U.S. patent application number 13/428350 was filed with the patent office on 2012-07-12 for stimulation system, in particular a cardiac pacemaker.
This patent application is currently assigned to UNIVERSITAT DUISBURG-ESSEN. Invention is credited to Erhard KISKER, Heinrich WIENEKE.
Application Number | 20120179219 13/428350 |
Document ID | / |
Family ID | 37836798 |
Filed Date | 2012-07-12 |
United States Patent
Application |
20120179219 |
Kind Code |
A1 |
KISKER; Erhard ; et
al. |
July 12, 2012 |
STIMULATION SYSTEM, IN PARTICULAR A CARDIAC PACEMAKER
Abstract
A stimulation system, an implantable electrode device and a
method for operating an implantable electrode device are proposed.
A simplified implantation, a simple construction and reliable
control are made possible by the electrode device being supplied
with energy, and controlled, in an exclusively wireless manner via
a time-variable magnetic field. The magnetic field is generated by
an implanted control device.
Inventors: |
KISKER; Erhard; (Dusseldorf,
DE) ; WIENEKE; Heinrich; (Essen, DE) |
Assignee: |
UNIVERSITAT DUISBURG-ESSEN
Essen
DE
|
Family ID: |
37836798 |
Appl. No.: |
13/428350 |
Filed: |
March 23, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12171955 |
Jul 11, 2008 |
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13428350 |
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PCT/EP2006/012193 |
Dec 18, 2006 |
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12171955 |
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Current U.S.
Class: |
607/7 ; 607/32;
607/9 |
Current CPC
Class: |
A61N 1/3787
20130101 |
Class at
Publication: |
607/7 ; 607/32;
607/9 |
International
Class: |
A61N 1/39 20060101
A61N001/39; A61N 1/362 20060101 A61N001/362 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 13, 2006 |
DE |
102006001968.7 |
Feb 17, 2006 |
DE |
102006007403.3 |
Sep 8, 2006 |
DE |
102006042850.1 |
Claims
1-24. (canceled)
25. A cardiac pacemaker or defibrillator stimulation system
comprising: an implantable control device and an implantable
electrode device for generating electrical impulses, which can be
one or more of supplied with energy and controlled by the control
device in an exclusively wireless manner by means of a time-varying
magnetic field, the electrode device being configured such that
two-stage triggering is required to generate an electrical impulse
by means of the electrode device, wherein pulse generation or
triggering is only made possible after respective previous
activation.
26. The stimulation system according to claim 25, wherein the
activation is effected by a signal different than a signal used for
pulse generation or triggering.
27. The stimulation system according to claim 26, wherein the
signal for activation has an opposite field direction of the
magnetic field.
28. The stimulation system according to claim 25, wherein the
triggering is not sensitive to interference to minimize the
possibility of triggering a next electrical impulse as a result of
an interference signal.
29. The stimulation system according to claim 25, wherein the
system is adapted such that the activation takes place shortly
before the generation of a next electrical impulse.
30. The stimulation system according to claim 25, wherein a
polarity of a coil core of the electrode device is reversed prior
to activation for triggering a next electrical impulse.
31. The stimulation system according to claim 30, wherein the coil
core polarity is reversed by an external magnetic field having an
opposite direction compared to that for generation or triggering
the electrical impulse.
32. The stimulation system according to claim 25, wherein the
control device is configured such that the magnetic field is
alternately generated with an opposite field direction for the
alternate generation of an electrical impulse and activating the
electrode device before generating the next electrical impulse.
33. An implantable electrode device for a cardiac pacemaker or
defibrillator stimulation system for generating electrical
impulses, wherein the electrode device is configured as one or more
of a wireless and compact structural unit and can be supplied with
energy and directly controlled exclusively by means of a varying
magnetic field, the electrode device being configured such that
two-stage triggering is required to generate an electrical impulse
by means of the electrode device, wherein pulse generation or
triggering is only made possible after respective previous
activation.
34. The electrode device according to claim 33, wherein the
electrode device is configured such that only after preceding
activation an electrical impulse is generated when a first minimum
field strength of the magnetic field is exceeded.
35. The electrode device according to claim 33, wherein the
electrode device is configured such that it generates and delivers
an electrical impulse each time the minimum field strength is
exceeded after activation.
36. The electrode device according to claim 33, wherein the
electrode device is configured such that an electrical impulse can
be generated in each case only following previous activation.
37. The electrode device according to claim 33, wherein the
electrode device is configured such that an electrical impulse can
be generated in each case only following previous activation by
exceeding a second minimum field strength of the magnetic field
having the opposite field direction to the field direction for the
generation of an electrical impulse.
38. The electrode device according to claim 37, wherein the second
minimum field strength is greater than the first minimum field
strength.
39. An implantable electrode device for a cardiac pacemaker
stimulation system for generating electrical impulses, wherein the
electrode device is configured as one or more of a wireless and
compact structural unit and can be supplied with energy and
directly controlled exclusively by means of a varying magnetic
field, wherein the electrode device comprises a coil device with a
coil core or a core element having a magnetization which varies
abruptly depending on an acting magnetic field strength.
40. The electrode device according to claim 39, which generates a
pulse-like induction voltage when a first minimum field strength of
the magnetic field is exceeded and is caused by the abruptly
varying magnetization of the core or core element.
41. The electrode device according to claim 39, wherein the coil
core or core element being at least partially magnetically soft or
ultra-soft.
42. The electrode device according to claim 39, wherein the coil
device has a layer arrangement of soft and hard magnetic
material.
43. The electrode device according to claim 39, wherein the coil
device core or core element is ferromagnetic.
44. The electrode device according to claim 39, wherein the
electrode device is configured such that each pulse-like induction
voltage is output as an electrical impulse via integrated
electrodes.
45. A method for operating an implantable electrode device, the
implantable device being a cardiac pacemaker or defibrillator for
generating electrical impulses, wherein the electrode device is
supplied with energy and directly controlled by means of a magnetic
field to generate the electrical impulses, wherein two-stage
triggering is required to generate an electrical impulse with the
electrode device, wherein pulse generation or triggering is only
made possible after respective previous activation.
46. The method according to claim 45, wherein the activation takes
place shortly before the generation of the next electrical
impulse.
47. The method according to claim 45, wherein the activation is
effected by a signal different than that used for pulse generation
or triggering.
48. The method according to claim 46, wherein the signal has the
opposite filed direction of the magnetic field.
49. The method according to claim 45, wherein the polarity of a
coil core or core element is reversed prior to activation for
triggering the next impulse.
50. The method according to claim 49, wherein the coil core
polarity is reversed by the magnetic field having an opposite
direction compared to that for generation or triggering of the
electrical impulse.
51. The method according to claim 45, wherein the magnetic field is
alternately generated with an opposite field direction for the
alternate generation of an electrical impulse and activating the
electrode device before generating a next electrical impulse.
52. The method according to claim 45, wherein a number of magnetic
field impulses is varied for variation of the duration of each
electrical impulse or a contiguous sequence of electrical
impulses.
53. The method according to claim 52, wherein the magnetic impulses
have a substantially sawtooth-shaped profile.
