U.S. patent application number 10/763989 was filed with the patent office on 2004-09-02 for implanted medical device/external medical instrument communication utilizing surface electrodes.
This patent application is currently assigned to Medtronic, Inc.. Invention is credited to Berg, Gary, Emst, Jill H., Nelson, Chester G., Stomberg, Charles, Wilkinson, Jeffrey D..
Application Number | 20040172104 10/763989 |
Document ID | / |
Family ID | 23677682 |
Filed Date | 2004-09-02 |
United States Patent
Application |
20040172104 |
Kind Code |
A1 |
Berg, Gary ; et al. |
September 2, 2004 |
Implanted medical device/external medical instrument communication
utilizing surface electrodes
Abstract
A medical device communications system uses subthreshold pulses,
modulated to provide relatively high speed electrical
communications with inexpensive external devices connectable to a
body with the implant by surface leads.
Inventors: |
Berg, Gary; (Edina, MN)
; Emst, Jill H.; (Apple Valley, MN) ; Nelson,
Chester G.; (Maple Grove, MN) ; Stomberg,
Charles; (Forest Lake, MN) ; Wilkinson, Jeffrey
D.; (Vadnais Heights, MN) |
Correspondence
Address: |
MEDTRONIC, INC.
710 MEDTRONIC PARKWAY NE
MS-LC340
MINNEAPOLIS
MN
55432-5604
US
|
Assignee: |
Medtronic, Inc.
|
Family ID: |
23677682 |
Appl. No.: |
10/763989 |
Filed: |
January 22, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10763989 |
Jan 22, 2004 |
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09423101 |
Oct 29, 1999 |
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6704602 |
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09423101 |
Oct 29, 1999 |
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PCT/US98/06103 |
Jul 2, 1998 |
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Current U.S.
Class: |
607/60 |
Current CPC
Class: |
A61N 1/3727
20130101 |
Class at
Publication: |
607/060 |
International
Class: |
A61N 001/18 |
Claims
What is claimed is:
1. A communications system for use with an implantable medical
device, comprising: a pulse generation circuit to deliver a first
electrical signal at a first threshold to electrically stimulate
body tissue; a control circuit coupled to the pulse generation
circuit, the control circuit to cause the pulse generation circuit
to deliver a second signal at a second threshold below the first
threshold to prevent substantial physiological effects in response
to the second signal, the second signal including encoded data; and
a receiving circuit to receive the second signal, enabling external
transfer of the encoded data.
2. The system of claim 1, wherein the second signal is a modulated
biphasic pulse.
3. The system of claim 2, wherein the biphasic pulses are delivered
during a refractory period.
4. The system of claim 2, wherein the second signal is an amplitude
modulated biphasic pulse.
5. The system of claim 2, wherein the second signal is a frequency
modulated biphasic pulse.
6. The system of claim 2, wherein the second signal is a
pulse-width modulated biphasic pulse.
7. The system of claim 6, wherein the pulse generation circuit
includes a circuit to perform pulse train modulation on the second
signal.
8. The system of claim 6, wherein the pulse generation circuit
includes a circuit to control the polarity of the biphasic
pulse.
9. The system of claim 1, further comprising a trigger circuit
coupled to the control circuit to receive a trigger signal from
outside the body, the control circuit causing the pulse generation
circuit to deliver the second signal in response to the trigger
signal.
10. A method of providing communication between an implantable
medical device and an external device, comprising the steps of:
generating a first electrical signal at a first threshold to
electrically stimulate body tissue; generating a second electrical
signal, including encoded data, at a second threshold below the
first threshold to prevent physiological effects in response to the
second signal; and sensing the generation of second signal at the
external device.
11. The method of claim 10, wherein the second signal corresponds
to multiple biphasic pulses.
12. The method of claim 11, wherein the second signal corresponds
to modulated biphasic pulses.
13. The method of claim 11, wherein the second signal is delivered
at a predetermined time relative to a cardiac cycle of the
heart.
14. The method of claim 10, further comprising the step of
providing an external signal from outside the body to trigger
delivery of the second signal.
15. The method of claim 10, wherein the second signal includes
marker channel data.
16. The method of claim 10, wherein the second signal corresponds
to patient data.
17. The method of claim 10, wherein the second signal corresponds
to device-specific data.
18. The communications system of claim 10, wherein the second
electrical pulses are transmitted during segments of time related
to one of a sensed cardiac event and a pacing pulse.
19. The system of claim 10, wherein the second signal is
transmitted during segments of time related to one of a sensed
cardiac event and a pacing pulse.
