FIG. 43 is a diagrammatic, front cross-sectional view of the
human head showing use of multiple deep brain probes which
incorporate an electronic switch of the present invention;
[0089] FIG. 44 is an enlarged sectional view taken generally of the
area illustrated by the line 44-44 in FIG. 43;
[0090] FIG. 45 is a view similar to FIG. 44, illustrating an
alternative stress relieving probe in electrode arrangement;
[0091] FIG. 46 is a sectional view similar to FIG. 44, except that
the probe containing the electronic switch is embedded under the
skull and then the skull bore hole is covered with a bone
encapsulant;
[0092] FIG. 47 is a cross-sectional view illustrating a
hermetically sealed electronic switch of the present invention
which has been adapted with a feedthrough hole for use with a
guidewire implantation system; and
[0093] FIG. 48 is a line schematic drawing showing an electronic
multiplexer which can be used to select optionally electronic
switches or L-C bandstop TANK filters.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0094] As shown in the drawings for purposes of illustration, the
present invention resides in the placement of electronically
activated switches, including MEMS switches, in series with lead
wires or circuits of active medical devices to protect the patient
and/or medical device from undesirable electromagnetic interference
signals, such as those generated during MRI and other medical
procedures. The present invention also resides in the design,
manufacturing, and installation of such electronic switches to be
used in the lead wires, inside the active implantable medical
device itself or at or in conjunction with a distal tip electrode
of AIMDs. As will be explained more fully herein, the invention is
applicable to a wide range of external medical devices, probes,
catheters, monitoring lead wires and the like that may be
temporarily inserted onto a patient or that a patient may be
wearing or connected to during medical diagnostic procedures, such
as MRI.
[0095] Another need which resides primarily within the
neuromodulation community is the need to perform real time MRI
guided placement of deep brain stimulator electrodes, spinal cord
stimulator electrodes and the like. Currently, because of the fear
of overheating such electrodes, surgical guidance of these
electrodes is quite difficult. What typically happens is the
patient is exposed to various MRI scans that are correlated with
marks that are placed on the patient's skull. There is a
stereotactic mechanical device that is then either affixed or
literally screwed to the patient's skull which guides the surgical
fixture. There is really no imaging done during this type of
surgical procedure. Everything relies upon the mechanical alignment
of the aforementioned tools. At the recent Neuromodulation Society
Conference held in December of 2007 in Acapulco, Mexico a number of
neurosurgeons mentioned the need for real time MRI guidance of such
electrodes. The present invention makes this very feasible. With a
novel electronic switch of the present invention in the open
position, real time MRI imaging can be conducted without any fear
of overheating the electrodes. Then, after the electrode is
accurately guided and placed in the correct location in the brain
or spinal cord tissue, the electronic switch can be closed so that
the surgeon can perform various tests of the patient to select the
appropriate electrode pairs. Advantages of real time MR guided
placement of electrodes include more accurate placement, less
discomfort to the patient by the elimination of the stereotactic
mechanical means for placement and the ability to reach small and
delicate physiologic areas that were previously too difficult.
[0096] Another application is in ablation catheters and ablation
probes. For example, an ablation catheter could be placed up
through the venous system and into one of the atria of the heart.
Currently such ablation procedures, for example to try to eliminate
atrial fibrillation, are done blind and require a great deal of
skill by the physician. It would be extremely desirable to have
this procedure real time MRI guided. In the present invention the
electronic switch could be open during the MRI guidance and then
closed at the moment the ablation pulse is applied. In fact, the
ablation pulse itself could create sufficient electrostatic forces
to close a MEMS switch and thereby, at the same time, close the
circuit and deliver the therapy to the appropriate location.
[0097] Yet another need resides in common surgical procedures.
Electrocautery is a very common surgical technique. The surgeon
often works with an instrument such as the Bovi knife that
cauterizes blood vessels as it cuts. However, the intense RF energy
from such electrocautery equipment can damage the sensitive
electronic circuitry of AIMDs. It is a particular advantage of the
present invention to incorporate electronic switches such that the
AIMD is disconnected during such procedures so that the AIMD is
well protected.
[0098] In a preferred embodiment, the electronic switch will be a
Micro-Electromechanical System (MEMS) type of electrostatic switch
device. The electronic switch would be preferentially opened just
prior to the patient receiving a therapeutic or diagnostic medical
procedure which has the potential to induce high currents or other
undesirable electromagnetic interference into the AIMD system. A
downside of opening the switch is that the therapy electrode(s)
will be disconnected. In other words, during the medical therapy or
diagnostic procedure, it will not be possible for the patient to
receive therapy from the AIMD. In general, this is not a problem
for most deep brain stimulator (DBS), spinal cord stimulator (SCS),
cochlear implant patients, urinary incontinence, diabetics, people
suffering from depression, people suffering from Parkinson's tremor
and the like. In other words, it is not a disadvantage for these
types of patients to be without therapy for the relatively short
time of the medical diagnostic imaging or therapeutic procedure.
There is a concern for patients who depend on each electrical pace
of the implanted pacemaker so that their heart will function
properly. For a pacemaker dependent patient, lack of a pacing pulse
is, of course, life threatening. However, there is still a way
around this and to still provide for safe medical diagnostic
imagine procedures. For this type of patient, it will be necessary
to place MRI compatible skin electrodes in conjunction with an
external pacemaker during the time that the medical procedure is
performed. In this case, the electronic switch in the patient's
lead wire systems would be open thereby making therapy from the
AIMD not possible. However, by properly externally pacing, one can
get around this without any threat to the patient. A novel
electronic switch structure as described herein also has a broad
application to other medical procedures and even procedures
involving telecommunications, military, space and the like.
[0099] In the present invention, the microelectronic switch and/or
MEMS switches ideally are of a type that do not require power other
than during the actual switching event. This is why a MEMS switch
is ideal. There are other types of RF switches that are also
mechanically functioning switches. These are electrically
controlled two-state switches that open and close contacts to
effect operation in the electrical system. When the switch is open,
this prevents therapeutic and sensing signals from passing, but
also prevents unwanted induced RF signals from passing which can
cause overheating or undesirable tissue effect. The present
invention includes many mechanical and solid state types of
switching. The preferred embodiment includes a MEMS switch which,
in general, includes micro-electrostatic and micro-magnetic relays
(open and closed switches). MEMS switches are termed electrostatic
MEMS switches if they are actuated or controlled during
electrostatic force which turns such switches on and off.
Electrostatic MEMS switches are advantageous due to their extremely
low power consumption because they can be actuated using
electrostatic force induced by the application of a voltage with
virtually no current. Some switches that are well known in the art
are used in a number of applications include variable RF phase
switches, RF switching signal arrays, switching tunable elements as
well as gang switching of voltage control oscillators. MEMS
switches are important building blocks in many wireless
communications systems. RF switches, including MEMS switches, are
found in many different communication devices such as cellular
telephones, wireless pagers, wireless infrastructure equipment,
satellite communications equipment, and cable vision equipment. The
most basic type of electronic switch, including certain MEMS
switches, is a single pole single throw (SPST), which contains a
single input and a single output. Other switches are available
which come in single pole multiple throw (SPMT) configurations. The
typical switch in accordance with the present invention will
contain two parts including an input and an output which are
controlled together (or may be separately controlled). In one
embodiment, the switch of the present invention is located at the
distal electrode and integrated within. As an alternative, it may
be placed close to the electrode. Alternatively, the electronic
switches could be placed anywhere in the AIMD lead wire system or
within the AIMD itself. Because of the way the RF field couples
from, for example, an MRI system, it is often advantageous to have
the switch as near the distal tip electrode(s) as possible. This is
because EMFs from the pulsed RF field of an MRI machine tend to be
distributed along the entire length of the lead system. These EMFs
are separated by the transmission line impedances of the lead wire
system. In lay terms, one can place such a switch within the AIMD
itself but still have substantial RF currents, overheating of
distal tip electrodes and tissue damage still occur. This is
because the distal tip is electrically isolated from the AIMD
itself at high frequencies. Alternative electronic switches could
consist of a number of solid state devices such as diodes and field
effect transistors (FETs). These types of devices are typically
fabricated using Gallium Arsenide (GaAs) technology. Solid state RF
switches are generally made using GaAs processes. All of these
would be acceptable in the present application, but not necessarily
ideal. The problem with GaAs devices are that they do require a
certain amount of power to operate. An advantage of the MEMS
technology is that other than during the actual switching, they
require no power at all from the system. This is extremely
important in an AIMD where power consumption is of utmost concern.
PIN diodes are semiconductor devices which can also be used in
switching applications. They have a high or very low resistance
value depending on the value of the biasing characteristics. PIN
diodes are typically fabricated in Si and GaAs technologies. One of
the more serious drawbacks of these switches is the necessity to
provide a constant DC current. This becomes impractical at the
distal tip electrodes of an AIMD because more lead wires would
generally be required. Again, this is why MEMS technology is an
advantage. In addition, using discrete PIN diodes would increase
both the size and the cost of the distal tip electrode assembly.
This is also why transistor type junctions are generally not the
preferred embodiment, but can definitely be used in switching
applications.
[0100] As stated, MEMS technology is the preferred embodiment for
fabrication of the electronic switches of the present invention.