54. A method for generating an electrical impulse in tissue for
operating a cardiac pacemaker or defibrillator, wherein a
pulse-like induction voltage is caused by an abruptly varying
magnetization of a coil core or core element when a first minimum
field strength of the magnetic field is exceeded.
55. The method according to claim 54, wherein the coil core or core
element is at least partially magnetically soft or ultra-soft
having a layer arrangement of soft and hard magnetic material or is
ferromagnetic.
56. The method according to claim 54, wherein a magnetization of
the at least partially soft or ultra-soft magnetic coil core or
core element is varied by an external or varying magnetic field in
order to vary the magnetic leakage flux for direct electrical
stimulation or generation of the electrical impulse.
57. The method according to claim 54, wherein the pulse-like
induction voltage is generated when a first minimum filed strength
of the magnetic field is exceeded.
58. The method according to claim 54, wherein the pulse-like
induction voltage is caused by the abruptly varying magnetization
of the coil core or core element.
59. The method according to claim 54, wherein the pulse-like
induction voltage is output as an electrical impulse via integrated
electrodes.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is U.S. CIP National Application under 35
U.S.C. 111(a) and claims priority to PCT Application WO/2007/087875
(PCT/EP2006/012193), filed Dec. 18, 2006, which claims priority to
German Applications, 10 2006 001 968.7, filed 13 Jan. 2006, 10 2006
007 403.3, filed 17 Feb. 2006 and 10 2006 042 850.1, filed 8 Sep.
2006, all of which are incorporated herein by reference in their
entirety.
[0002] An exemplary embodiment of the present invention relates to
a stimulation system, in particular for a cardiac pacemaker, an
implantable electrode device or stimulation device for a
stimulation system as well as a method for operating an implantable
electrode device or stimulation device, in particular a cardiac
pacemaker.
[0003] In the following description of the invention, the focus is
primarily on a cardiac pacemaker. However, the present invention is
not restricted to this particular solution, but in general can be
applied to other stimulation devices which operate electrically and
in particular deliver electrical impulses for stimulation.
BACKGROUND OF THE INVENTION
[0004] Cardiac pacemakers stimulate the heart beat by means of
electrical impulses which are introduced into the muscle tissue of
the heart. For this purpose, a cardiac pacemaker is usually
implanted, for example, near the shoulder of the thoracic cage, at
least one probe or electrical lead being guided from the implanted
cardiac pacemaker via a vein into the atrium or the chambers of the
heart and anchored there. The electrical lead is problematical or
disadvantageous. This runs over a length of about 30 cm in the
blood circulation system and can thereby cause undesirable or even
fatal physical reactions. Furthermore, the risk of failure of the
probes or leads due to material fatigue as a result of the severe
mechanical stressing during body movements is particularly high.
Another complication frequently encountered is dislocation of the
probes triggered by movements of the patient.
[0005] Stimulation by magnetic impulses has been proposed, for
example, in U.S. Pat. No. 5,170,784 A in order to avoid the
electrical lead and the electrode. However, purely magnetic
stimulation does not function satisfactorily so that magnetically
stimulating cardiac pacemakers have not been generally
accepted.
[0006] U.S. Pat. No. 5,411,535 A discloses a cardiac pacemaker with
an implantable control device and a separate electrode device.
Electrical signals of 10 MHz to a few GHz in particular are
transmitted without wires between the control device and the
electrode device for controlling the electrode device. The actual
power supply of the electrode device is provided via a battery
integrated in the electrode device. Such cardiac pacemakers with a
separate electrode device have not been widely accepted so far.
This may be because the electrode device is of a considerable size
and has a limited operating time because of the battery.
[0007] The article "A Surgical Approach to the Management of
Heart-Block Using an Inductive Coupled Artificial Cardiac
Pacemaker" by L. D. Abrams et.al., published in the journal "The
Lancet", 25 Jun. 1960, pages 1372 to 1374, describes a method for
stimulating a heart where an external control device comprising a
coil to be located externally on the body is inductively coupled to
a coil implanted between the skin and the ribs. Two electrical
leads led from the implanted coil to two electrodes in the heart
muscle. Apart from the fact that an external control device is
generally problematical and not desirable, the wiring between the
implanted coil and the electrodes at a distance therefrom results
in the same problems as in the usual cardiac pacemaker described
above where at least one electrode is connected to the implanted
cardiac pacemaker via an electrical lead through a vein.
Furthermore, the implantation of a pacemaker system requires
opening the thoracic cage and involves an open-heart operation.
Moreover, the implanted coil is very sensitive to external
electromagnetic fields so that undesirable interfering voltages are
induced and appear at the electrodes.
[0008] JP 06 079 005 A discloses an implantable cardiac pacemaker
whose battery can be inductively recharged from outside via a
coil.
[0009] U.S. Pat. No. 5,405,367 A discloses an implantable
microstimulator. The microstimulator comprises a receiving coil, an
integrated circuit and electrodes. It can be supplied with energy
and with control information via an external magnetic field
generated by an external coil having an allocated oscillator and an
allocated stimulation control device. Such a microstimulator is not
suitable for cardiac stimulator or as a cardiac pacemaker since it
is relatively large for sufficient capacity and requires an
external energy supply.
[0010] WO 2006/045075 A1 relates to various configurations of
systems that employ leadless electrodes to provide pacing therapy.
In particular, a single magnetic pulse is used to generate an
electrical pulse in an electrode device. This is problematic, in
particular due to magnetic saturation.
SUMMARY OF THE INVENTION
[0011] One exemplary aspect of the present invention to provide a
stimulation system such as a cardiac pacemaker, an implantable
electrode device or stimulation device for a stimulation system as
well as a method for operating an implantable electrode device or
stimulation device, wherein in particular an electrical lead to the
electrode device is not required in the implanted state, wherein
the electrode device can have a simple and compact structure and/or
wherein an energy supply and/or control insensitive to external
influences can be achieved.
[0012] The above aspect is achieved by a stimulation system
according to claim 1, an electrode device according to claim 8, a
stimulating device according to claim 16 and a method according to
claim 19 or 23. Advantageous further developments are the subject
matter of the dependent claims.
[0013] Another aspect of the present invention resides in the fact
that the implantable electrode device for generating electrical
impulses can be supplied with energy and/or preferably directly
controlled in an exclusively wireless or leadless manner by means
of a time-varying magnetic field. This permits a very simple and
compact structure of the electrode device, whereby in particular no
wiring of the electrode device is required so that implantation is
simplified and the risk of failure of an electrical lead is avoided
and in particular, whereby the use of an energy storage device such
a rechargeable battery, a battery or similar in the electrode
device can be avoided. Furthermore, substantially greater freedom
in the placement of the electrode device is obtained.
[0014] The magnetic field is preferably generated by an implantable
control device so that an external controller can be avoided. This
is particularly desirable when the stimulation system is used as a
cardiac pacemaker and is substantially more reliable in use than
control by an external, i.e. non-implanted, control device.