20. The method of claim 10, wherein the second electrical signal is
generated during segments of time related to one of a sensed
cardiac event and a pacing pulse.
Description
CROSS REFERENCE TO PRIOR APPLICATION
[0001] This application is a continuation of prior U.S. patent
application Ser. No. 09/423,101, filed Oct. 29, 1999, entitled
"Implanted Medical Device/External Medical Instrument Communication
Utilizing Surface Electrodes".
FIELD OF THE INVENTION
[0002] This invention is related to inter-device communications
between medical devices and most particularly to systems that
employ sub stimulation threshold pulses for such
communications.
BACKGROUND
[0003] The high cost and general level of difficulty in
communicating with an implanted medical device using a low cost
external instrument has prevented widespread usage of the data
which is currently available from pacemaker and other implantable
medical devices to augment traditional transtelephonic home
follow-up.
[0004] Health care systems are increasingly emphasizing and
rewarding those products which reduce the cost of obtaining,
communicating, and managing patient data. Therefore inexpensive
devices for remotely monitoring the essential status of pacemaker
patients and patients with other implantable medical devices is
highly desirable. Even small improvements may have significant
economic and medical benefit.
[0005] Difficulties arise in transferring large amounts of data
between an implanted medical device and external monitors or other
medical communications systems. Telemetry using RF or E fields and
H fields is commonly practiced in, for example, the field of
implantable devices such as pacemakers and
defibrillator/cardioversion devices in communicating information
between the implant and the external transceiving device for
example, a programmer. This has limitations as well, primarily on
the cost for the external device which goes up considerably if it
needs to receive telemetry. Also, the energy cost of transmitting
information from the implanted device to outside the patient's body
is higher than using subthreshold electrical pulses and this
therefore depletes the implant's battery, weighing against using
telemetry too. The overriding consideration for employing external
devices to receive data through skin contact electrodes is the
simplicity and low cost of the one way (receiving) device. (The
receiving device could even be worn like a wrist watch and receive
subthreshold communications for later retransmission).
[0006] Therefore to enable better device transmitted communications
as the data amounts and transfer rates are desirably increased, a
communications protocol and implementing hardware that facilitates
such communications has been developed and is the subject of this
document.
[0007] A list of references where similar or related inventions in
the same or other unrelated fields were contemplated follows, and
is incorporated into this disclosure by this reference thereto.
1 Davis et al. US Pat. No. 5,544,661, Spinelli et al. US Pat. No.
5,413,593, Coppock et al. US Pat. No. 5,503,158, Yomotov, et al. US
Pat. No. 5,313,953, Fujii et at. US Pat. No. 5,411,535, Nappholz et
al. US Pat. No. 5,113,869, Nolan et at. US Pat. No. 5,404,877,
Prutchi et al. US Pat. No. 5,556,421, Funke US Pat. No. 4,987,897,
and Strandberg US Pat. No. 4,886,064.
[0008] Additionally the Cardiac Telecom HEARTTrac(tm) cardiac
monitoring system may provide additional information about such
communications but at this date the inventors have not had an
opportunity to review this matter.
[0009] There still is a need for a very inexpensive method of
getting large amounts of data from an implanted device to an
external device that is as yet unsatisfied by this art. This is
especially true in rural areas and in places where sophisticated
telemetry systems may be difficult to use or obtain.
SUMMARY OF THE INVENTION
[0010] In general this invention provides a way for an implantable
medical device to communicate a limited amount of stored data or
sensor or status data such as battery status and lead condition to
an inexpensive external instrument. Additionally it would be an
advantage to be able to also transmit marker data for
electrocardiograms. Rather than relying on the more traditional
telemetry communications channel which requires a large amount of
support circuitry and so forth, we are using certain subthreshold
electrical pulsing capability present in some current implantable
medical devices for this purpose. This subthreshold pulsing may be
delivered along different pathways for minute ventilation, lead
impedance, and capture detection, as well as for this new
communications purpose. In a preferred embodiment this circuit 10
outputs pulses at rates up to 125 Hz. By modulating a series of
such pulses we can easily send data at 10 to 100 bps or even higher
data rates. Preferably, communication occurs on a dedicated set of
such pulses.
[0011] The pulse train can be by modulated to include data in
several ways. The form (its amplitude or width for example) of the
wave of the communications pulse may be varied in discrete steps.
Including or omitting pulses at a given time in a segment length of
time can represent various forms of data. Pairing of pulses to send
a data bit may be employed. For example, a zero (0) bit could be
represented by a pulse followed by a missing pulse, while a one (1)
would be represented by a missing pulse followed by a pulse. By
limiting ourselves to having at least one missing pulse every two
pulse locations, we eliminate the possibility of a 00 or 11
configuration and enhance reliability in reading and allows for
easier synchronization by this limitation too. Again, since it is
so much less costly we make the communication be only one way.