These are miniature devices that can be manufactured in a wide
variety of mechanical forms. MEMS devices are inherently both
mechanical and electrical devices that are subject to wear and
contamination and can suffer from limited lifetimes. However, in
accordance with the present invention, this is not particularly
important. Patients will only undergo typically a few therapeutic
or diagnostic procedures in their lifetime. They will certainly not
undergo millions of such procedures which starts to approach the
lifetime of a typical MEMS device. A MEMS system is typically
fabricated using a semiconductor integrated circuit type of
fabrication technology. MEMS switches tend to be ultra-small type
devices. MEMS switches in the present application are ideal in that
they have a very high off impedance with a very low off capacitance
and a relatively low on impedance with a high on capacitance. These
are important advantages for use in the lead wire system in an
active implantable medical device. When the MEMS switch is on, it
is very important that the impedance be low so that there is no
degradation of biological sensing signals or therapeutic signals
such as a pacemaker pacing pulse. A resistive type MEMS switch is
preferred. In a capacitive type MEMS switch, a dielectric layer is
deposited on the first conductor in an area opposite to the
underside of the two-movable bridge, with this area on the
conductor acting as the pull down electrode. With this arrangement,
the full pull down voltage appears across the dielectric area
resulting in a relatively high electric field across the
dielectric. The problem with this, for active implantable medical
devices, is that a relatively high impedance would occur across
this capacitive layer at low frequencies. Because of this, in the
present invention, a resistive type MEMS switch is preferred.
[0101] MEMS switches can be activated through the application of a
static field or when they include ferrite materials by the
activation of a magnetic field. The present invention includes MEMS
switches that would automatically activate in the presence of the
very powerful fixed magnetic field (B.sub.0) of an MRI scanning
system.
[0102] Another advantage of MEMS switches is that they are very
cost effective and can be built in very small geometries. This
makes them ideal for packaging within a hermetic distal tip.
[0103] In the present invention, it is preferable that the MEMS
switch or equivalent electronic switch be designed such that it
does not require additional lead wires in the implanted AIMD lead
wire system. That is, the MEMS switch will be placed in series with
the electrode delivery lead wire(s). It is therefore important that
the MEMS switch not be inadvertently activated by the application
of therapy (for example, pacemaker pacing pulses). For example, for
an electrostatic type MEMS switch, it will be important that a
voltage be applied in the AIMD lead wire systems sufficient to
create an electrostatic field to activate the MEMS switch and cause
it to switch from its normally closed to its open position. The
voltage that is applied is typically at a higher voltage than those
of normal therapeutic pacing pulses. However, this voltage should
be at a level below which will it have a deleterious effect on body
tissues. Accordingly, it is a feature of the present invention that
the voltages and/or pulses required to activate the electronic
switch be desirably below a therapeutic or tissue stimulation
threshold voltage. What is more important is that the switch
activation voltages and currents be below a threshold which would
cause dangerous tissue stimulation or damage. Fortunately, there is
a considerable window between the relatively low therapeutic
neuromodulation thresholds and the threshold where actual tissue
damage would occur.
[0104] As mentioned, the preferred embodiment is a MEMS switch
which is electrostatically activated. This approach is ultra low
power because typically only a nano-joule of power is required for
each switching event and no power is consumed when the switch is
either in the closed or open state. This approach is far better
suited to power sensitive applications such as those for AIMDs than
the more power hungry magnetic switch activation approach that is
traditionally used in mechanical relays. This is also superior to
other electronic switches such as diodes or FETs which also require
a constant supply of energy. The switching alternative products
that are most similar to MEMS switches and who have very high
reliability, are high density interconnect (HDI) switches and
conventional electro-mechanical relays. As previously mentioned,
negatives to these approaches are that they are relatively large in
size and they also consume energy. When properly designed, MEMS
switches operate with much lower forces and much lower power
consumption with much longer lifetimes. Off-the-shelf electronic
switches or MEMS switches are ideal for a variety of low voltage
neurostimulator, pacemaker and cochlear implant applications.
However, there are certain high voltage and high power applications
such as implantable cardioverter defibrillator (ICD) applications
which require a special type of electronic switch. Fortunately,
high power RF MEMS switches can be designed and fabricated. In
general, these switches are composed of a matrix of ohmic contact
cantilevers and bridges. The fabrication processes have been
developed to improve planarization on the MEMS switch contact
surfaces and thereby reduce residual stresses in switch beams,
which ensure that a very flat and smooth surface results. This is
important so that the MEMS switch can operate at high power and low
activation voltage.
[0105] The two major suppliers in the MEMS switch market are
Microlab and Teravicta, although there are other suppliers. An
ideal switch for the present invention is Teravicta's TT612 switch
which is a MEMS device with a micromachined cantilever that rapidly
bends like a diving board from an on to an off position in response
to an electrostatic signal. The present invention includes existing
RF switches which are really solid-state semiconductor devices that
turn on and off electronically. However, as previously explained,
the MEMS type of switch has performance advantages and power
consumption advantages over the solid-state type devices. There are
other types of MEMS switches other than the ones that use
electrostatic actuation. The most common one is one that uses a
magnetic latching and switching mechanism that eliminates the need
for the static power supply and that can latch on or off with zero
power required to hold this state. The switch is nonvolatile and
bi-stable. Because of the long range magnetic forces, the switch
requires very low operation voltage. Such a switch, in accordance
with the present invention, is designed to be automatically
activated when placed in the presence of the powerful static
magnetic field of a magnetic resonance imaging system. Such a
switch can operate at 0.5 Tesla, 1.5 Tesla, 3 Tesla and the like.
Multiple magnetically activated MEMS devices can be placed in
various x, y and z geometries so that it does not matter what their
orientation is within the bore of the MRI system. In other words,
by using multiple MEMS devices, it will always be assured that at
least one or more of them switches.
[0106] The principle behind the latching characteristics is the
preferential magnetization of a cantilever made of soft magnetic
material (for example, permalloy). In a nearly perpendicular
magnetic field, a cantilever can have a clockwise or counter
clockwise torque depending on the angle between the cantilever in
the magnetic field, which leads to the bi-stability. To switch the
relay, a second magnetic field, in this case generated by a short
current pulse through a coil, realigns the magnetization of the
cantilever causing it to flip. A static external magnetic field,
such as that from an MRI scanner, instantly latches the switch in
the closed or open position (whichever is desired), respectively.
The switch maintains this state until the next switching signal
realigns the cantilever. The relay consumes no power to maintain
the latched state. Accordingly, with such a magnetically activated
MEMS switch, the physician or the radiology technician can apply an
external magnet or coil over the AIMD electrode to switch the
switch back into the desired therapy delivery state. Whereas
previously noted, this can be done automatically by placement of
the patient into the powerful static field of the MRI scanner. Once
the scanning sequence is completed and the patient is removed, then
it is a relatively easy matter to apply an opposite torque using a
simple device consisting of a coil which will apply the opposite
magnetic effect.
[0107] There are also electronic switches in a particular MEMS
technology that will suppress high voltage arc phenomena. Arc
suppressors can be put across the MEMS switch, for example, to make
them more resistant to external defibrillation. In addition, this
will make them more robust and able to withstand undesirable
electrostatic discharge (ESD). Accordingly, it is a feature of the
present invention that any of the electronic, electromechanical or
MEMS switches that can be used in the present invention could also
be combined with various forms of arc suppression (including zener
diode, Transorb technology, and the like) in order to protect them.
The types of switches that can be used in accordance with the
present invention include micromachined electromagnetic switches
such as those described in U.S. Pat. No. 5,475,353; micro
electromechanical RF switches as defined in U.S. Pat. No.
5,578,976; micro electromechanical switches as described in U.S.
Pat. No. 6,160,230; and MEMS switches as described in U.S. Pat. No.
6,218,911. There are also a number of other switches that can be
used that are cited in the referenced patents on the first page of
this application. In general, in the present invention, a method of
performing electrical switching in the lead wire or distal tip
electrode of the AIMD is included such that it would be
disconnected from body tissue in the presence of powerful emitters
such as those used in medical diagnostic, therapy or imaging
equipment. In the preferred embodiment, the method of performing
electrical switching resides in a MEMS type device where either a
static, magnetic field, or RF field (RFID) is used to change the
switch state. Other than during switching, such devices require
little to no power.
[0108] In another embodiment, the MEMS type switch or other
electronic switch can be activated from the field, for example, of
an MRI machine, through an antenna. This is very similar to the
type of antennas used in the RFID industry. Passive RFID tags
require no battery or no power source. They obtain all of their
power to activate their microcircuit from the external
electromagnetic field. For an RFID device, the external magnetic
field is provided by the RFID reader. In the present application,
this can certainly be done, however, for an MRI device, the
external electromagnetic field can be provided by the MRI machine
itself. In a preferred embodiment, for a MEMS type switch, the
preferred movable metallic conductor, inclusive electrical
switching element would be of a cantilever type configuration.
Maintaining an electromechanical type switch or MEMS switch in the
open position can be accomplished through the action of a spring
tension resident in the movable arm portion. Such normally closed
type switches are known in the prior art (see U.S. Pat. No.
6,373,007).
[0109] FIG. 1 illustrates various types of active implantable and
external medical devices 100 that are currently in use. FIG. 1 is a
wire formed diagram of a generic human body showing a number of
implanted medical devices. 100A represents a family of hearing
devices which can include the group of cochlear implants,
piezoelectric sound bridge transducers and the like. 100B
represents a variety of neurostimulators, neuromodulators and brain
stimulators. Neurostimulators are used to stimulate the Vagus
nerve, for example, to treat epilepsy, obesity and depression.