[0015] The electrode device is particularly preferably controlled
directly by the time-varying magnetic field. "Direct" control is to
be understood in the present patent application in that the
electrical impulses are generated in direct dependence on the
magnetic field, for example, depending on the magnitude of the
magnetic field, the polarity of the magnetic field and/or the rate
of change of the magnetic field, particularly preferably without
any active electronic component being interposed in the electrode
device. Consequently, in the preferred direct control, electrical
impulses or stimulations are generated so that they are only
temporally correlated to the magnetic field. This also permits a
very simple and in particular compact structure of the electrode
device and/or a very reliable defined control.
[0016] Another aspect of the present invention includes configuring
the electrode device such that an electrical impulse is only
generated when a minimum field strength of the magnetic field is
exceeded. This very simply permits reliable control which in
particular is not sensitive to interference when the minimum field
strength is selected as suitably high, since strong magnetic fields
occur very rarely but alternating electromagnetic fields having
various frequencies are very common.
[0017] According to one aspect of the present invention, the
electrode device must first be activated before a further
electrical impulse can be generated. This activation is effected in
particular by another signal, preferably by the opposite field
direction of the magnetic field, shortly before triggering and
generating the next electrical impulse. Thus, two-stage triggering
or signal generation is required to generate an electrical impulse
by means of the electrode device. This two-stage property results
in particularly reliable triggering, i.e., not sensitive to
interference.
[0018] The aforesaid triggering safety can be further improved or
enhanced whereby the activation of the electrode device always
takes place shortly before the generation of the next electrical
impulse. Accordingly, the possibility that an electrical impulse as
a result of an interference signal (external magnetic field with
corresponding field orientation and exceeding the minimum field
strength) can lead to undesirable or premature triggering of the
next electrical impulse is so minimal that there is no risk for a
patient.
[0019] According to another aspect of the present invention, a coil
device having a high number of turns, that is a coil having many
turns, is used to generate an electrical impulse having a high
voltage of at least 0.5 V, preferably substantially 1 V or more and
having a relatively long duration of at least 0.05 to 2 ms. In this
case, the coil device can in particular have a soft-magnetic or
ultrasoft magnetic core. The high number of turns, in particular at
least 1,000 turns, of a suitably insulated wire made of, for
example, Cu, Ag or Al in particular having a diameter of about 0.01
to 0.1 mm permits the generation of a strong and long electrical
impulse in said sense.
[0020] According to a further aspect of the present invention, when
the magnetic field is switched on, no continuous or persistent, for
example, sawtooth-shaped ascending magnetic field pulse is
generated by the control device but a plurality of short magnetic
field pulses, in particular so that the core of the coil device or
electrode device always varies its magnetization far below the
saturation state. Thus, a minimal energy consumption can be
achieved, in particular if the largest possible temporal flux
variation takes place in the core of the coil device or electrode
device throughout the entire duration of the stimulating pulse
(optionally a contiguous sequence of electrical impulses of the
electrode device; in the present invention, this sequence is
considered as a single electrical impulse for stimulation). This
can be achieved by short magnetic field pulses.
[0021] The magnetic field pulses can be unipolar or bipolar when
using soft-magnetic core material. When using bistable materials
(in particular Wiegand or pulsed wires), bipolar magnetic fields
must be used.
[0022] According to an additional further aspect of the present
invention, instead of an electrode device, direct electrical
stimulation by a magnetisable element can take place. The element
in particular comprises a coil core without coil or the like. This
means that a coil for transforming the magnetic field into electric
current can be omitted. Instead, the magnetisable element generates
directly the desired electric impulse for stimulation.
[0023] Accordingly, an implantable stimulation device comprises the
magnetisable, preferably ferromagnetic element, the magnetization
of the element being varied by an external or varying magnetic
field so that the magnetic leakage flux of the element results in
the desired electrical stimulation or generation of an electrical
impulse in the surrounding tissue. This permits a particularly
simple structure where electrical contact electrodes are omitted
and the associated problems can be avoided.
[0024] The proposed electrode device or another electrode device
can be used alternatively or additionally to convert the
self-action of the heart, in particular a movement of the heart
and/or electrical activity of the heart, into a magnetic impulse or
another, in particular, electrical signal which can preferably be
detected by the stimulation system or another receiving unit.
[0025] As has already been explained, the implantable electrode
device is used in particular for generating electrical signals to
stimulate the heart. However, the present invention is not
restricted to this. Rather, the electrode device can generally
generate any type of electrical impulse(s) or electrical signals in
the human or animal body. The terms "electrode device" and
"stimulation system" should accordingly be understood in a very
general sense so that other applications and uses, such as for
example to influence the brain, can also be understood.
[0026] The preceding is a simplified summary of the invention to
provide an understanding of some aspects of the invention. This
summary is neither an extensive nor exhaustive overview of the
invention and its various embodiments. It is intended neither to
identify key or critical elements of the invention nor to delineate
the scope of the invention but to present selected concepts of the
invention in a simplified form as an introduction to the more
detailed description presented below. As will be appreciated, other
embodiments of the invention are possible utilizing, alone or in
combination, one or more of the features set forth above or
described in detail below.
[0027] Further advantages, properties, features and aspects of the
present invention are obtained from the following description of
preferred exemplary embodiments with reference to the drawings.
BRIEF DESCRIPTION OF THE FIGURES
[0028] In the figures:
[0029] FIG. 1 is a schematic diagram of a proposed stimulation
system comprising a control device and an electrode device in the
implanted state according to this invention;
[0030] FIG. 2 is a schematic view of the control device according
to this invention;
[0031] FIG. 3 is a schematic view of the electrode device according
to this invention;
[0032] FIG. 4 is a block diagram of the electrode device according
to this invention;
[0033] FIG. 5 is a schematic section view of a core element of the
electrode device according to this invention;
[0034] FIG. 6 is a schematic diagram of a magnetization curve of a
coil device of the electrode device according to this
invention;
[0035] FIG. 7 is a schematic diagram of the time profile of a
magnetic field and an induced voltage according to this
invention;
[0036] FIG. 8 is a schematic section of another electrode device
according to this invention;
[0037] FIG. 9 is a schematic section of another stimulation or
electrode device according to this invention;
[0038] FIG. 10 is a schematic block diagram of a further proposed
stimulation system comprising control device and electrode device
as well as comprising a charging device according to this
invention;
[0039] FIG. 11a-c is a schematic diagram of the time profile of
trigger pulses, a generated magnetic field and a generated
electrical impulse according to this invention;
[0040] FIG. 12 illustrates an example of an exemplary magnetization
according to this invention;
[0041] FIG. 13 is a diagram for choosing optimized operation
parameters according to this invention; and
[0042] FIG. 14 is a schematic diagram of an exemplary circuit of
the electrode device according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0043] In the figures the same reference numerals are used for the
same parts or parts of the same type, components and the like,
where corresponding or similar advantages and properties are
obtained even if a repeated description is omitted.
[0044] FIG. 1 is a schematic sectional view of a proposed
stimulation system 1 which is in particular configured as or works
as a cardiac pacemaker in the example shown. However, the present
invention is not restricted to this. For example, the stimulation
system 1 can additionally or alternatively operate as a
defibrillator or be used for other purposes and at other locations
in the human or animal body.