However, so that the implanted device is not communicating
constantly to a turned off or disconnected receiver, it is also
preferable to trigger a communications episode or session from
external to the implanted device. This can be done with a simple
"telemetry system" or a substitute for one like a magnet and an
internal reed switch that is in the implant device circuitry and
which when triggered by the presence of the magnet, begins a
communications episode. (Of course, if a more sophisticated
external device is used this sub threshold communication may run
simultaneously with or be triggered by the H or E field telemetry.
But the preferred embodiments will use simple triggers like sounds
or magnets or externally applied electrical pulses, or a short
burst of H or E field signal produced by an inexpensive external
trigger device.) More specifically, each pulse is adapted to avoid
pacing, or any tissue stimulation, and to avoid or minimize its
effect on the lead to tissue interface. The size of the electrical
pulse energy is therefore below the threshold required for cardiac
or skeletal muscle stimulation. These pulses can be safely applied
by a pacemaker electrode in a pattern which makes them easily and
reliably detectable and interpretable by a simple external
device.
[0012] A few modifications to currently known devices for
delivering subthreshold pulses allows for delivery of modulated
pulses. A simple detection algorithm can be implemented in external
receivers which normally read electrograms of the patient by use of
skin electrodes. The data read can be translated, error-checked, or
otherwise modified to transmit the aata to the external device. The
external device can store this or transmit it to other devices or
employ it directly to display diagnostically useful information or
device related information for attending technicians or
physicians.
[0013] In general then the invention is a communications system for
communicating between an implanted medical device and a device
external to a living body containing said implanted medical device
wherein communications of data from within said implanted medical
device to said external device is accomplished by a communications
circuit for producing modulated biphasic subthreshold pulses in a
pattern of modulations predetermined to represent data and
insufficiently energetic to cause a physiologically significant
reaction in living body tissue, and wherein said modulated pulses
are transmitted across two electrodes electrically connected to
said implanted device, said electrodes linkable in an electrical
circuit from said communications circuit through tissues of said
living body, such that said transmission can be received by an
external device through a plurality of electrodes connected to said
external device when such external device electrodes are in contact
with the surface of said body, and the modulations of said
subthreshold pulses will be at least one of the set of modulations
comprising (adjustments to timing between delivery of pulses,
changing amplitude of pulses, absence of a pulse or pulses in a
train of pulses, altered or alternating polarity of pulses, and
alterations in pulse width).
[0014] It has a medical information device for receiving modulated
biphasic subthreshold electrical pulses in a pattern of modulations
predetermined to represent data and insufficiently energetic to
cause a physiologically significant reaction in living body tissue
through electrodes for affixation to a living body surface having a
detecting circuit for detecting said subthreshold pulses through
said electrodes, comprising an amplifier circuit connected to said
electrodes and producing an amplified output signal representing an
electrical waveform composed substantially of said modulations of
said pulses, and having a detecting circuit output for sending said
amplified output signal, a decoding circuit comprising a circuit
for reading each pulse modulation in said representation of the
electrical waveform sent on said detecting circuit output, and for
determining a data bit pattern representing data decoded from said
modulations in said representation of said electrical waveform, and
a conversion circuit for producing a signal representative of the
useful information in said bit pattern.
[0015] In one preferred form, the decoding circuit determines one
data bit value based on based on whether a paired sequence of
pulses is in a present-then-absent order, and an opposite data bit
value based on an absent-then-present order, in another, the
decoding circuit determines a data bit value based on the order of
the polarity of a biphasic pulse, in yet another, the decoding
circuit determines a data bit value based on whether a biphasic
pulse is relatively wide or narrow, and in still another form, the
decoding circuit determines a data bit value based on a measure of
relative amplitude of a biphasic In fact, the decoding circuit
could determine data bit values based on a combination of
modulations in said subthreshold pulses.
[0016] The useful information communicated can represent marker
channel information, data representing physiologic data about a
patient or information about a device sending the subthreshold
communications from within a body.
[0017] The system operates via a method for communicating between
an implantable medical device and an external device, starting with
some data within an implantable medical device, sending a
triggering signal to an implantable medical device, activating said
implantable medical device in response to said triggering signal so
as to encode and send a modulated set of subthreshold electrical
pulses from said implantable device in accord with a protocol
having for each data packet a header followed by substantive
information, receiving the subthreshold pulses across a pair of
electrodes on the surface of the body and decoding modulations of
said subthreshold pulses so as to produce a data output
representative of the data transmitted by the implantable medical
device.