Brain stimulators are pacemaker-like devices and include electrodes
implanted deep into the brain for sensing the onset of the seizure
and also providing electrical stimulation to brain tissue to
prevent the seizure from actually occurring. As noted above, it
would be a significant improvement in surgical technique if the
lead wires associated with a deep brain stimulator could be placed
using real time MRI imaging. 100C shows a cardiac pacemaker which
is well-known in the art. 100D includes the family of left
ventricular assist devices (LVAD's), and artificial hearts,
including the recently introduced artificial heart known as the
Abiocor. 100E includes an entire family of drug pumps which can be
used for dispensing of insulin, chemotherapy drugs, pain
medications and the like. Insulin pumps are evolving from passive
devices to ones that have sensors and closed loop systems. That is,
real time monitoring of blood sugar levels will occur. These
devices tend to be more sensitive to EMI than passive pumps that
have no sense circuitry or externally implanted lead wires. 100F
includes a variety of bone growth stimulators for rapid healing of
fractures. 100G includes urinary incontinence devices. 100H
includes the family of pain relief spinal cord stimulators and
anti-tremor stimulators. 100H also includes an entire family of
other types of neurostimulators used to block pain. 100L includes a
family of implantable cardioverter defibrillators (ICD) devices and
also includes the family of congestive heart failure devices (CHF).
This is also known in the art as cardio resynchronization therapy
devices, otherwise known as CRT devices. 100J illustrates an
externally worn pack. This pack could be an external insulin pump,
an external drug pump, an external neurostimulator or even a
ventricular assist device. 100K illustrates the insertion of an
external probe or catheter. These probes can be inserted into the
femoral artery, for example, or in any other number of locations in
the human body. 100L illustrates one of various types of EKG/ECG
external skin electrodes which can be placed at various locations.
100M are external EEG electrodes placed on the head.
[0110] FIG. 2 illustrates a prior art active implantable medical
device (AIMD) 100. In general, the AIMD 100 could, for example, be
a cardiac pacemaker 100C which is enclosed by a titanium housing
102 as indicated. The titanium housing is hermetically sealed,
however, there is a point where lead wires 104 must ingress and
egress the hermetic seal. This is accomplished by providing a
hermetic terminal assembly 106. Hermetic terminal assemblies are
well known and generally consist of a ferrule 108 which is laser
welded to the titanium housing 102 of the AIMD 100. Referring once
again to FIG. 2, four lead wires are shown consisting of lead wire
pair 104a and 104b and lead wire pair 104c and 104d. This is
typical of what's known as a dual chamber bipolar cardiac
pacemaker.
[0111] The ISI connectors 110 that are designed to plug into the
header block 112, such as in ports 118, are low voltage (pacemaker)
connectors covered by an ANSI/AAMI standard IS-1. Higher voltage
devices, such as implantable cardioverter defibrillators (ICDs),
are covered by a standard known as the ANSI/AAMI DF-1. There is a
new standard under development which will integrate both high
voltage and low voltage connectors into a new miniature connector
series known as the IS-4 series. These connectors are typically
routed in a pacemaker application down into the right ventricle and
right atrium of the heart 114. There are also new generation
devices that have been introduced to the market that couple lead
wires to the outside of the left ventricle. These are known as
biventricular devices and are very effective in cardiac
resynchronization and treating congestive heart failure (CHF).
[0112] Referring once again to FIG. 2, the bipolar lead wires 104a
and 104b could be routed, for example, into the right ventricle.
The bipolar lead wires 104c and 104d could be routed to the right
atrium. There is also an RF telemetry pin antenna 116 which is not
connected to the IS-1 or DS-1 connector block. This acts as a short
stub antenna for picking up telemetry (programming) signals that
are transmitted from the outside of the device 100.
[0113] It should also be obvious to those skilled in the art that
all of the descriptions herein are equally applicable to other
types of AIMDs. These include implantable cardioverter
defibrillators (using the aforementioned DF-1 connectors),
neurostimulators (including deep brain stimulators, spinal cord
stimulators, cochlear implants, incontinence stimulators and the
like), and drug pumps. The present invention is also applicable to
a wide variety of minimally invasive AIMDs. For example, in certain
hospital catheter lab procedures, one can insert an AIMD for
temporary use such as an ICD. Ventricular assist devices also can
fall into this type of category. This list is not meant to be
limiting, but is only an example of the applications of the novel
technology currently described herein.
[0114] FIG. 3 is a general diagram of a unipolar active implantable
medical device system 100. The housing 102 of the active
implantable medical device 100 is typically titanium, stainless
steel or the like, and acts as one conductive electrode. Inside of
the device housing are the AIMD electronics. Usually AIMDs include
a battery, but that is not always the case. For example, a Bion may
receive its energy from an external pulsing magnetic field. A lead
wire 104 is routed in insulative relationship with the AIMD housing
to a point 120 where it is embedded in body tissue. In the case of
a spinal cord stimulator 100H, the distal TIP 120 could be in the
spinal cord. In the case of a deep brain stimulator 100B, the
distal electrode 120 would be placed deep into the brain tissue,
etc. In the case of a cardiac pacemaker 100C, the unipolar distal
electrode 120 would typically be placed in the cardiac right
ventricle.
[0115] FIG. 4 is very similar to FIG. 3 except that it is a bipolar
system. In this case, the return path is between the two distal
electrodes 120a and 120b. In the case of a cardiac pacemaker 100C,
this would be known as a bipolar lead wire system with one of the
electrodes known as the distal TIP 120a and the other electrode
which would float in the blood pool known as the RING 120b (see
FIG. 5). In contrast, the return path in FIG. 3 is between the
distal electrode 120 through body tissue to the conductive housing
102 of the implantable medical device 100.
[0116] FIG. 5 further illustrates a bipolar lead wire system with a
distal TIP 120a and RING 120b typically as used in a cardiac
pacemaker 100C In all of these applications, the patient could be
exposed to the fields of an MRI scanner or other powerful emitter
used during a medical diagnostic procedure. RF currents that are
directly induced in the lead wire system 104 can cause heating by
Ohmic (I.sup.2R) losses in the lead wire system or by heating
caused by current flowing in body tissue. If these currents become
excessive, the associated heating can cause damage or even
destructive ablation to body tissue.
[0117] The distal TIP 120a is designed to be implanted adjacent to
or affixed into the actual myocardial tissue of the heart. The RING
120b is designed to float in the blood pool. In a pacemaker cardiac
chamber, the blood is flowing (i.e. perfusion) and is thermally
conductive, therefore RING 120b structure is substantially cooled.
However, the distal TIP 120a is thermally insulated by surrounding
body tissue and can readily heat up due to the RF pulsed currents
of an MRI field.
[0118] Accordingly, for a cardiac pacemaker application, the novel
electronic switch concepts of the present invention will be more
directed to the distal TIP 120a as opposed to the RING 120b
electrode (although the concepts of the present invention can be
applied to both). For poorly perfused areas, such as is typical in
neurostimulator electrodes, then both TIP and RING electrodes must
have an electronic switch circuit of the present invention.
[0119] FIG. 6 shows an electronic filter module 122 shown in series
with the lead wire 104 of any of the unipolar or bipolar AIMD
systems as previously illustrated in FIGS. 3, 4 and 5. One can see
that there are control leads 126 and 128 where power can be
supplied to the electronic switch module 122. Also, by varying the
power or adding electronic pulses, the switch module 122 can be
opened and closed. FIG. 7 gives a schematic diagram of the
equivalent circuit of the switch 122.
[0120] FIG. 8 is an improved switch module 130 which incorporates a
MEMS technology switch 124 in accordance with the present
invention. In this case, the power and control lead wires 126 and
128 are no longer needed. Again, FIG. 7 is the equivalent circuit
model for the MEMS switch of FIG. 8.
[0121] FIG. 9 is a tracing of an actual patient X-ray from the
Association for the Advancement of Medical Instrumentation (AAMI)
PC69 Pacemaker EMC Task Force. This particular patient required
both a cardiac pacemaker 100C and an implantable cardioverter
defibrillator 1001. The corresponding lead wire system 104, as one
can see, makes for a very complicated antenna and loop coupling
situation. The reader is referred to the article entitled,
"Estimation of Effective Lead Loop Area for Implantable Pulse
Generator and Implantable Cardioverter Defibrillators" provided by
the AAMI Pacemaker EMC Task Force.
[0122] Referring again to FIG. 9, one can see that from the
pacemaker 100C, there are electrodes 120a-c in both the right
atrium and in the right ventricle. Both these involve a TIP and
RING electrode. In the industry, this is known as a dual chamber
bipolar lead wire system. Accordingly, at a minimum the electronic
switches 124 of the present invention should be placed at the
distal TIP 120c in the right atrium and the distal TIP 120a in the
right ventricle from the cardiac pacemaker. The implantable
cardioverter defibrillator (ICD) 1001 is typically implanted
directly into the right ventricle. Its shocking TIP and sense
electrodes 120 would incorporate an electronic switch 124 either
into the electrode 120 or preferably placed adjacent to the
electrode 120 and in series with the lead wire 104 so that MRI
exposure cannot induce excessive currents in that lead wire system.
Modern implantable cardioverter defibrillators (ICDs) incorporate
both pacing and cardioverting (shock) features. Accordingly, it is
becoming quite rare for a patient to have two discrete AIMD
systems, as shown in FIG. 9. However, the number of electrodes
remain the same. There are also newer combined pacemaker/ICD
systems which include biventricular pacemakers (pacing of the left
ventricle). These systems can have as many as nine to even twelve
lead wires.
[0123] FIG. 10 is a line drawing of an actual patient cardiac X-ray
of one of the newer bi-ventricular lead wire systems. The new
bi-ventricular systems are being used to treat congestive heart
failure, and make it possible to implant leads outside of the left
ventricle. This makes for a very efficient pacing system; however,
the lead wire system 104 is quite complex. When a lead wire system
104, such as those described in FIGS. 3, 4 and 5, are exposed to RF
fields, electric currents can be induced into such lead wire
systems. For the biventricular system, electronic switches 124
would be incorporated at each of the three distal TIPs 120a-c so as
to be in series with the respective lead wire.