[0045] The stimulation system 1 preferably comprises an implantable
control device 2 and an implantable electrode device 3 separate
therefrom. In the example shown, the control device 2 is implanted,
in particular in the thoracic cage between the skin 4 and the ribs
5.
[0046] The control device 2 can be implanted as in present-day
cardiac pacemakers. However, it is not absolutely essential to
implant the control device 2. In principle, the control device 2
can also be used in the non-implanted state, that is, as an
external device for controlling the electrode device 3.
[0047] Depending on the configuration, the electrode device 3 can
also be used independently of the control device 2. For example, it
is possible in principle that the electrode device 3 can be
supplied with energy and/or controlled by another device,
optionally even by a nuclear spin tomograph or the like, with
suitable matching. Thus, further possible uses are obtained which
go substantially beyond the possible uses of conventional cardiac
pacemakers or other stimulation systems.
[0048] The electrode device 3 is preferably implanted in the heart
6 or the heart muscle of the patient, who is shown only
schematically and in part. The electrode device 3 can be implanted,
for example, as described in U.S. Pat. No. 5,411,535 A.
[0049] FIG. 2 is a schematic sectional view of the control device
2. In the example shown the control device 2 comprises a coil 7 for
generating a magnetic field H, a control 8 and preferably an energy
storage device 9 such as a rechargeable battery. The coil 7 can
optionally be provided with a ferromagnetic, soft-magnetic or
ultrasoft magnetic core or a half-sided cladding or another shoe or
conducting element to concentrate the magnetic flux.
[0050] The control device 2 or control 8 can preferably receive or
take up the required heart information via means not shown and/or
via the coil 7 so that the generation of electrical impulses by the
electrode device 3 to stimulate the heart 6 can be controlled in
the desired manner. For example, reference is also made here to
U.S. Pat. No. 5,411,535 A. For example, electrodes, not shown, can
also be connected directly to the control device 2, in particular
to detect ECG signals or the like.
[0051] If necessary, the control device 2 or its energy storage
device 9 can be inductively recharged in the implanted state. Thus,
in particular when the energy consumption is high, an otherwise
necessary operation to change the battery or changing the control
device 2 can be avoided. The coil 7 provides a way to generate the
magnetic field H, and is preferably used for the inductive
charging. However, another induction device not shown can also be
used for charging.
[0052] FIG. 3 shows the proposed electrode device 3 in a schematic
sectional view. The electrode device 3 is preferably constructed
only of passive structural elements and/or without an energy
storage device such as a battery. In the example shown, this
preferably comprises a coil device 10, an optional pulse forming
device 11 and preferably at least one electrode 12, preferably at
least two electrodes 12, as well as preferably a common housing 13.
The components and electrodes 12 are preferably integrated in the
electrically insulated housing 13 or attached thereon.
[0053] The electrode device 3 is very compact and in particular is
configured as substantially rod-shaped or cylindrical. In the
example shown, the length is 10 to 20 mm, in particular
substantially 15 mm or less. The diameter is preferably at most 5
mm, in particular substantially 4 mm or less. A retaining device
can be attached to the electrode device 3, preferably an anchor or
a screw which allows the electrode device 3 to be anchored in the
heart muscle.
[0054] The electrode device 3 is configured to generate electrical
impulses for the desired stimulation or signal generation. The
electrical impulses are delivered, for example, via the electrodes
12. In the example shown, the electrodes 12 are located on opposite
sides. However, the electrodes 12 can also be arranged
concentrically or otherwise, for example, at one end or at the
opposite ends of the electrode device 3 or the housing 13.
[0055] FIG. 4 shows a schematic block diagram of the electrode
device 3 according to the described and preferred exemplary
embodiment. In this case, the pulse forming device 11 preferably
comprises a capacitance 14, in particular in the form of a
capacitor, and a resistance 15. Additionally or alternatively, an
inductance not shown, such as a coil can also be used for pulse
forming.
[0056] The pulse forming device 11 is used for forming or reforming
a pulse-like induction voltage which is generated or delivered
under certain circumstances, as will be described in further detail
hereinafter, by the induction or coil device 10. The reformed
electrical impulse can then be output directly for stimulation via
the connected electrodes 12.
[0057] Further structural elements are not required in principle
but are possible. Furthermore, the electrode device 3 can also be
implemented by other structural elements having a corresponding
function.
[0058] The induction or coil device 10 is preferably configured
such that a pulse-like induction voltage is generated when a
minimum field strength of the, i.e., external magnetic field acting
on the electrode device 3 or coil device 10 is exceeded. For this
purpose, the coil device 10 particularly preferably has a coil core
16 which exhibits an abrupt change in the magnetization, i.e.
bistable magnetic properties, when the minimum field strength is
exceeded. This abrupt change in magnetization or magnetic
polarization results in the desired pulse-like induction voltage in
an allocated coil 17.
[0059] In order to achieve the aforesaid bistable magnetic behavior
of the coil core 16, as shown in the diagram according to FIG. 6 as
an example, in the example shown the coil core 16 is preferably
constructed of at least one core element 18, preferably of a
plurality of core elements 18.
[0060] The core elements 18 preferably run parallel to one another
so that the coil core 16 has a bundle-like structure of the core
elements 18. If necessary, however, only a single core element 18
can be used to form the coil core 16, especially if the energy of
the electrical impulse to be generated is relatively low or a
different arrangement, for example, comprising a plurality of coil
devices 10 is used.
[0061] FIG. 5 shows a preferred exemplary embodiment of the core
element 18 in a sectional schematic view. The core element 18 is
preferably configured as wire-like.
[0062] The coil core 16 and/or the core element 18 preferably have
a layer arrangement of soft and hard magnetic material. In the
example shown, an inner layer such as the core 19 and an outer
layer such as the cladding 20 comprise of at least magnetically
different materials, namely soft magnetic material on the one hand
and hard magnetic material on the other hand. The differences
therefore lie in the coactive field or in different hysteresis
curves of the (magnetically) different materials. The coupling as a
result of the layer structure then results in the desired
magnetically bistable behavior or the desired abrupt change in the
magnetization of the core element 18 or all the core elements 18
and therefore the coil core 16.
[0063] The individual core elements 18 preferably have a diameter
of about 50 to 500 .mu.m, in particular substantially 100 .mu.m
and/or a length of 5 to 20 mm, in particular substantially 15
mm.
[0064] The core elements 18 are particularly preferably so-called
Wiegand wires as described in U.S. Pat. No. 3,820,090 and/or
supplied by HID Corp., 333 St. Street, North, Heaven, Conn. 06473,
USA under the trade name "Wiegand Effect Sensors" or so-called
impulse wires as supplied by Tyco Electronics AMP GmbH,
Siemenstrasse 13, 67336 Speyer, Germany. In the Wiegand wires the
soft and hard magnetic layers are formed of the same material, the
different magnetic properties being achieved in particular by
mechanical reforming.