[0018] Preferably, the encoding further adds in error correcting
code data to the modulated subthreshold pulses in each packet.
[0019] On the other side of the communications system is the
implantable medical device which has a memory for storing data to
be transmitted to an external device and a communication circuit
for transmitting subthreshold signals representing data stored in
said memory across electrodes external to but electrically
connected to the communications circuit, wherein said
communications circuit has a generating circuit for producing a
biphasic pulse having a modulatable characteristic, said producing
circuit adapted to configure each biphasic communications pulse in
a pulse train in accord with a value represented by a modulation
information signal, a conversion circuit for providing to said
generating circuit said modulation information signal to control
the modulation of said biphasic pulses, and a configuration circuit
for translating data signal values from said memory into modulation
signal values for sending said modulation values to said conversion
circuit in a stream of values representative of an encoded
translation of said data values in said memory. It should also have
a trigger circuit for receiving a trigger signal from outside a
body and for producing an internal trigger signal on such an
occurrence, and an initiation circuit to receive said internal
trigger signal from said trigger circuit and on such receipt to
initiate program control of functioning of said generation,
translation, and configuration circuits so as to send a stream of
translated, converted and modulated biphasic communications pulses
across said electrodes.
[0020] Of course, any communications to the external device could
be done so as to later be sent by the external device across a
telephone or other communications network to a medical information
group located at a distant receiver.
[0021] Numerous other features and advantages are described with
reference to the following drawings.
[0022] FIG. 1 is a graph of a generalized biphasic pulse for use
with this invention.
[0023] FIG. 2 is a graph of four broken segments showing pulse
modulation features in accordance with preferred forms of this
invention.
[0024] FIG. 3 is a heuristic block diagram.
[0025] FIG. 4 is a graph of a preferred biphasic pulse for use with
this invention.
[0026] FIG. 5 is a graph of a pair of preferred biphasic pulses for
use with this invention
[0027] FIG. 6 is a graphic drawing of a pulse stream in accord with
a preferred form of this invention.
[0028] FIG. 7 is a graphic representation of a timing diagram with
cardiac event features and time periods highlighted as features
thereon.
[0029] FIG. 8 is a graphic representation of a timing diagram like
FIG. 7.
[0030] FIG. 9a is a simplified block diagram representing the
features an implanted device may have in a preferred form of the
invention.
[0031] FIG. 9b is a simplified block diagram representing the
exterior (outside of the patient) device as would be used with a
preferred form of this invention.
[0032] FIG. 10 is a block diagram representation of a protocol.
[0033] FIG. 11 is a block circuit diagram.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0034] The basic pulse waveform is shown in FIG. 1 by line 10. In
general it can be described by an amplitude expressed in voltage
(V.sub.pw) and either side of the biphasic pulse can define a
specific time period (T pw)' (For physiologic reasons, the net
energy delivered to the muscle must be zero.) In order to code data
using these pulses, characteristics of each individual pulse may be
modulated, the relationship between pulses may be modulated, or
some combination of techniques used. Each modulation technique can
be used to include multiple bits per pulse to raise the
transmission rate. Combining multiple techniques can additionally
raise the information transfer rate.
[0035] FIG. 4 describes a single subthreshold waveform 40 having
been positive peak at 41 and a negative peak at 42, and timing
measurements described by arrows 43-48.
[0036] FIG. 5 is being graph 50, of two adjacent pulses 51 and 52,
having the same time measurement values on pulse 51 and
additionally describes a new time parameter illustrated by arrow
53.
[0037] Individual pulses can vary by width (T.sub.pw) or amplitude
(V.sub.pw)' Additionally by choosing complementary pair electrode
hardware (e.g. delivering the pulse, tip to can first and then can
to tip in a pacemaker configuration), the polarity relationship of
the pulse phases can be changed from positive/negative to
negative/positive. Such variations as just described are
illustrated in FIG. 2. Note on the figure the changes in height of
the pulses on line 3, the width of the pulses on line 4, the change
in the order or phase polarity on line 5 of the pulses, and the one
combined form of amplitude, width, and polarity modulation in line
6. The interval between pulses can also be used to include data
bits-of information. The repetition rate (pulse frequency
modulation) or missing pulse configuration in a constant rate pulse
train (another form of pulse frequency modulation) may be used to
encode information into the pulse stream of subthreshold
pulses.