[0124] FIG. 11 illustrates a single chamber bipolar cardiac
pacemaker lead wire showing the distal TIP 120a and the distal RING
120b electrodes. This is a spiral wound (coaxial) system where the
RING coil 104b is wrapped around the TIP coil 104a. There are other
types of pacemaker lead wire systems in which these two leads lay
parallel to one another (known as a bifilar lead system).
[0125] FIG. 12 is a schematic illustration of the area 12-12 in
FIG. 11 which incorporates the features of the present invention.
In the area of the distal TIP 120a and RING 120b electrodes,
electronic switches 124a and 124b have been placed in series with
each of the respective RING and TIP circuits. The RING circuit wire
104b has been drawn straight instead of coiled for simplicity.
Accordingly, at an MRI pulsed RF frequency, an open circuit will be
presented thereby stopping the flow of undesirable MRI induced RF
or gradient currents. The TIP 120a is designed to be inserted into
intimate contact with myocardial tissue. Over time it becomes
encapsulated and fully embedded or buried within such tissue.
However, the RING 120b is designed to float within the blood pool,
for example, in the ventricle or atrium. With the constant blood
perfusion, the RING 120b is somewhat cooled during medical
diagnostic procedures, such as MRI. However, the TIP 120a which is
embedded in the myocardial tissue, is thermally insulated in
comparison. It can't always be assumed that a RING electrode that
is floating in the blood pool will be adequately cooled by the flow
of blood. There are certain types of patients that have illnesses
that lead to very low blood flow rates and perfusion issues.
Accordingly, the operation of the novel electronic switch 124 is
more important in the TIP 120a than it is in the RING 120b in order
to prevent distal TIP heating and associated tissue damage. In most
cardiac applications, only a TIP electronic switch 124a is required
for MRI compatibility.
[0126] FIG. 13 illustrates an equivalent circuit diagram for a
bipolar pacemaker lead wire system. On the left is the housing 102
of the pacemaker 100. The 0.5 ohm impedance represents the
approximate reactance of a typical filtered feedthrough capacitor
at high frequencies (around 128 MHz). As one can see, there are
distributed inductances and resistances along the length of each
lead wire 104a and 104b shown as L.sub.1, R.sub.1 and L.sub.2,
R.sub.2. There are actually an infinite number of these as the lead
wire system forms a transmission line. One can also see similar
reactances L.sub.3, R.sub.3 and L.sub.4, R.sub.4 in the return path
of the RING circuit. There is also distributive capacitance
C.sub.1, C.sub.2 and C.sub.3 between the closely wound or closely
routed TIP and RING wiring 104a and 104b. One can see also that
there are voltages V.sub.1, V.sub.2, V.sub.3 and V.sub.4 which
represent electromotive forces (EMFs) induced by the gradient and
RF pulsed fields of an MRI imaging machine, or similar imaging
equipment or similar therapeutic equipment.
[0127] There are a number of choices for placement of the novel
electronic switch 124 of the present invention. Choice A.sub.1
would put the electronic switch 124 inside the housing 102 of the
cardiac pacemaker 100. This would certainly be advantageous in
order to protect the electronic switch 124 circuitry from body
fluids. However, as one can see, the impedances of the lead wire
system 104 isolate the pacemaker 100 from many of the induced
voltages. In other words, it really doesn't matter if you have an
open switch 124 at location A.sub.1. The voltages induced in the
MRI field at V.sub.2 could still cause unacceptable heating or
gradient currents to flow into body tissue. In this case, body
tissue is myocardial tissue of a human heart 114. A better approach
would be to put a switch 124 at locations A or B. However, the
really optimal location that makes sure currents do not flow into
body tissue or the human heart 114 in this example, would be
location C. Accordingly, a preferred embodiment is to place the
electronic switches 124 of the present invention either at or close
to the distal TIP electrode 120.
[0128] FIG. 14 is similar to FIG. 2 and shows locations where
electronic switches 124 may be placed. First of all, the electronic
switches 124, 124', 124'' and 124''' could be placed in the header
block 112 of the AIMD 110 which is outside the titanium shield 102.
Location here allows for convenient activation by RF radio
frequencies (such as RFID pulses). Another location would be inside
of the AIMD housing 102 shown as locations 124A, B, C, D or E.
Locations B, C, D and E are in series with the pacing and sensing
outputs of the cardiac pacemaker. In order that the RF telemetry
116 circuits not be interfered with during MRI, or that
electromagnetic interference from MRI not undesirably enter through
the RF telemetry channel, it is also optional to have an electronic
switch 124 in accordance with the present invention located at
position A. This would have the effect of turning off pacemaker
telemetry during the MRI procedure. Older pacemakers and older
implantable medical devices often incorporate internal or external
low frequency magnetic loop telemetry antennas. It will be obvious
to those skilled in the art that an electronic switch 124A could
also be placed in such a telemetry loop to defeat the possibility
that it could carry on telemetry while in an MRI or similar imaging
or therapeutic environment.
[0129] Referring once again to FIG. 14, one can see that there is
an optional adapter 132 which would plug into the ports 118 and
have electronic switches 124 in accordance with the present
invention. Prior art lead wires, including their plugs 110, could
then plug into the adapter. This would allow for convenient
retrofitting of any prior art active implantable medical device and
any prior art lead system to incorporate features of the present
invention. Additional information can be learned from U.S. Patent
Application Publication No. US 2008/0195180 A1 entitled LOW LOSS
BAND PASS FILTER FOR RF DISTANCE TELEMETRY PIN ANTENNAS OF ACTIVE
IMPLANTABLE MEDICAL DEVICES, the contents of which are incorporated
by reference herein, which describes in more detail the operation
of RF telemetry pin 116 circuits.
[0130] When placing the novel electronic switch filters 124 in a
module 132 such as shown in FIG. 14, there are two options. One is
to encapsulate the electronic switches 124 in a hermetic structure
such that they are not exposed to body fluids. Another option is to
make all of the materials and design of the electronic switches or
MEMS switches 124 biocompatible. A discussion of this can be found
in U.S. Pat. No. 6,985,347, the contents of which are incorporated
by reference herein. In general, MEMS switches are the preferred
embodiment which would utilize platinum or gold contacts and
associated biocompatible structures.
[0131] Referring once again to FIG. 14, one can see that there are
RFID chips 134, 134', and 134'' that may be placed at different
locations. It is noted that an RFID chip 134 is not shown inside of
the shield housing 102 because it would not be possible for an RF
interrogator to send signals to said RFID chip. RFID chips are well
known in the prior art. They can be either powered (with a battery)
or externally powered. The externally powered RFID chip is the
preferred embodiment for the present invention. In this case, the
electromagnetic field which is sent by the reader couples through
an embedded antenna to power the microcircuit in the RFID chip. The
RFID chip 134 can be affixed anywhere along the lead wire system
104 or the lead pocket. The RFID chip 134' can be incorporated
within the header block 102 of an AIMD 100. Or the chip 134'' can
be included within a lead wire adapter 132 as shown. The purpose of
the RFID chip 134 would be to disclose the presence of the MRI
compatible electronic switches 124. In this way, a physician or
radiology technician could very quickly determine if the patient's
AIMD 100 and/or associated lead wire system 104 is outfitted with
the electronic switches 124 and is therefore suitable for
conducting an MRI examination. One is directed to U.S. Patent
Application Publication No. US 2006/0212096 A1, entitled RFID
DETECTION AND IDENTIFICATION SYSTEM FOR IMPLANTABLE MEDICAL
DEVICES, the contents of which are incorporated by reference
herein, for a more thorough explanation of these concepts. One is
also directed to U.S. Patent Application Publication No. US
2008/0065181 A1, entitled RFID DETECTION AND IDENTIFICATION SYSTEM
FOR IMPLANTABLE MEDICAL LEAD SYSTEMS, the contents of which are
incorporated by reference herein.
[0132] FIG. 15 is an illustration of placing electronic switches
124a and 124b inside of housing 102 of an active implantable
medical device 100. In this case, the switches 124a and 124b would
be activated through the external programmer which is used to
communicate with the active implantable medical device 100.
Drop-down menus could be incorporated into the software of the
external programmer in order to put the AIMD into an MRI
conditional or MRI safe mode. This would be done prior to the
patient's exposure to a medical diagnostic or therapeutic procedure
such as MRI.
[0133] FIG. 16 illustrates an active implantable medical device 136
that may or may not incorporate implanted lead wires. This is known
in the industry as a Bion. Bions generally come in two different
categories. That is, certain Bions have an internal battery and are
a stand-alone stimulation device used for urinary incontinence and
other applications. These are generally large needle injectable
systems. The Bion 136 generally is encased in a ceramic tube 138
that has an end cap electrode 140. The end cap electrode 140, for
example, could be titanium or platinum and is generally welded or
brazed to the ceramic tube 138 to make a hermetic seal thereby
protecting the sensitive electronics that are inside of the ceramic
tube 138 from damage due to body fluid. There is also an opposite
polarity RING electrode 142 as shown. This particular device
stimulates body tissue between the cap electrode 140 and the RING
electrode 142.
[0134] Other types of Bions have no battery, but instead have a
resonant coil. The device picks up its energy from an externally
worn or externally placed pulsing magnetic field pack. A patient
can wear some sort of a device around his or her waist or shoulder,
for example, with a large battery and circuit coil that produces
this field. The Bion would get its energy by coupling with this
field. No matter whether the Bion 136 is passive or has an internal
battery, it is still important to protect the internal circuits of
the Bion from temporary or permanent malfunction due to the RF
pulse frequency of MRI systems. There are also cases where the
diameter of the Bion 202 is too large for it to effectively make
contact with a precise location within a nerve or muscle. In this
case, the Bion 136 may have an associated lead wire 104 with a
distal TIP 120, as shown in FIG. 16. In this case, the end cap 140
and the lead wire 104 would be insulative wherein the electrical
connection to body tissue would occur at distal TIP 120. A small
diameter of lead wire 104 and distal TIP 120 allows the surgeon to
tunnel the lead wire 104 into a precise location and have the Bion
TIP 120 placed at a location within muscle, nerve or other body
tissue where its location can be precise. However, lead wire 104
can act very much like pacemaker lead wires, in that it could act
as an antenna and pick up undesirable RF fields from MRI.