[0065] With regard to the possible structure and/or the materials
used, reference is made supplementarily, additionally or
alternatively to the article "Power Generating Device Using
Compound Magnetic Wire" by A. Matsushita et al. published in the
journal "Journal of Applied Physics", Vol 87, No. 9, 1 May 2000,
page 6307 to 6309 and to the article "A Soft Magnetic Wire for
Sensor Applications" by M. Vazquez et al. published in the journal
"J. Phys. D: Appl. Phys.", Vol. 29, 1996, pages 939 and 949, which
are introduced as additional disclosure.
[0066] Various properties, features and operating modes of the
proposed method, the proposed electrode device 3 and the proposed
stimulation system 1 are explained in detail hereinafter.
[0067] The electrode device 3 for generating electrical impulses is
preferably supplied with energy and/or controlled by means of a
magnetic field H which can be generated in particular by the
control device 2 in an exclusively wireless manner. In particular,
the electrode device 3 requires no energy storage device such as a
battery which restricts the lifetime of usability of the electrode
device 3.
[0068] The electrode device 3 is configured such that an electrical
impulse is only generated and delivered when a (first) minimum
field strength of the magnetic field is exceeded. Furthermore, this
or another pulse generation or triggering is preferably only made
possible after respective previous activation.
[0069] The impulse generation and triggering preferably takes place
as a result of the external magnetic field H acting on the coil
device being varied in time so that when the first minimum magnetic
field strength H1 is exceeded, an abrupt change in the
magnetization of the core elements 18 or the coil 16 takes as shown
in the schematic magnetization curve according to FIG. 6. As a
result of the inverse Wiedemann effect, this abrupt change in the
magnetization results in a pulse-shaped induction voltage (pulse P
in FIG. 7) in the allocated coil 11. This first minimum field
strength H1 is therefore a switching threshold.
[0070] The induced voltage pulses P can have an amplitude of up to
about 5 V and are about 5 to 100 is long. In order to achieve a
preferably longer pulse duration, as is usual for cardiac
stimulation, the optional pulse forming device 11 is preferably
used. The induced voltage pulse P can thus in particular be
stretched in time. Alternatively or additionally, a longer pulse
duration can also be achieved by bundling a plurality of core
elements 18 in the coil 17, in particular so that the pulse forming
device 11 can be completely omitted.
[0071] Additional core elements 18 can be provided in the coil core
16 to increase the pulse power. Alternatively or additionally, a
plurality of coil devices 10 can be connected in parallel or in
series to increase the pulse power.
[0072] Alternatively or additionally, other magnetic, in particular
permanent-magnetic elements can be used in the coil core 16 to
achieve the respectively desired magnetic properties of the coil
core 16.
[0073] The magnitude of the minimum field strength H1 depends on
various factors, in particular the manufacturing conditions of the
core elements 18. The minimum field strength H1 is preferably
between 0.5 and 20 mT, in particular between 1 to 10 mT and is
quite particularly preferably about 2 mT. These values are already
substantially above the values for magnetic fields usually
permissible in public so that any triggering of an electrical
impulse by interference fields usually expected is eliminated.
[0074] The individual core elements 18 or the coil core 16 having
the bistable magnetic properties, in particular in the preferred
but not absolutely essential structure of layers having alternately
soft and hard magnetic properties, can be used in various ways. In
the example shown, preferably asymmetrical behavior is achieved on
running through the magnetization curve or hysteresis. For
resetting or attaining the starting point, that is activation for
the triggering of the next impulse, the polarity of the coil core
16 is (completely) reversed by the external magnetic field H having
the opposite direction when the second minimum field strength H2 is
exceeded, as can be deduced from the magnetization curve in FIG. 6.
It should be noted that in said processes in each case only the
polarity of the soft magnetic material layers is reversed whilst
the magnetization of the hard magnetic material layers is thus
retained. In principle, however higher magnetic fields H can also
be used to reverse the polarity of the hard magnetic layers if
required.
[0075] In the example shown, the external magnetic field H, in
particular generated by the control device 2, is used both for
controlling (triggering) the generation and delivery of an
electrical impulse by the electrode device 3 and also for supplying
the electrode device 3 with the energy necessary for generating the
electrical impulse. In addition, the magnetic field H is preferably
also used for said activation of the electrode device 3 for the
possible generation of the next electrical impulse. However, this
can be also be effected in another manner or by another signal.
[0076] The external magnetic field H preferably runs at least
substantially parallel to the longitudinal direction of the coil
core 16 or the core elements 18.
[0077] FIG. 7 shows schematically a preferred time profile V1 of
the external magnetic field H acting on the electrode device 3 and
the corresponding time profile V2 of the voltage U induced in the
electrode device 3 or its coil 17.
[0078] The magnetic field H is preferably generated intermittently
and/or as an alternating field. The magnetic field H preferably has
a switch-on ratio of less than 0.5, in particular less than 0.25,
particularly preferably substantially 0.1 or less.
[0079] The field strength of the magnetic field H has a
substantially ramp-shaped or sawtooth-shaped time profile, at least
during the switch-on times as indicated in FIG. 7.
[0080] The magnetic field H is alternately generated with an
opposite field direction for alternate generation of an electrical
impulse and activation of the electrode device 3 before generation
of the next electrical impulse. The activation preferably takes
place only shortly before generating the next electrical impulse,
as indicated in FIG. 7.
[0081] The frequency of the magnetic field H is preferably only a
few Hz, in particular less than 3 Hz and corresponds in particular
to the desired frequency of the electrical impulses to be
generated.
[0082] The ramp-shaped increase in the field strength of the
magnetic field H is preferably relatively steep in order to achieve
only short switch-on times and only a low switch-on ratio. This is
advantageous in regard to minimizing the required energy and a
defined triggering with few interfering influences.
[0083] According to the minimum field strength to be achieved, the
maximum field strength of the magnetic field H in the region of the
electrode device 3 preferably reaches substantially 1 to 20 mT, in
particular 2 to 10 mT.
[0084] It can be seen from FIG. 7 that the negative magnetic field
ramps on reaching the second minimum field strength H2 in each case
only induce a very small electrical impulse which is negligible
compared to the pulses P at the abrupt change in magnetization. The
magnitude of these small pulses depends substantially on the rate
of change in the magnetization during resetting, that is during the
activation of the electrode device 3 for generation of the next
electrical impulse.
[0085] According to a further development not shown, a plurality of
electrode devices 3 can be used which in particular can be
controlled and supplied with energy by a common control device 2.
The electrode devices 3 can then be implanted at different
locations, for example. As a result of different first minimum
field strengths H1, different coil devices 10 and/or pulse forming
devices 11 or the like, desired phase shifts, energy differences or
the like can then be achieved in the electrical impulses or signals
delivered by the individual electrode devices 3.
[0086] It should be noted that the preferred synchronization of the
stimulation of the heart 6 with the heat beat can be achieved, for
example, by evaluating the electric voltage induced in the coil 7
of the control device 2 by the movement of the electrode device 3,
optionally in conjunction with the ECG voltage which can be
detected galvanically via the housing of the control device 2 or a
relevant electrode.
[0087] Particular advantages of the invention reside in the
possibility that the wireless electrode device 3 can be implanted
in more suitable regions for stimulation, in particular, of the
heart muscle, than is possible with wire-bound electrodes.
Moreover, a plurality of electrode devices 3 can be implanted at
different locations whereby improved stimulation and in particular
better cardiac dynamics can be achieved.