[0038] Also, missing pulse modulation is difficult to combine with
the modulations illustrated in FIG. 2 since pulses would frequently
be omitted in the pulse stream. This would make interpretation
difficult. In general the specific technique or techniques must be
decided by the user of this invention as a result of considering
trade-offs between increasing data rate with more complex
demodulation, lower cost external instruments, and achieving a
specific level of reliability.
[0039] For the purpose of explanation, we describe a simple
frequency pulse modulation scheme that is easily decoded and
produced, employing a constant space between times for pulses to
possibly occur and the absence or occurrence of a pulse during such
times indicating a data "`1" or "`0". However we also describe how
to enable numerous other modulation schemes which employ the
available features of the subthreshold pulse we can deliver. One of
ordinary skill in this art can employ the heuristic principles
described with reference to the simple frequency modulation scheme
we describe to the other forms of pulse modulation available
without difficulty. The designer of a device in accord with this
disclosure will have to consider that the more complex the
modulation scheme employed, the more complex and expensive the
receiver will probably have to be. Accordingly it is expected that
the person of ordinary skill have some knowledge of the use of
biopotential amplifiers.
[0040] FIG. 7 illustrates an 8 millisecond segment 20 of
information under this simple modulation scheme. In this space of 8
mS, 5 pulses can be sent, but only 4 are, pulses 10a-d. (The
segment may also be considered 10 mS long if you include the full
time for the fifth pulse to end before the 6the pulse may be
allowed to occur). The time between each same sized and polarity
simple biphasic subthreshold pulse is expected to be (for this
modulation scheme) 2 mS. There is one space for a pulse missing
between pulse 10c and 10d. Thus, the segment would be read, in its
simplest form as a digital data stream of 11101. As is well known
to those of ordinary skill in the communications art, this could be
a part of a series of allowable pulse configurations, for example
where only one "`zero" is allowed and must be positioned in either
the second or fourth position, thus yielding only three bits of
information from a five pulse long code. Such pulse code schemes
are used to enhance through redundancy the ability of a receiver to
guess at the correct data where, for example one or
another>pulse might be lost in a noisy environment. We use
checksum data for example, but other redundancies and well known
schemes used for the same purposes should be considered to be
within the ambit of this invention.
[0041] Another preferentially designed feature is the limitation of
transmission times to segments of time related to a sensed cardiac
event or a pacing pulse. (Use of this feature is preferred
especially where the receiver cannot distinguish communications
pulses from physiologic signals very well, or more importantly,
where there may be doubt about the inability of the communications
pulses to trigger physiologic reaction in the patient's cells).
FIG. 8 illustrates such a preferred embodiment. The segments such
as segment 20 of FIG. 7 are limited in time to a period wherein the
tissue is refractory to responding to stimulation after the
delivery of a pulse (A or VP point on the line 25) or after an
equal amount of time following a cardiac natural event (A or VS).
These preferred transmission time periods are referenced with
numerals 21 and 22.
[0042] Referring to FIG. 8, there is a single channel transmission
in the atrial channel having marker channel information in each
transmission. The atrial channel is on line A and the ventricular
channel is line V. In this illustrated scheme, a redundant
transmission occurs after an event, here VP2, was triggered by the
occurrence of an Atrial Sense event AS. Since it occurred in the
transmission frame of the Amarker D3, the same Amarker data will be
retransmitted.
[0043] Such periods are chosen to be set for the period of time the
cardiac tissue is refractory to stimulation, thus even
communication of subthreshold pulses near the stimulation threshold
for the tissue will not cause a depolarization. These times of
absolute tissue refractoriness are well known in the pacemaker
art.
[0044] When used in this manner, simple but powerfully descriptive
marker channel information can be transmitted. (The seminal
disclosure regarding marker channel information generally is U.S.
Pat. No. 4,374,382 issued to Markowitz and incorporated herein by
this reference). Thus, for a 24 bit data stream in each space
following a pacing pulse the available amount of information for
transmission using our simple preferred scheme for modulation is
2.sup.24 messages. Thus, the receiving device could have a lookup
table with 2.sup.24 entries, which could be used for transmitting
that much information regarding the present state of the implanted
device, it's history, the patient's physiological event history and
in fact, any data usefully used outside the body where the implant
resides. It is of course, important to recognize that with the
inclusion of framing and error checking information as integral
parts of the bit stream, substantially less than this amount of
data will be available. Thus, the size of the table could be
reduced to include spacer information, headers, or other
redundancies to ensure correct receipt of the intended transmitted
information, as might be designed into the table by one of ordinary
skill. Or, the protocol information can be used by a preprocessing
circuit or program to send the remaining substantive data to the
table look-up circuit or program. The receiving device could use
this information to print marker channel information on the moving
electrocardiograph it is making, and/or store the information for
later retrieval or transmission to a more empowered device where
the information can be interpreted for diagnostic or research
purposes.