Accordingly, overheating of lead wire 104, in conjunction with the
distal TIP 120, and/or coupling of electromagnetic interference
into the circuits of the Bion 136 are a concern. Accordingly, it is
a feature of the present invention that novel electronic switches
124 could be placed in series with lead wire 104, as shown in FIG.
16, and/or be placed internal to the Bion 136, as shown in FIG.
17.
[0135] Referring to FIG. 17, one can see an application of the
present invention where inside of the Bion device 136 a novel
electronic switch circuit 124 can be placed at the end cap
electrode 140. This would prevent selected pulse RF frequencies,
for example, those from a 3 Tesla MRI system from entering into and
disrupting or damaging the sensitive electronics of the Bion 136.
This placement is the preferred embodiment for the electronic
switch 124 because not only will it protect the internal circuits,
but it will also prevent MRI pulsed currents from flowing into the
associated body tissues, or in the case of an external lead wire
104, it would prevent RF currents from flowing in that lead wire
104 as well.
[0136] Alternatively, the novel electronic switch circuit 124 of
the present invention could also be placed at the ground electrode
142. One could also place electronic or mechanical switch circuits
124 at both the cap 140 and the ground 142 electrodes. This would
make the Bion 136 resistant to 0.5 Tesla, 1.5 Tesla, 3 Tesla and
even other MRI systems. This also eliminates currents from MRI low
frequency gradient fields as well as the pulsed RF frequencies. The
Bion 136 is just one example of an AIMD that may or may not have
implanted lead wires. Other examples include drug pumps and the
like. Accordingly, the present invention is very useful to protect
the electronic circuits of active medical devices that do not have
associated lead wires, from the high fields involved with certain
hospital and other medical diagnostic procedures such as MRI.
[0137] FIG. 18 illustrates a hermetic package incorporating a
distal TIP electrode 120 into which an electronic switch module 144
having a switch 124 is placed. This package is extremely convenient
as it places the electronic switch module 144 literally at the
distal TIP electrode 120. This is optimal to prevent any current
flow at all into body tissue. It also eliminates currents in this
location such that overheating at the distal TIP 120 is
prevented.
[0138] With reference to FIG. 18, in one embodiment, the lead wire
104 from the AIMD 100 is conductively coupled to a lead wire 146
extending from the switch module 144. This can be accomplished, for
example, by having both lead wires 104 and 146 extending through
one or more apertures 148 of an end cap 150, and conductively
coupling either the lead wires 104 and 146 to one another, and/or
to the end cap 150, such as by a laser weld 152, gold braze or the
like. Another lead wire 154 extends, typically generally opposite
the lead wire 146, through a passageway 156 of the TIP electrode
120 and is conductively coupled, such as by laser weld 158 thereto,
although other means of conductively coupling the switch module 144
to the AIMD lead wire 104 and TIP 120 are within the scope of the
invention. In this manner, the switch 124 of the switch module 144
can be selectively opened to prevent transmission of current and
electrical signals between the distal TIP electrode 120 and the
AIMD 100. The electronic switch module 144 is preferably a MEMS
ohmic switch, although it could be a MEMS capacitive switch, or
other type of electronic switch. Another alternative would be a
mechanical switch which is externally activated.
[0139] Typically, the invention is embedded or inserted into bodily
fluids or tissue. For example, the lead wire 104 could be part of a
bipolar lead wire from a cardiac pacemaker. In this case, this
would be the lead wire 104 to connect the distal TIP 120 (fixation
clips not shown) which would contact myocardial tissue. In such
cases, it is preferable to hermetically seal the switch module 144
from bodily fluids. This is done by encasing the module 144 in a
hermetically sealing material, tube, or the like. FIG. 18
illustrates one possible hermetic seal arrangement. The end cap 150
could be laser welded or gold brazed 160 to an outer tube or
cylinder 162. So as to electrically isolate the connection between
the end cap 150 and the exterior cylinder 162, insulative washers
164 and 166 could be used. The outer cylinder 162 could then be
coupled, such as by adhesion, laser weld, or the like 168 to an
inner cylinder or sleeve 170, which would contain the switch module
144. A thermosetting polymer or the like could be used to
mechanically hold the electronic switch module 144 in place within
the inner cylinder or sleeve 170. Of course, it will be understood
that a single cylinder or tube could be used to form a hermetic
connection and seal between the end cap 150 and the tip 120.
Alternatively, all of the materials used to construct the switch
module 144 could be comprised of biocompatible materials, although
this increases the cost of the switch module 144. It will be
appreciate that the distal TIP 120, end cap 150, and outer cylinder
162 would need to be comprised of, or coated with, a biocompatible
material.
[0140] FIG. 18A is a schematic drawing of the hermetically sealed
electronic switch module previously described in FIG. 18.
[0141] FIG. 19 describes an alternative method of forming a
hermetically sealed enclosure for the electronic module 144. In
this case, the electronic switch module 190 incorporates an antenna
structure 174 which activates an RFID circuit 176. The pulse from
the RFID circuit 176 is sufficient to cause a MEMS switch 124 to
activate. In this case, there is a distal TIP electrode 120 which
is designed to intimately contact body tissue. An insulative
cylinder 178 is preferably of either machined or pressed alumina
ceramic, and includes sputtering 180 (such as a titanium-molybdenum
layer) suitable for receiving a gold braze preform 182. The
cylinder 178 is attached to biocompatible electrically conductive
(preferably metallic) RINGS 184 and 186 as shown. Since RINGS 184
and 186 will be exposed to body fluid, they must be biocompatible
and made of materials such as titanium, platinum or the like. The
distal TIP 120 is laser-welded at 188 to the electrically
conducting RING 184. The hermetic package shown by FIG. 19 is best
suited for a rectilinear electronic switch module 150. Electrical
attachment is made between the distal TIP 120 and the termination
surface of the module 190 by means of a solder or thermal setting
conductive adhesive 192. A gold sputter layer 194 or equivalent
makes good electrical contact between the conductive attachment
material 192 and the distal TIP 120. The electronic switch module
190 end terminations are shown at 196 and 198. A thermal setting
conductive adhesive, solder, braze or the like 200 is used to form
an electrical connection between the electronic switch termination
198 and a sputtered layer 202 on the back side of metallic end
plate 204. The typically titanium or platinum end plate 204 has
attachments 206 and 208 to lead wire 104. The end plate 204 is
designed to be laser-welded at 210 to the biocompatible
electrically conductive housing piece or RING 184. Material 212 is
a non-conductive thermosetting polymer or adhesive. Its purpose is
to provide mechanical support to the switch module 190. It can fill
the entire space inside the hermetically sealed container or only a
portion, as illustrated. A schematic diagram for the structure of
FIG. 19 is shown in FIG. 20, wherein the electronic switch 124 is
shown to be activated by RFID chip 176.
[0142] Referring to FIG. 20, one can see that the lead wire from
the AIMD 104 enters into the switch 124. In this case, the switch
124 is an electrostatic MEMS switch. The electrostatic MEMS switch
is activated from a signal or pulse from the RFID chip circuitry
176 which causes it as a cantilever to flip from a normally closed
to an open position. The other end of this switch arrangement is
connected to the distal TIP electrode 120. One can see that the
MEMS switch 124 is shown open after it has received an
electrostatic signal from the RFID chip 176. As previously
mentioned, this could also be a magnetically activated MEMS switch
in accordance with the present invention. In the case of
magnetically activated MEMS switch, the antenna structure 171 and
associated RFID chip 176 would not be needed. However, the MEMS
switch components would include ferrite material such that they
became sensitive to switching in a magnetic field.
[0143] FIG. 21 illustrates an alternative hermetically sealed
package 214 that contains a novel electronic switch module 216
incorporating a switch 124 and RFID circuit 176, similar to that of
FIG. 19. In this case, the electronic switch module 216 has a
different type of RFID antenna configuration 218 as shown. In this
case, there is a helix active fixation electrode 220 which is
designed to be screwed into body tissue. This type of electrode 220
is well known in the prior art and is used to firmly affix a distal
TIP, for example, into myocardial tissue. There is a biocompatible
metallic end plate 224 which has been laser welded at 226 to the
insulative tube 228 and also laser welded at 230 to the helix tip
lead wire 220. At the opposite biocompatible metallic end RING 232
has been pre-attached by hermetic brazing or laser welding at 234
to the insulative tube 228. Biocompatible metallic end plate 236 is
typically made of titanium, platinum or the like. It will be
obvious to those skilled in the art for any of the inventions
herein, that other biocompatible materials, such as tantalum,
niobium and the like could also be used. Referring once again to
end plate 236, one can see that there is a sputter area 238 in
order to provide a highly conductive and oxide free surface. This
sputter layer 238 would typically be gold, platinum or the like.