[0088] FIG. 8 is a schematic section of a further embodiment of the
proposed electrode device 3. In this case, the coil device 10 can
comprise a coil core 16 or core elements 18 made of a soft magnetic
material or ultrasoft magnetic material, for example in the form of
wires or strips. Such a material has a very low coactive field
strength which corresponds to the minimum field strength H1 and in
particular is less than 0.1 mT. The saturation field strengths of
the material are less than about 0.01 to 3 mT. The coil core 16
consists of non-magnetic or completely or partially of said soft
magnetic or ultrasoft magnetic material or a combination of various
such magnetic materials.
[0089] In this case, the electrode device 3 or coil device 10
comprises a coil 17 preferably having a high number of turns, in
particular at least 1,000 turns, particularly preferably 2,000
turns or more. In the example shown, the coil 17 has substantially
3,000 turns or more.
[0090] In the example shown, the coil inside diameter D1 is
preferably 1 to 3 mm, the coil outside diameter D2 is preferably 2
to 6 mm and the coil length L1 is preferably 10 to 30 mm.
[0091] In general, ferrites or ferromagnetic metal powder materials
can be used as core materials or soft magnetic materials. An
advantage is that as a result of the poor electrical conductivity,
these materials only exhibit low eddy current losses.
[0092] In general, the bobbin-like coil shown in FIG. 8 or its core
16 or only the central rod or only a rod-shaped core 16 or a
plurality of core elements 18 can be constructed of soft or
ultrasoft materials in the form of a stack of films electrically
insulated from one another to reduce the transverse conductivity,
to minimize eddy current losses. The same applies to the use of
ferrites or other materials having corresponding properties.
[0093] The proposed electrode device 3 or coil device 10 permits
the generation of relatively strong electrical impulses, in
particular an impulse having a voltage of at least 1 V and a time
duration of substantially 0.1 ms or more. This can be achieved in
particular by the bobbin-like coil configuration shown and/or by
the high number of turns. In particular, this relatively strong and
relatively long-lived electrical impulse can also be achieved with
the soft magnetic core material. A magnetic resetting pulse as with
the Wiegand wires or the like is not absolutely necessary. However,
a combination with the other magnetic materials or structures is
possible.
[0094] As a result of the special RLC properties (impedance) of the
primary coil 7, the exciting magnetic field H can only increase
relatively slowly (typically from 0 to a maximum of, for example,
0.1 to 2 mT in 0.1 to 5 ms). In the proposed coil device 10 and
under loading with a characteristic resistance for the heart muscle
of, for example, about 1 kOhm, a relatively broad or long-lived
impulse having a duration of at least 0.1 ms, in particular of
substantially 0.25 to 2 ms, can be generated. This can possibly be
attributed to the alternating current properties of the LRC
arrangement (or the coil device 10, high inductance and high
winding capacity of the coil) and/or to the retroactive effect of
the coil current on the core 16.
[0095] The electrode device 3 described hereinbefore is preferably
again combined with the control device 2 already described or
another control device 2 and/or is controlled and/or supplied with
energy preferably exclusively by means of an external or varying
magnetic field H, as already described.
[0096] FIG. 9 shows another embodiment of the proposed electrode
device 3. More precisely, this is not an electrode device 3 but a
stimulation device 21 since no electrodes 12 are required as in the
preceding embodiments. However, the stimulation device 21 can be
used instead of the electrode device 3 or for the stimulation
system 1 described previously. The reasoning so far relating to the
use and insertion of the electrode device 3 therefore fundamentally
apply accordingly for the stimulation device 21.
[0097] The stimulation device 21 has a magnetisable element 22
which is preferably surrounded by an optional cladding 23.
Electrodes 12 or the like as in the electrode device 3 are
preferably not required.
[0098] The element 22 can be magnetized by an external or varying
magnetic field H, in particular, the magnetic field H is generated
by the control device 2 or in another suitable manner.
[0099] Variation of the magnetic field H causes a change in the
magnetization of the element 22. Accordingly, the magnetic leakage
flux of the element 22 in the tissue surrounding the stimulation
device 21 in the implanted state, such as the heart 6, varies in
time so that an electrical field strength or an electrical
stimulation is generated. Consequently, an electrical stimulation
or an electrical impulse is generated in the tissue, such as the
heart 6, without electrodes 12.
[0100] The element 22 is preferably ferromagnetic, in particular at
least substantially or exclusively made of ferromagnetic material.
Alternatively or additionally, the element 22 can also be
constructed as described with reference to FIG. 5 and/or it can be
constructed as a Wiegand wire or the like and/or from a plurality
or a bundle of core elements 18.
[0101] The stimulation device 21 in particular brings about an
amplification of the external magnetic field H at the location of
the stimulation device 21, that is at the implanted site. This
makes it possible to achieve specific electrical stimulation in the
desired area and/or depending on the magnetic field H.
[0102] FIG. 10 shows another embodiment of the proposed stimulation
system 1 comprising the control device 2, the electrode device 3
and an external charging device 24 in a schematic diagram similar
to a block diagram. In this embodiment a plurality of short
magnetic field pulses are generated as a sequence by the control
device 2 during the switch-on time of the magnetic field H, i.e.
during the switch-on phases. In particular, it is thus achieved
that the coil arrangement 10 or its coil core 16 always changes its
magnetization far below the saturation state. Thus, a minimum
energy consumption can be achieved since the largest possible flux
variation in the core of the coil arrangement 10 of the electrode
device 3 is present or produced during the entire switch-on time of
the magnetic field H and therefore substantially during the
generation of the electrical impulse.
[0103] The magnetic field pulses can be unipolar or bipolar when
using soft magnetic core materials. Bipolar magnetic field pulses
are used when using bistable materials.
[0104] In the example shown according to FIG. 10, bipolar magnetic
field pulses are preferably generated by means of a bridge of
switching transistors M1 to M4 (e.g. MOSFETS, also in complementary
design) or other switching semiconductor components. Also indicated
in FIG. 10 are the coil 7, the control 8 and the energy storage
device 9 of the control device 2. The control 8 can, for example,
comprise one or two signal generators V2 and V4. Preferably
connected in parallel to the energy storage device 9 is a smoothing
capacitor 25. In addition, separating electronics 26 such as a
switch or the like can be provided.
[0105] The control device 2 or its coil 7 is preferably configured
such that the control device 2 or its energy storage device 9 can
be inductively charged in the implanted state, in particular via
the coil 7. For generating the required electromagnetic field
during charging the charging device 24 is equipped with a suitable
coil 27 and a corresponding power supply, in particular an
alternating current supply 28.
[0106] In one exemplary embodiment, multiple magnetic field pulses
are used to control the electrode device 3 and to generate the
respectively desired electrical impulses, i.e. multiple magnetic
field pulses form one single electric pulse for one
stimulation.
[0107] The exemplary electrode device 3 can comprise a rectifier
(in FIG. 10 formed by the shown diodes or any other components
diodes, in particular with a means for smoothing the resulting
electrical voltage, here in the form of a capacitance). Thus, a
single electrical impulse can be generated as desired, in
particular as discussed in the following with regard to FIG.