[0045] In our preferred embodiment, we developed a specific
integrated circuit for varying the parameters described across a
range of values in a series of discrete steps. See Table 1 below
for these values. A designer of systems employing this invention
can make changes in these selections and ranges within the ambit of
this invention so long as the changes continue to provide
distinguishable features for the receiver and so long as the pulses
are modulated to remain below the threshold which would adversely
affect body tissue through electrical stimulation.
[0046] Just to detail the clear implications for data transmission
again; with a simple modulation scheme as we are detailing here for
a preferred form, for example, using a single binary modulation at
2 mS/pulse area, the data rate is about 50 bits per second; or
using a similar single pulse modulation scheme such as phase
polarity at 125 bits per second, thus the raw data or bit rate is
limited to 125 bps. By using some of the independent modulation
schemes described above, nine bits per pulse can easily be achieved
with a resulting raw data rate of 1125 bps. However, using such
high data rates requires a more sophisticated reading device to
parse the information from the analog encoding of small power
signals, and since for the present moment, price is the main
consideration, the simpler modulation schemes are preferred.
[0047] Since this data transmission scheme is for transmitting data
from between implanted device within a patient's body and an
inexpensive external device similar to an electrocardiogram
receiver/recording device, some type of redundant transmission
information is useful to ensure good transmission of data through
noisy environments and less than ideal conditions. Redundancy is
also important because there is little or no opportunity to inform
the implant that its data is not understood, even if the
inexpensive receiver could determine that the data is not good by
itself. Multiple transmissions of the same data, and/or various
forms of error correction are both classes of useable redundancy
that may be employed for this. In one preferred embodiment we send
a message having an error correcting code incorporated into the
message and use a decoding circuit to correct any errors located in
the message. Depending on the complexity of this added redundancy,
which will need to be included, the amount of data that can be sent
in a given time period will be reduced by from about 5 to 70%.
[0048] In another preferred embodiment, we transmit data
continuously once the transmission is activated without regard to
refractory periods since the size of the pulses is too small to
stimulate the tissue response. In this preferred embodiment,--much
more data can be transmitted in the same period of time since we
don't have to wait for refractory periods.
[0049] Specifically, in our preferred example embodiment,
information is transmitted as words that are 24 bits in length. We
could design this in numerous ways, but for marker channel
information a word of approximately this length or shorter should
be used if transmission time is limited to refractory cardiac
times, and is using something close in date rate to the example
modulation scheme. Our preferred marker channel words are 21 bits
in length. A word can represent data file header, data file
segments, or marker channel information. For unipolar lead
configuration one word is transmitted per pacing cycle. For bipolar
lead configuration up to four words are transmitted per pacing
cycle. Marker channel information is transmitted with one word per
pace or sense event. All bits within a word need to be transmitted
without interruption. If the transmission of a word is interrupted
the entire word must be retransmitted at the next available
opportunity. A preferred structure for the transmitted word is as
shown.
[0050] It should be noted that where the implanted device has no
concern about the potential to stimulate tissue, say for example,
because it is merely a subcutaneous implant monitoring a local
physiologic condition incapable of sending large stimulation
pulses, than much longer! shorter or just different data structures
could be used, as will by now be apparent to the reader.
Additionally, the localization of the external electrodes near the
subcutaneous device would obviate any concern about isolating the
communication pulses from physiologically produced electric
signals.
2 Data Bit Notes Data File Header DO, D1 11.sub.2 Used for clock
synchronization. DO is first bit of word D2-D6 00001.sub.2
Indicates that this is a data file header D7-D8 00.sub.2 to
11.sub.2 Used to identify up to 30 data file segments as a group.
D9-D13 00000.sub.2 to 11111.sub.2 Indicates the number of data file
segments to be transmitted in a group. D14 Complete Transmission
complete indicator, 1 indicates that this is the last group to be
transmitted. D15-D18 00000.sub.2 to 11111.sub.2 Unassigned data
file header bits. D19-D23 ECC Error detection and correction
information. Data File Segment DO, DI 112 Used for clock
synchronization. D2-D6 00000.sub.2 to 11111.sub.2 Used to order
data file segments for reconstruction of date file. 30 segments can
be ordered. D7-D18 Data (12 bits) Data field for a data file
segment D19-D23 ECC Error detection and correction information.