Instead of sputter, this could also be applied by gold brazing,
plating or other techniques. A solder or thermosetting conductive
adhesive preform 240 is used for the seating of end plate 236. The
end plate 236 is seated against this perform 240 and is then cured
such that an electrical connection is made between the end plate
236 and the end metallization surface 242 of the module 216. The
helix TIP 220 has an electrical attachment 244 to the opposite end
metallization surface 246 of the module 216. The end plate 236 is
designed to be laser welded into the inside diameter (counter-bore)
of the mating RING 232 forming a hermetic seal. The lead wire 104
coming from the implantable medical device is then attached by
laser welding to a stub on the end plate 236 at point 246 and
optionally at point 248. Additional attachment points can be added
for additional mechanical strength. Material 250 is typically a
non-conductive thermo setting polymer or adhesive, which provides
mechanical support to the switch module 216. The polymer or
adhesive can fill the entire space inside the hermetically sealed
container 214, or only a portion, as illustrated.
[0144] FIG. 22 illustrates a distal electrode pad 252 applicable to
a wide variety of neurostimulator applications. Neurostimulators
include cochlear implants, deep brain stimulators, spinal cord
stimulators, incontinence stimulators, general pain control
stimulators, Vagus nerve stimulators, Parkinson's tremor control
stimulators and the like. Typical prior art stimulators often come
with a variety of pads such as that shown in FIG. 22. Three
neurostimulation electrodes 254, 256 and 258 are shown, however,
these can vary anywhere from one, ten or even 20 or more
neurostimulation electrodes. For example, in cochlear
neurostimulators, there are commonly sixteen wires, which are
inserted in a bundle of electrodes to make contact to the auditory
nerves. Referring back to FIG. 22, one can see that there is a lead
wire bundle 260 which contains three wires that are connected to an
external or implanted active medical device 252.
[0145] FIG. 23 is a cross-sectional view taken generally along line
23-23 of FIG. 22, and illustrating one form of the novel electronic
switch 124 of the present invention. One can see that there is an
electronic MEMS switch 124 located within the electrode assembly
254. The electronic switch 124 can be a prior art electronic or
MEMS switch or equivalent. In this case, the distal TIP electrode
pad 254 has a laser weld or equivalent biocompatible electrical
attachment 262 to the surrounding metallization 264 of the
cylindrical ceramic insulator structure 254. The electrode pad 254
is electrically connected to the cathodic contact 266 of the MEMS
switch 124. The cathodic contact 266 is rigid in this case and is
not free to move. Lead wire 268 is then routed through the flexible
neurostimulator pad insulation/encapsulant 270 (also as shown in
FIG. 22) and is electrically connected to the anodic MEMs contact
cantilever 272 as shown. In FIG. 23, the MEMS switch 124 is shown
in the up or open state wherein no currents can flow through it.
This is the ideal position for MRI imaging or other procedures such
as surgical electrocautery (Bovi knife for example) requiring high
RF energy.
[0146] FIG. 23A shows the MEMS switch 124 of FIG. 23 in the down or
closed state. In this position, biological sensing and therapy
signals freely pass through the switch 124.
[0147] FIG. 24 illustrates the novel electronic switch assembly 124
of the present invention shown in an encapsulated switch assembly
or module 274 convenient for location anywhere in the lead wire 104
circuit. In this case, the entire module 274 has been overlaid with
a suitable glass seal 276. This glass 276 can be deposited as a
frit or molten and then sintered at high temperature. The glass 276
is designed to adhere to the substrate 278 and the wire bond pads
280 and 282 such that it forms a hermetic seal over the entire
switching circuitry 274 of the present invention. Material 276 can
be any number of borosilicate or compression glasses or even
polymer sealants such as silicone and the like.
[0148] FIG. 25 is a representation of how one could also glass
hermetically seal 276 any of the novel switch assemblies 274'
incorporating electronic switches 124 of the present invention. It
will be obvious to those skilled in the art that the hermetic seal
276 could be of ceramic, sapphire, glass and can be sealed in a
variety of ways to prevent the intrusion of body fluids into the
sensitive capacitor or inductor element embedded therein. The
hermetic seal structure 276 shown in FIG. 25 is easiest done by
glass. There are a number of prior art glass sealing processes that
are used for capacitors and diodes and the like. In the art, many
of these are known as DAP sealers.
[0149] FIG. 26 shows a prior art pacemaker active fixation distal
TIP 284. A novel electronic switch module 286 is shown in close
proximity to the helix tip 288. By having the electronic switch 286
in immediate proximity to the distal TIP 282, one can prevent the
flow of MRI induced gradient and RF pulsed currents into myocardial
tissue. One can see the lead wire 104 is routed from the output of
the implantable medical device. Typically, this lead wire 104 would
be constructed of biocompatible alloy MP-35N and laser welded at
points 290 and 292 as shown. Optional stop 294 is shown and can be
laser welded 296 in place on the spline 298. This would typically
be done after assembly. A novel feature of the present invention is
that assembly of the entire novel electronic switch 286 can be done
with the helix 288 outside of its housing 300. This allows for
easier electrical and mechanical connections (assembly), and
enables high reliability screening of the switch 286, such as
thermal shock, burn in, and the like. This is very important so
that the electronic switch 286 will be highly reliable in the
patient application. Once all of this testing has been done, the
entire assembly consisting of helix 288, electronic switch 286,
flange 654 and spline 298 can be inserted by screwing it past guide
302 and into the cylinder 300 from the right side. Then, novel end
cap 304 is placed over the spline shaft 298 and laser welded 306 in
place. It is not necessary that laser weld 306 be 360 degrees (only
spot attachments are required). Subsequently, stop 296 can be laser
welded 296 in place onto the shaft 298 as shown. Then the lead wire
104 consisting of MP-35N alloy can be laser welded at points 290
and 292 as shown, which completes the assembly.
[0150] FIG. 27 illustrates applying the novel electronic switch
assembly 286 to a prior art active fixation distal TIP 284
previously described in FIG. 26. In this case, the electronic
switches 124 are MEMS switches in package 286 such that it has
convenient external gold metallization surfaces 306 and 308. This
is convenient because this entire assembly 286 can be directly
brazed via braze performs 310 and 312 into the active fixation
assembly. This protects the relatively sensitive MEMS switch 124
within the hermetically sealed package 286. As the physician
applies torque along the shaft 298, this entire assembly and its
associated helix tip 288 is designed to be screwed into body
tissue. The hermetic package 286 is very strong and robust thereby
protecting the sensitive electronic switches 124 from the torsion
during insertion into body tissues.
[0151] Referring once again to FIG. 27, one can see the attachment
from the metallization 306 of novel electronic switch module 286
shown attached to the spline and spline pedestal 298 and 314. This
is typically accomplished by a gold braze perform 312. In this
case, the pedestal 314 has been counterbored to receive the end of
the novel electronic switch assembly package 286. This allows the
gold braze material 312 to angle up along the sides of the
electronic switch package 286, thereby adding sheer strength to the
overall assembly. It will be obvious to those skilled in the art
that this same counterbore could also be applied to the helix
pedestal post assembly 316. This would allow gold braze material
310 to also come around the sides of the novel electronic switch
module 286 to provide sheer strength in that area also. A similar
gold braze preform 310 is used to attach a distal TIP helix
pedestal 316 to the metallization 308 of the novel electronic
switch 286. Of particular advantage is that the novel electronic
switch 286 substrate can be constructed entirely of low k, very
high strength ceramics. In this case, pure alumina or porcelain
would be preferred embodiments. These have the advantage of being
mechanically very rugged and also very rugged to thermal shock such
that it would take pure gold brazing. By use of all biocompatible
materials, the assembly is greatly simplified in that it need not
be hermetic. It would also be possible to replace the gold brazes
310 and 312 with equivalent laser welds.
[0152] Referring once again to FIG. 27, one can see that the end
cap 304' has been modified in a novel way such to make it flush
with the outside diameter of the active fixation TIP assembly 300.
This allows one to increase the inside diameter allowing room for
the novel counterbore as previously described in pedestal 314. The
metallic end cap 304' has been stepped 316 so that it is seated for
convenient fixturing and also provides a countersink 316 for
convenient gold brazing or laser weld material 306.
[0153] FIG. 28 shows a prior art deep brain stimulator or other
type of neuromodulation electrode. There are electrode RING
assemblies shown at locations 542, 542', 542'' and 542'''.
[0154] FIG. 29 is an enlarged view of an electrode embodying the
present invention and similar to area 29-29 from FIG. 28. One can
see that there are MEMS electronic switches 146 and 146' that are
placed in a position to put them in series with the lead wire 104
before it contacts the electrode surfaces 542 and 542'. These
switches, of course, can be any electronic switch in accordance
with the present invention. In this case a matrix of multiple MEMS
cantilevers are used to facilitate higher currents.
[0155] FIG. 30 illustrates the hermetic seal 236 of an AIMD. Lead
wires 552 are typically routed from the outside (body fluid side)
through the hermetic seal to the inside. In this case, lead wire
104 has been broken into discontinuous nail head segments 104 and
104'. Lead wire 104 extends through an alumina ceramic 334 which is
fixed to the end of lead wire 104 at location 340, such as by laser
weld, gold braze or the like. The alumina ceramic 334 is also
attached to a ferrule 342 at location 344, by gold braze or the
like. Although not shown, the housing of the AIMD is typically
welded to the ferrule 342, so as to create a hermetic seal
therewith. The switch assembly 336 is electrically connected to the
lead wire segments 104 and 104'. It may be disposed within a plate
or substrate 346, which can be an alumina ceramic, plastic or any
other insulative material suitable for placing the switch assembly
336 therein. The plate 346 could also be an inductor slab as
described in U.S. Pat. No. 6,999,818. The electrical attachment
between the lead wire segments 104 and 104' to the switch assembly
336 is made by electrical connection material 348 and 350.