11.
[0108] FIG. 11a) is a schematic diagram showing a possible pulse
sequence (voltage over time t) generated by the control 8 and
allowing optimum triggering of the bridge. The trigger pulses, in
this case for the bridge of switching transistors, are preferably
only generated during the switch-on time t.sub.on to t.sub.off,
i.e. when the magnetic field H is switched on. For example, the
trigger pulses each last less than 50 .mu.s. After a first pulse 1
(shown by the continuous line) and a certain delay time of, for
example, .DELTA.t.sub.1 of about 1 to 10 .mu.s, an opposite pulse 2
then follows for the duration t.sub.2 which in particular
corresponds to the first duration t.sub.1, and which reverse the
primary coil voltage (voltage of the coil 7) via the bridge. This
alternating generation of trigger pulses is repeated n times until
a sufficient number of pulses consisting of positive and negative
paired single pulses has been delivered.
[0109] As a result of the inductance of the coil 7, the trigger
pulses or pulse sequences shown results in a sequence of in
particular at least substantially sawtooth-shaped, preferably
bipolar magnetic field pulses (shown as current through the coil 7
over time t in FIG. 11 b) which act on the electrode device 3 or
its coil device 10 (secondary coil) as the magnetic field H in the
sense of the present invention and there bring about the generation
of an electrical impulse (or a sequence of electrical impulses for
each single stimulation) for stimulation. FIG. 11c) shows an
electrical impulse (in particular a superposition of partially
smoothed individual impulses) generated by the magnetic field
pulses or the pulse-like varying magnetic field H as a schematic
diagram of voltage over time t. In particular, the length of the
electrical impulse depends on the length of the switch-on time of
the trigger pulses or the magnetic field pulse and substantially
corresponds particularly preferably to the switch-on time.
[0110] Similar behavior can be achieved with a unipolar sequence of
magnetic field pulses. In this case, for example, the left part of
the bridge and the generator V2 in FIG. 10 as well as the dashed
pulse sequence 2 in FIG. 11 c) can be omitted.
[0111] The duration between two trigger pulses .DELTA.t should be
selected so that the second pulse is triggered when the primary
coil current which initially decreases quasi-linearly towards zero,
reaches the zero level. This time interval depends both on the R/L
value of the coil 7 and on the R/L value of the secondary circuit,
in particular the coil arrangement 10. For the primary circuit
(control device 2) substantially the winding resistance and the
inductance of the coil device 10 determine the R/L ratio whilst the
resistance of the coil device 10 is determined by the winding
resistance and the loading resistance (tissue resistance of the
stimulated part of the heart muscle or the like which is present at
the electrodes 12) and the inductance is determined by the winding
inductance taking into account the preferably ferromagnetic core
16. Here R designates the electrical resistance in general and L
designates the inductance.
[0112] As has been explained, the impulses induced in the coil
device 10 at times t or t' have different signs, i.e. a pulse
sequence of bipolar pulses is obtained (both in the case of
unipolar and bipolar excitation by magnetic field pulses). Unipolar
electrical impulses are preferably required and generated for
stimulation. These are rectified by a rectifier, in particular a
bridge or diode rectifier, in the electrode device 3. The rectifier
is preferably connected between the connections of the oil device
10 and the electrodes 12, as indicated in FIG. 10. This results in
unipolar sequences of electrical impulses with peak values. Between
the peak values the voltages can be close to zero. A small
smoothing capacitor C2 (of, for example, 1 to 100 nF) connected in
parallel to the stimulation electrodes can smooth this pulsating
voltage sequence if necessary. The capacitance can be optimally
matched to the properties of the entire system.
[0113] With regard to FIG. 10, it should be noted that the
electrode device 3 is preferably only constructed of passive, in
particular, few components such as one or a plurality of diodes, in
particular Schottky diodes D2, D5, D8, D9 to form the rectifier
and/or the capacitor C2.
[0114] The duration of the respective electrical impulse (a single
stimulation) generated by the electrode device depends on the
respective switch-on time of the magnetic field H, in particular on
the number of trigger pulses generated in a sequence and thus on
the number of magnetic field pulses generated by the control device
2. Consequently, the control device 2 controls the generation of
the electrical impulse or the electrode device 3 by the magnetic
field H directly in the initially specified sense of the present
invention.
[0115] The schematic diagram according to FIG. 11 c) shows the
influence of the rectifier and the R/L ratio of the coil device 10
of the electrode device 3. When the R/L ratio is large (e.g. very
small L), the coil voltage follows the derivative of the primary
coil current dl/dt, which preferably increases or decreases
quasi-linearly here as a consequence of the smaller R/L ratio of
the primary coil (coil 7) when the polarity of the primary coil
voltage is reversed. When the R/L ratio of the coil device 10 is
small (including the tissue resistance present at the electrodes
12), as is realistic on account of the preferred high number of
turns (in particular about 1,000 turns or more) and the preferred
presence of the ferromagnetic core 16, the induced coil voltage
(measured as the voltage at the load resistance of the coil 17--in
particular therefore at the tissue resistance present at the
electrodes 12) only increases relatively slowly.
[0116] The proposed method of using relatively short, closely
following, rectified electrical impulses as a result of a sequence
of short magnetic field pulses or trigger pulses according to FIG.
11 to stimulate a single heart beat or the like offers the
possibility of adapting the stimulation pulse duration (the total
length of the electrical impulse during a switch-on time of the
magnetic field H, substantially the switch-on time t.sub.on to
t.sub.off) to the needs of a particular patient by suitably
adjusting the number n of pulse pairs of the trigger pulses by
acting externally on the control 8 equipped with at least one
suitable sensor. However, other electrical or electrotechnical
design solutions are also possible.
[0117] FIG. 12 shows a B(H) curve (schematic). .DELTA.H corresponds
to the current variation through the primary coil produced by
applying a voltage pulse to its leads. Symmetry to H=0 is achieved
by using a sequence of a positive and a negative voltage pulse of
equal amplitudes (cf. FIG. 11). This is advantageous to the case of
using a unipolar voltage pulse producing the same .DELTA.H since
dB/dH is monotonically decreasing along the hysteresis curve.
Hysteresis effects have been omitted in the drawing since core
materials with very small hysteresis are to be preferred to avoid
BH-losses.
[0118] A constant voltage suddenly applied to the primary coil
results in a monotonically increasing current through the coil (Eq.
1) and hence a proportionally increasing magnetic field at the site
of the electrode device 3 the rate governed by the time constant
L/R of the coil circuit.
i = U / R ( 1 - - t L / R ) ( Eq . 1 ) ##EQU00001##
Since the induced voltage in the coil device 10 of the electrode
device 3 is proportional to the change of the induction dB/dt in
the core element 18 which is a function of H, the induced voltage
decreases with time during the time the voltage pulse at coil 7 is
on. This means a reduction of the efficiency of conversion of the
electric power consumed by coil 7 into a voltage occurring at the
posts 12 the longer the voltage is applied to coil 7. Therefore,
for optimal efficiency, H should be kept small which is reached by
switching off or reversing the voltage applied to coil 7 by using
short pulse duration times.