Marker Channel DO, D1 112 Used for clock synchronization. D2-D6
000002 Indicates that this is marker channel information D7 Lead 0
= Atrial lead, 1 = Ventricular lead D8 Event 0 = Sense event, 1 =
Pace event D9 Sense type 0 = Non refractory sense, I = Refractory
sense DIO-D15 Correction The number of 10 ms 1 OmS periods prior to
the first bit of this frame that the marker event occurred. Result
rounded to the nearest 10 msmS. D16-D20 ECC Error detection and
correction information.
[0051] In one example embodiment that limits transmission to
refractory periods but includes marker channel information, data
files consist of a data header file and up to 30 data file
segments. Such segments can be broken across the refractory periods
used in the marker channel transmission times if desired, but this
may result in a slower transmission of large amounts of data. On
the other hand, by only transmitting in the refractory period, the
implanted device is assured of not capturing the cardiac tissue.
All information is transmitted twice to allow for the recovery of
missed information. If the reading device is expecting the
information after the pace (or sensed event) pulse, there is no
need for a header. Similarly, marker channel information does not
require a header. Marker channel transmission occurs once per
event. Incorrect information that can not be corrected with the
checksum information will be discarded by the receiver. The data
file is constructed as shown:
3 Cumulative Transmission File Transmitted Pace Event Time at 85
BPM Unipolar leads Data header file (group 0) 1 Data header file
(group 0) 2 Data file segment (0) 3 Data file segment (1) 4 Data
file segment(n), n .ltoreq. 29 32 for n = 29 23 seconds Data file
segment (0) 33 Data file segment(n), n .ltoreq. 29 64 for n = 29 45
seconds Bipolar Leads (Example shows two groups transmitted) Data
header file (group 0) 1 (Transmission complete bit = 0) Data header
file (group 0) 1 Data header segment (0) 1 Data header segment (1)
1 Data file segment(n), n .ltoreq. 29 8 for n = 29 5.6 seconds Data
file segment (0) Data file segment(n), n .ltoreq. 29 16 for n = 29
11.2 seconds Data header file (group 1) Transmission complete bit =
1) Data header file (group 1) Data file segment (0) Data file
segment(n), n .ltoreq. 29 Data file segment (0) Data file
segment(n), n .ltoreq. 29 32 for n = 29 22.4 seconds
[0052] Additionally one may wish to employ a more detailed
protocol. An example protocol for data communications is described
with respect to FIG. 10 wherein a Marker Frame 101 and a Data Frame
102 structure can coexist in a single transmission. Here the data
file bit stream 103 is broken across the two frames 101 and 102 and
it resides in chunks of the segment data within the protocol
marked
[0053] Segment #1-N. A synchronization portion 105, a marker space
which is zero in one frame and one in the next to distinguish one
frame form another 106, segment' number or marker type 107 marker
time correction data 108 error correcting code 109, and final
synchronization space 110, transmitted in the order shown, make up
the overall protocol, allowing for easy decoding by a compatible
reading device. In a segment having other data than marker data
such as frame 102, the segment 108 contains the data. One of
ordinary skill in the data communications art will be able to
produce innumerable protocol arrangements and the specifics are
best left to the designer of the specific devices. Error correcting
codes are well known in that field as well. See for example, Error
Control Coding: Fundamentals and Applications by Lin and Costello,
Prentice Hall, Inc., Englewood Cliffs, N.J., Copr. 1983, and Error
Correcting Codes by Peterson and Weldon, 2nd Edition, MIT Press,
Boston, Copr. 1972.
[0054] The preferred circuitry is described in overview with
reference to FIG. 3. A bus 31 connects a microprocessor 32 with the
memory 33 and the pulse generator and measurement circuit 34 which
develops the subthreshold communications pulses (as it also can
develop other subthreshold pulses for purposes such as determining
minute ventilation through impedance measurements as was described
in U.S. Pat. No. 4,702,253 issued to Napholtz, among others. Such
pulses can have other alternative uses as well which may be
employed by the same circuitry for generating these pulses any time
they are not being used for communications as they are for this
invention). A microprocessor or other control circuitry 32 formats
a set of register values to be sent to the excitation control
register. These register values set the parameters of each
individual pulse and its timing to include the desired data values
and redundancy. To start communication, the microprocessor writes
the first value to the control register under firmware control.
Subsequent values are automatically transferred from memory to the
control register by either the microprocessor or a the direct
memory access (DMA) controller circuit in the microprocessor. A
program in memory may control the processor circuit 32 to encode
the data sent with the appropriate conversions to the transmission
code and include any protocol features that may be required.