Typically, the ends of the switch assembly 336 are metalized 352
and 354 so as to create an electrical connection therebetween. The
plate or slab 346 and a feedthrough capacitor 338 are coupled to
the ferrule 342 at location 356. Typically, the adhesive 356 is a
conductive adhesive such that the grounding electrodes are
conductively coupled to the metallic ferrule 342, as is well known
in the art. The novel electronic switch 336'' has been placed in
accordance with the present invention between the lead wire 104 and
the typical prior art EMI feedthrough capacitor 338. It will be
obvious to those skilled in the art that the electronic switch
336'' could also be placed in other locations within the AIMD.
However, referring back to FIG. 30, this particular location is the
preferred embodiment as when the switch 124 is open it then becomes
impossible for high frequency RF signals from medical diagnostic or
therapeutic equipment, such as MRI, from entering into the shielded
housing of the AIMD. Once such EMI is inside, it can readily cross
couple or re-radiate to adjacent sensitive circuits, such as
pacemaker sensing circuits. This can cause disruption or even the
delivery of inappropriate therapy. There have been cases of
inadvertent ICD discharge during MRI procedures, for example.
Accordingly, it is a feature of the present invention that the
novel electronic switch 124'' can be placed at or near the input or
point of ingress or egress of lead wires 104 to the AIMD. As
previously described, it would also be preferable to place an
electronic switch 124 near the distal TIP electrode 120 to prevent
induced currents from flowing into body tissue.
[0156] FIG. 31 is a diagrammatic representation of a human heart 14
which includes right and left subclavian veins 358 and 360
respectively, the superior vena cava 362 and the aorta 364. A lead
wire 104, which is typically routed from a biventricular cardiac
pacemaker or a biventricular implantable cardioverter defibrillator
(ICD) (which are not shown), is routed through a catheter 366 and
directed, in this case, through the left subclavian vein 360 and
then down through the superior vena cava 362 and into the coronary
sinus 368. The lead wire 104 must first enter the coronary sinus
ostium 370 where the implanting physician selects the correct
location. The coronary sinus 368 is actually divided into two
zones: the first part (on the left) is known as the coronary sinus
368; and the second part (on the right) is called the great cardiac
vein 372. The great cardiac vein 372 wraps around the back of the
left ventricle. The novel electronic switch 124 of the present
invention is intended to be placed ideally near the end of the
great cardiac vein 372 where it breaks into several venous branches
374 and 376.
[0157] Referring once again to FIG. 31, one can also see the right
ventricle 378 and the right atrium 380, and the heart 14 also
includes the left atrium 382 and the left ventricle 384. The ideal
location for a proximal electronic switch 124 is shown. An ideal
length for the proximal electronic switch 124 would be between 5
and 7.5 mm in length. At this particular location, at the end of
the great cardiac vein 372, cardiac motion is relatively small and
fibrotic tissue will tend to encapsulate the electronic switch 124
and its lead wires 104 and thereby attach it/fixate it in position
in this relatively low motion region. This is a particular
advantage to the present invention, in that the lead 104 will
remain highly reliable and resistant to breakage. Because of the
relatively large diameter of the coronary sinus 368 and the great
cardiac vein 372, this portion of the lead wire system, including
the electronic switch 124, can be of much larger diameter (for
example, 7 or 8 French). Beyond this point, where the great cardiac
vein 372 branches, the venous systems become much smaller. In
general, these branches are below 6 French in diameter and ideal
electrode sizes go all the way down to 3 French. FIG. 32 shows the
relationship between French size, millimeters and inches. Since
left ventricular pacing is important for cardiac resynchronization
and treatment of congestive heart failure, it is a feature of the
present invention that a lead wire reduction occurs at the point of
egress of the electronic switch 124 allowing insertion of
electrodes into the small diameter venous system in the proper
position outside the left ventricle 384.
[0158] The primary benefit of locating the electronic switch 124 in
the coronary sinus 368 and/or great cardiac vein 372 is that the
diameter of the electronic switch 124 itself can be larger making
it much easier to manufacture. The distal portion 386 of the lead
104 from the electronic switch 124 is smaller (3 to 6 French size)
for easier employment and navigation into the branch veins of the
left ventricle 384. Secondary benefits beyond the diameter of the
electronic switch 124 include the length of the electronic switch.
Entering into and navigating the coronary sinus 368 and great
cardiac vein 372 generally involve larger bend radii compared to
accessing and navigating the branch vessels. Therefore the portion
of the lead 386 that traverses through and resides in the branch
vessels must be very small and very flexible, not having a stiff
section longer than approximately 1.5 mm as a rule of thumb. Rigid
sections of the lead 104 measuring longer than 1.5 mm can impede
the ability to navigate around the tight corners and bends of the
branch vessels. In the coronary sinus 368 and great cardiac vein
372, however, there is substantially more latitude, and stiff
sections of the lead could approach 5 mm or even 7.5 mm without
drastically impeding deliverability. A secondary benefit of
locating the electronic switch 124 in the coronary sinus 368 or the
great cardiac vein 372 has to do with MRI image artifacts. Although
the image artifact will be quite small due to avoiding the use of
ferromagnetic materials, it is still beneficial to locate the
electronic switch 124 away from the coronary arteries, ventricular
wall motion or other anatomies/physiologies/pathologies of most
interest. Location of this area is particularly important for
example for a magnetically activated MEMS switch. By definition a
magnetically activated MEMS switch incorporates ferrite materials
in its cantilever beam system. The presence of ferrite materials
will introduce a certain amount of image artifact. Therefore by
locating the switch away from the anatomies of most interest,
imaging of the left ventricle can still be accomplished. If the
electronic switch 124 is located in the coronary sinus 368,
however, it could generate small artifact in the vicinity of the
valves. Another benefit of having the electronic switch 124 located
in the coronary sinus 368 or the great cardiac vein 372 is that its
rigidness provides a foundation on which fixation fixtures may be
more strategically utilized. For example, one or more tines could
originate from the region of the lead where the electronic switch
124 resides. Additionally, rigidness of the electronic switch 124
makes the tines more effective in their engagement of the vessel
walls. Alternatively, a rigid portion of the lead 104, skillfully
navigated beyond a corner or bifurcation, can function as a
fixation mechanism that proves difficult or requires skill to track
the lead.
[0159] FIG. 33 is an enlarged perspective view of the lead wire
system 104 of FIG. 31. One can see that there is a guide wire 388
which is common in the prior art for inserting into position prior
to sliding the lead wire system 104 down over it. A terminal pin
390 is designed to plug into the implantable medical device, such
as a pacemaker or ICD. The electronic switch 124 is shown at the
point where the lead wire 104 would be reduced from 6-9 French down
to 3-6 French. Optional fixation tines 392 are shown which may be
affixed to the electronic switch 124. By way of reference, the
French scale is related to both diameter in millimeters (mm) or
inches. For example, 7 French is 2.3 mm (0.092 inch) in diameter
and 3 French is only 1 mm in diameter (0.039 inch). The length (l)
of the reduced diameter lead wire section 386 can be adjusted in
accordance with the branch vein into which the lead system is being
inserted in the desired location of the electrodes 394. Below the
electrodes 394 is the other end of the guide wire 388. Once the
electrodes 394 are in the proper position and the system has been
tested, the guide wire 388 is typically removed. A particular
advantage of the lead system 104 as shown in FIG. 32 is that no new
deployment instruments or catheters are required. In other words,
this system that includes the novel electronic switch 124 is
backwards compatible with all known deployment systems. It is also
very important that the lead wire system 104 is designed to be
extracted in the case of a broken lead, defective lead or infected
lead. The lead wire system illustrated in FIG. 33, is also
backwards compatible with current (prior art) mechanical and laser
lead extraction technologies.
[0160] FIG. 34 is a diagrammatic representation of the human heart
14 similar to that illustrated in FIG. 31. However, in this case,
external (epicardial) electrodes 396 are attached outside and to
the left ventricle 384 by means of epicardial leads 398. A
sutureless myocardial lead 398 is shown affixed to the outside of
the left ventricle. This methodology is well known and generally
involves an insertion between the ribs outside of the heart and a
screwdriver type feature to affix the sutureless epicardial lead
tip electrode 396 in place. Epicardial leads may also have a suture
feature which is also well known in the art, which can have a
helical or other configuration type tip. It will be obvious to
those skilled in the art that the present invention can be extended
to any type of external (epicardial) electrode or satellite pacer
affixed to the outside of the heart, particularly outside of the
left ventricle.
[0161] FIG. 35 is a cross-sectional view taken generally along line
35-35 of FIG. 34, illustrating an epicardial lead electrode
assembly 396 which includes an electronic switch 400. In this case,
the electronic switch is shown in the novel MEMS coaxial structure
with multiple cantilevers 402 and 404 as shown. This will be useful
in high voltage or high current applications such as for an ICD.
Other types of MEMS switches or electronic switches can be easily
adapted to this device. The electrode 396 includes a helix
electrode 406 designed for insertion into body tissue. In the prior
art, the epicardial lead electrode assembly 396 is typically
over-molded with silicone rubber 408. The assembly shown in FIG. 35
is self-affixing to the myocardial tissue by a helical electrode
structure 406. Typically this electrode is affixed into the
myocardium by 31/2 mechanical turns and is made of platinum-iridium
alloy or equivalent biocompatible material. The helical electrode
tip 406 is affixed into the myocardial tissue by a screwdriver type
turning surgical tool. With reference to FIG. 35, it can be seen
that the helical electrode 406 is electrically connected to both
the anode cantilevers as well as the cathode cantilevers 404 so as
to be in electrical series therewith.
[0162] FIG. 36 illustrates a split cylinder cuff electrode 410
embodying two electrodes, anode 412 and cathode 414. This is
designed to be inserted by a physician around a nerve. It is a
bipolar system typically consisting of a 6 to 8 French diameter
lead body 416. A double electronic switch module 418 (two discrete
electronic switches 124 in parallel) otherwise known in the art as
a double pole-single throw switch (DPST) is used in accordance with
the present invention is located as shown. In general, the cuff 410
is sized to match the diameter of the nerve which passes through
its center. The lead body 416, after the double electronic switch
418, is of a reduced diameter, generally in the 3 to 4 French
range. Not shown is a closing suture which is typically used to
draw the cuff together after it's installed.