[0119] The amplitude and duration of the induced voltage pulse in
the coil device 10 is adjusted by choosing a proper pulse voltage
applied to coil 7, a suitable pulse duration and frequency. A very
high frequency becomes undesirable to one part because of an
increased impedance of the stimulator coil (Z=.omega..times.L),
resulting in reduced pacing pulse amplitudes. For a given design of
the coil 7 and the electrode device 3 at a given mutual geometric
arrangement (including the distance of the electrode device 3 from
the plane of the coil 7, the angle between the coil axis and the
distance of the electrode device 3 from the axis of the coil 7)
details of the burst pulse sequence are optimized for minimal
energy consumption at a desired pacing pulse shape. The energy
consumption is given by
E=1/2C(U.sub.1.sup.2-U.sub.2.sup.2)
where U.sub.1 is the voltage at the charging capacitor C before
firing a pulse burst and U.sub.2 the voltage after firing a pulse
burst after the power supply has been disconnected from C.
[0120] Since the electronic properties of the electrode device are
strongly non-linear and only approximately known a priori, the
optimal operational parameters of the pacing system have to be
determined experimentally. This is performed for a single pacing
pulse preferably according to the diagram shown in FIG. 13.
[0121] The timing sequence of the voltage pulses comprising a burst
applied to the coil 7 can be chosen to produce almost arbitrary
pacing pulse shapes. For instance, a ramp-like increase of the
pacing pulse is obtained with sequentially increased voltage pulse
amplitudes.
[0122] Especially, the pacing pulse can be made to change sign for
some arbitrary fraction of time. This may be achieved by using a
one-way rectifier 29 or diode D1 instead of the bridge rectifier
depicted in FIG. 11 and attaching or connecting a Zener diode 30
(or other devices exhibiting a breakdown characteristic like
four-layer-diodes, thyristors, etc.) parallel to the rectifying
diode D1 with an adequate Zener voltage larger than that of the
normal rectifying diode, as shown in the exemplary embodiment in
FIG. 14.
[0123] Whenever the induced voltage, now with an opposite sign as
compared the normal pulse, is increased beyond the Zener breakdown
voltage (or the Off-State Voltage of the mentioned other types of
semiconductors) a reversed pulse polarity is obtained. This
requires producing an asymmetrical rate of current increase or
decrease through the coil 7, i.e., producing di/dt values differing
in sign and amplitude. Hence the amplitudes of positive and
negative induction voltages in the electrode 3 will be different,
enabling the selective application of positive or negative pacing
pulses. The differing di/dt values are obtained by applying voltage
pulses U (cf. Eq. 1) of different amplitudes and polarities to the
coil 7. The described possibility is advantageous since the pacing
voltage is reported to increase with time when pacing with unipolar
pulses. This potentially undesirable effect is largely reduced by
employing a bipolar pacing pulse. The possibility to produce
arbitrary bipolar pacing pulses persists when the normal diode D1
is omitted, using the forward and breakdown characteristics of the
Zener diode.
[0124] In particular, the following aspects of the present
invention can be realized independently or in any combination:
[0125] An exemplary embodiment of the present invention makes use
of energy recovery by the magnetic field.
[0126] An exemplary embodiment of the present invention uses
parameters and operations such that core magnetic saturation in the
electrode 3 is avoided. This reduces energy consumption
significantly.
[0127] The pulse shape can be adjusted arbitrary for the most
effective stimulation with respect to the pacing pulse height and
width by using a programmable sequence of amplitudes, durations and
delay times of the individual burst pulse voltages (voltage source
9, our FIG. 10) applied to the primary coil. The importance of
choosing an optimal pulse shape has been described in U.S. Pat. No.
5,782,880 A.
[0128] This very flexible design also provides the possibility to
generate bipolar pacing pulses by controlling the di/dt rate and
sign of the current sent through the primary coil 7 and making use
of Zener diodes or other rectifiers with selectable breakthrough
voltages.
[0129] The burst-pulse sequence is optimized with respect to
duration, repetition rate and time delay to achieve minimal energy
consumption for a given pacing pulse amplitude an duration. If,
e.g., the delay times .DELTA.t.sub.1 or .DELTA.t.sub.2 are too
small, the energy consumption can increase dramatically.
[0130] Use of Cu cladded Al strand (Litz) wire in the primary coil
is preferred and of advantage for significantly reducing the weight
of the coil of the electrode device. It also provides--as also does
Cu strand wire--a large degree of mechanical flexibility. Due to
the skin effect present because of the alternating current sent
through coil 7 the effect of the smaller conductivity of the Al as
compared that of Cu is reduced but the weight is determined largely
by the aluminum. In an experiment, the energy consumption using the
Co cladded Al Litz wire was close to that of using pure Cu Litz
wire with similar dimensions.
[0131] Metallic soft or ultrasoft magnetic cores might preferably
be used for the electrode device and provide a larger saturation
magnetization as compared to ferrite. Accordingly, a lower exciting
magnetic field will be needed. Due to the transients of the
magnetic field pulses eddy current losses occur in the core
material. They are essentially reduced by lamination of metallic
cores which is preferred.
[0132] Magnetically soft cores can be achieved in particular by
lamination of multiple isolated layers. Magnetically ultra-soft
cores can be achieved in particular by using amorphous or
nanocrystalline magnetic materials.
[0133] Only one cap at the side of the stimulator that points
toward the primary coil on the core instead of two, making it
smaller and only slightly less efficient.
[0134] It is preferred to use energy recovery of the magnetic field
by using high capacity buffering (supercaps) of the power supply
together with fast switching diodes parallel to the MOSFETs
comprising the H-bridge (FIG. 11) in case these are not already
implemented in the MOSFETs. This also extends the lifetime of the
batteries since the large peak currents are delivered by the
supercaps instead of the battery.
[0135] The control device is preferably in a flexible housing as it
should be implanted directly above the heart near the thoracic
wall. To achieve this flexibility the control device can be
embedded in a silicon cushion, however other soft materials can
also be used.
[0136] For magnetic field concentration towards the electrode 3 a
flux concentrator might be used contained within the interior of
the inner surface of the preferably soft housing, preferably
silicon cushion. Experiments had shown an increase in magnetic
field strength at the pacing site when the coil 7 was halfway
surrounded by a thin Mumetal cover the collar of which pointing to
the pacing site. Other shapes might be used.
[0137] To guarantee flexibility the power supply should be
preferably provided by tailor made, flexible, lithium polymer
batteries. However also other types of power supplies might be used
(thermoelectric using body heat, fuel cells, cells using body
fluids, or the like).
[0138] An exemplary embodiment, the electrode devices comprises a
flexible housing and/or means for magnetic field concentration at
the inner surface of the housing as described above.
[0139] The induction pacemaker technology described can also be
used in combination with conventional cardiac pacemaker technology.
In this connection, the use for left-ventricular stimulation within
the framework of resynchronization therapy is particularly
appropriate.
[0140] Individual features, aspects and elements of the individual
embodiments and variants can be arbitrarily combined with one
another or used in other stimulation systems or electrode
devices.
* * * * *