Microprocessor and program control are the most flexible way to set
this operation us, however one could use fixed analog circuitry to
avoid use of registers and other memory devices if desired, but
that would not be preferred.
[0055] An example preferred excitation control register 125 is
shown in FIG. 11. As it is well known how to convert values in a
register to signal values to modulate a waveform no detailed
description is provided here. It is sufficient to say that a larger
number of elements (125.sub.1 . . . n) provides more flexibility in
range between the two polar values of a given pulse modulation
characteristic (such as amplitude or pulse width). But since in our
preferred embodiment we only determine whether a pulse or non pulse
condition will occur at the time for a next pulse during a
communication, the flexibility provided by such a register is
surplussage for this simple embodiment. If however one prefers to
enable more forms of modulation, the diagram of FIG. 11 should be
referenced. There, the value in register 125 would program an
output circuit 126 to produce the pulse modulated for the
characteristics defined by the data in the register 125.
[0056] FIG. 9a represents the shell of the implanted device in
dotted line 40, here having two surface electrodes 47 and 48,
electrically isolated from each other. There may also be electrodes
such as an indifferent electrode employing the exterior metal can
or housing 14, and electrodes 16a 16b 17a and 17b on leads located
so as to provide stimulation within specific tissues, as
illustrated here, in a heart right atrium RA and right ventricle
RV. These devices could be pacemakers, cardioverter/defibrillators,
drug pumps, or any implanted device which can generate subthreshold
pulses for communication in accord with this description. The form
of the implanted device is relevant to the choice of modulation and
data transmission schemes as has been explained throughout this
document. For example, a simple two to four electrode subcutaneous
electrocardiogram recording device has no chance of accidentally
causing physiologic changes in tissue during use of the
communications pulses, so continuous rather than only refractory
time communication would be preferred. The systems with more
electrode choices may be used to enhance the signal received by the
reading device through experiment and the preferred transmission
set of electrodes may be fixed at the time of implant.
[0057] In FIG. 9a, only the relevant features of a typical
implanted device which could be used with this invention are shown.
The pulse generator circuit 74 creates the waveform pulse and
sequence of pulses in accord with parameters written by the
microprocessor 75 under program control to the control register CR
of circuit 74. The Microprocessor 74 may transfer these parameters
through a DMA circuit or across bus 18. The output of circuit 74 is
applied to the electrode switching circuit 71 in accord with the
preferred sending path to the selected electrode pair. The
configuration of the switches in circuit 71 is determined by values
in its control register (not shown) which are in turn selected by
the microprocessor under program control. The data communicated
will generally reside in a specific area of the memory circuit 10,
having been stored thereby the implanted device during its normal
operation for this purpose. The application of the waveform pulse
across a pair of electrodes causes a current to flow and be
detectable by an external reading device via electrodes affixed to
the skin of a patient. The initiation of a communications session
as just described is preferably performed by the activation of some
internal switch such as a reed switch of Hall-effect sensor by a
magnet placed near the implanted device, or by some kind of
telemetered wakeup signal generated by a programmer or a simple
activator device capable of transmitting a simple activation
sequence This function is illustrated here by the use of a
"telemetry" block in dotted line within the shell 40 of device
41.
[0058] FIG. 9b. illustrates an external reading device 60 connected
electrically to a patient's skin SK by electrodes PE1 and PE2. The
signals received by these electrodes (which could be any
combination of known electrocardiogram type electrodes) is fed into
a receiving sense amplifier circuit 62, and commonly will produce
an analog display of an electrocardiogram 64 which represents the
varying signal value found between any two of the leads on the
patient's body. Additionally, the input signal is sent to a
decoding circuit 63 that detects the bit stream in any of the
manners described above, depending on the design of the reading
device 60. The data from that stream is fed to a memory and output
management circuit 65 for storage and use through communications
circuits 66 or by adding to the display or printing an additional
display via circuits 67, if desired. Additionally the data may be
received in a coded format that requires a decoder circuit to do
error correcting and accommodation to redundancies or intradata
modulation techniques. Further a microprocessor circuit 68 may have
a program that operates on the received data to perform diagnostic
or other reporting functions, and a telephonic transmission or
other transmission circuit may send the relevant data received
and/or digested by the programs to some other devices for further
use. Commonly a programmer device 61 will be a receiving device for
such information and may perform additional operations on the data.
The trigger for the transmission by the device 40 may be from an
attached or separate trigger device 86, here a simple magnet, which
acts upon the circuit 77 in an appropriate manner to the circuit 77
design. A separate programmer device 61 could also provide the
trigger to start the transmission by the implant 40.
* * * * *