[0163] FIG. 37 illustrates helical nerve cuffs 420 which are
self-sizing. These incorporate electrode foils 422 which are well
known in the prior art. The lead body 424 is attached to an
electronic switch module 426 of the present invention. This can be
unipolar or bipolar. The electrode foil 422 can either be etched or
stamped and then the termination point where the conductor attaches
to the foil is either prepared or fabricated. This is the location
to where the electronic switch 426 is electrically attached and
incorporated. The conductors and the foil 422 on the electronic
switch 426 are laid into a split mold and assembled and then
silicone is injected into the mold. FIG. 37 illustrates one leg of
a bipolar or multipolar lead. Obviously, an electronic switch 124
would be required for each electrode foil 422 in the multipolar
configuration.
[0164] FIG. 38 is an enlarged sectional view taken generally along
line 38-38 of FIG. 37.
[0165] FIG. 39 illustrates a larger multiple cuff nerve electrode
428 for current steering in a large nerve trunk. Various electrodes
can be stimulated by trial and error to obtain the optimal result.
For example, for pain control, one can try various electrodes and
various types of electrical stimulation by trial and error until
pain is minimized or eliminated. The multiple parallel electronic
switches 430, similar to that described previously, can be placed
in conjunction with each one of the electrodes 432 as shown.
[0166] FIG. 40 illustrates an alternative in that a multiple
electronic switch array 434 is shown in series with the lead bodies
436 and 438. This can be built as an integrated circuit, hermetic
package or other technologies which are well known in the prior
art. This can in turn be connected to the cuff electrode 428 as
previously described in FIG. 39 or to the multiple cuff electrodes
410 and 420 as previously illustrated in FIGS. 36 and 37. It can
also be adapted to the multiple single electrodes 436 as
illustrated in FIG. 41. The multiple electronic switches 434 as
illustrated in FIGS. 40 and 41 can be made in a variety of ways
from any of the technologies that are known in the prior art. This
is accomplished by putting the devices on a substrate next to each
other and arranged so that they are in series with each lead wire.
It will be obvious to those skilled in the art that the structures
as illustrated in FIGS. 40 and 41 can be over-molded with silicone,
glass or the like to provide reliable protection against body fluid
and mechanical attachment.
[0167] FIG. 42 is a diagrammatic cross-sectional view of the human
head, showing the skull 440 and the brain 442. A burr hole 444 is
drilled through the skull 442 for placement of deep brain probe 446
with associated electrodes 448. One can see that there is a lead
wire body 450 which has been tunneled up underneath the skin and
attaches to the deep brain probe 446. One or more electronic
switches 452 of the present invention are incorporated into the
brain probe 446, electrodes 448, or lead wire body 450.
[0168] FIG. 43 is a diagrammatic cross-sectional front view of the
human head, showing that there can be multiple deep brain probes
446 . . . 446n placed as previously described in connection with
FIG. 42. In a preferred embodiment, the top of the deep brain probe
446 and associated electronic switches 452 would be flush with the
top of the skull 440. The lead wire 450 is generally connected to a
pulse generator or transmitter (not shown) which is either
implanted or can sit outside the skin. There can also be a receiver
(not shown) which typically sits on the skin. The deep brain probe
446 can also have a nail head or nail shank.
[0169] FIG. 44 is an elongated sectional view of the area indicated
by line 44 in FIG. 43, of the deep brain probe 448. Shown is the
location of the electronic switch assembly 452, the skin 454 which
covers the skull 440, the lead wire 450, the burr hole 444, dura
layer 456 and the brain 442. At the end of the deep brain probe 448
are electrodes 448.
[0170] FIG. 45 illustrates an alternative view wherein a highly
flexible region 458 is connected between the electronic switch
module 126 and the electrodes 132. This flexible section 458
accounts for the relative motion between the skull 440 and the
brain 442.
[0171] FIG. 46 illustrates an alternate configuration taken from
FIG. 44 wherein the top of the electronic switch module 452 is
below the skull bone 440. The burr hole 444 is covered with a bone
encapsulant 460. It will be obvious to those skilled in the art
that the deep brain probe 448 of the present invention can be
placed in various locations that are convenient for the
physician/surgeon.
[0172] FIG. 47 is a hermetic package consisting of the distal
electrode trip 462 and a convenient pedestal 464 for connection of
an AIMD lead wire 104. Typically the AIMD lead wire 104 is attached
electrically and mechanically to the metal pedestal by laser welds
466 and 468. The metal cap 470 is typically of platinum, platinum
radium, titanium or other suitable biocompatible material. A
hermetic seal is formed by gold braze 472 to the insulative ceramic
coaxial substrate 474. Hermetic seals are also formed at braze seal
joints 476 and 478 to completely encapsulate the electronic switch
assembly 480 of the present invention. In this case the electronic
switch 480 is a special coaxial MEMS switch with a through hole 482
through the center as shown. In this case there are multiple
cantilevers 484 and 486 shown for high current applications.
Multiple cantilevers 484 and 486 are shown for redundant circuit
paths thereby allowing high currents to flow. One set connected to
the outside diameter metallization 488 of the MEMS cantilevers
switch contacts 484 are rigid and not free to bend. The other set
of contacts 486 which are connected to the inside diameter
metallization 490 are free to bend in the presence of electrostatic
forces which is a principle well known in MEMS technology. The
switch assembly is preferably sandwiched between insulator
substrates 492 and 494 such that the metal cap 470 is electrically
connected only to the outer metallization 488 and the inner
metallization 490 is electrically connected to the electrode TIP
462, such as by means of conductive adhesive 496 or the like. In
this manner, the electronic switch assembly 480 resides in series
between the lead wire 104 and the electrode TIP 462. The novel
discoidal geometry of the switch assembly 480 is new and not known
in the prior art.
[0173] Referring once again to FIG. 47 one can see that there is a
guide wire 498 which extends through an opening 500 of the top
plate 470, through the throughhole 482 of the switch assembly 480,
and a passageway 502 of the TIP electrode 462. During implantation
the physician threads the guide wire 498 through the venous system
until it is in the desired location. This is a lot easier than
threading the much larger diameter lead wire system. Once the guide
wire 498 is suitably placed then the physician simply slips the
lead system including its electrode tip 462 down over the guide
wire. At this point guide wire 498 is withdrawn.
[0174] In all of the previously described embodiments, it is
preferable that the electronic switches be as close to the
electrode-to-tissue interface as possible. The reason for this is
that lead wire systems act as transmission lines in that they have
series inductance and resistance and also stray capacitance from
lead to lead. This tends to cause them to decouple into various
loops at MRI pulsed frequencies. It is for this reason, for example
in a cardiac pacemaker, that placing an electronic switch at the
housing of the cardiac pacemaker will not provide adequate cooling
at the end of, for example, a 52 cm lead wire whose electrode tip
is inside, for example, the right ventricle. The impedance of the
lead wire would tend to decouple the electronic switch by
presenting a very high impedance at the MRI RF pulsed frequencies.
Accordingly, it is a feature of the present invention that the
electronic switches be placed in relatively close proximity to the
delivery electrodes as illustrated in the accompanying
drawings.
[0175] This principle varies with the RF pulsed frequency of the
MRI machine. For example, a 0.5 Tesla machine has an RF pulsed
frequency of 21 MHz. In this case, the wavelength is long enough
where the electronic switches could be a considerable distance away
from the delivery electrode and still be quite effective. However,
when one gets up around 3 Tesla with an RF pulsed frequency of 128
MHz, then the electronic switch must be much closer to the delivery
electrode. This is because the impedance along the series lead wire
tends to increase at higher frequencies.
[0176] With reference to FIG. 48, in certain AIMD applications,
there can be as many as 6, 10, 12, 16 or more electrodes. There is
largely a trial and error process after implantation as to which
electrode pairs or which electrodes the physician will decide to
use. For example, in a deep brain application, the physician will
experiment using different electrodes until the desired patient
responses is achieved. In a cochlear implant application, which
typically uses 16 electrodes, the Audiologist will experiment with
various electrodes until optimal patient hearing is achieved. A
downside of these AIMDs is that a novel electronic switch would be
required in series with each one of those electrodes. FIG. 48
illustrates a novel electronic multiplexing circuit 504. By way of
illustration, a novel single MEMS switch 124 is shown. After the
physician determines which of the multiple mini electrodes 506 is
the optimal one by using the multiplexing function 508, the
physician or technician can switch in the electronic switch 124 in
series with that particular circuit. This has the effect of greatly
reducing the number of electronic switches required.
[0177] As an alternative, instead of switching in an electronic
switch, one could put a novel band stop filter as previously
described in United States Patent Application Publication Nos. US
2007/0112398 A1; US 2008/0132987 A1; US 2008/0049376 A1; US
2008/0116997 A1; and US 2006/0247684 A1, the contents of which are
incorporated herein. It will be obvious to those skilled in the art
that by the use a multiplexing network, that the number of bandstop
filters can be significantly reduced. Referring back to FIG. 48,
one can see, if for example, there were 16 electrodes, one might
get away with only 4 electronic switches (or alternatively,
bandstop filters consisting of a parallel inductor and capacitor
element) to service selectively all 16 electrodes.
[0178] Although several embodiments have been described in detail
for purposes of illustration, various modifications may be made
without departing from the scope and spirit of the invention.
Accordingly, the invention is not to be limited, except as by the
appended claims.
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