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Description  |
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FIELD OF THE INVENTION
The present invention relates to an implantable heart stimulator and
related method, and more particularly to a highly versatile, externally
programmable and implantable heart stimulator capable of functioning in
various modes of operation to perform a variety of therapeutic routines in
response to recognizable heart disorders or arrhythmias.
BACKGROUND OF THE INVENTION
In recent years, substantial progress has been made in the development of
techniques for providing effective medical response to various heart
disorders or arrhythmias. The types of contemplated disorders or
arrhythmias have typically been treated in the past by drug therapy, or by
devices such as pacers, defibrillators, cardioverters, etc.
More recent efforts have resulted in the development of electronic standby
defibrillators, such as disclosed in U.S. Pat. No. Re. 27,652 of Mirowski
et al (based on original U.S. Pat. No. 3,614,954) and U.S. Pat. No. Re.
27,757 of Mirowski et al (based on original U.S. Pat. No. 3,614,955).
Most recently, efforts have been directed toward the development of
miniaturized defibrillating, cardioverting and pacing devices amenable to
implantation in the body of a patient subject to heart disorder or
arrhythmia. An example of one such implantable device is contained in U.S.
Pat. No. 3,952,750 of Mirowski et al (which discloses a command atrial
cardioverting device). The utilization of an implantable automatic
defibrillator is referred to in U.S. Pat. No. 4,030,509 to Heilman et al.
Moreover, U.S. Pat. No. 4,164,946 to Langer discloses a fault detection
circuit for a permanently implanted cardioverter.
Despite the developments of the recent past, there remains much room for
advancement in this area of medical technology. For example, it is
considered highly desirable to develop a single implantable heart
stimulator having the capability of selectively performing any one of the
various techniques for responding medically to recognizable heart
disorders or arrhythmias, that is to say, the development of a single
implanted heart stimulator capable of performing defibrillating,
cardioverting, and pacing functions on a selective basis.
It also is highly desirable to develop an implantable heart stimulator and
related method capable of selectively performing any one of these
techniques on an automatic basis, that is, automatically in response to
detection of the occurrence of the corresponding heart disorder or
arrhythmia.
Moreover, an extremely advantageous feature of such a device and method
would reside in the capability of externally programming the device to
perform various operations, or sequences of operations, in accordance with
defined parameters. Further elaboration on this point, including a
background discussion, is appropriate at this point.
It is known that the human heart requires coordinated electrical activity
to successfully supply the body with a sufficient flow of blood. This
coordinated activity is produced by a specialized conduction system
contained in the body. A description of this system can be seen by
reference to The CIBA Collection of Medical Illustrations, Heart, by Frank
Netter, M.D., pp. 49-49, 1974 (ISBN 0-914168-07-X, Library of Congress
Catalog No. 53-2151). Malfunctions of the conduction system produce a
variety of human disease conditions up to and including death (see Netter,
op. cit., pp. 66-68).
Recently, an implantable automatic defibrillator has been developed. The
defibrillator automatically delivers a large electrical pulse to the
fibrillating ventricles to abolish fatal malfunction and, thus, may be
lifesaving in the case of ventricular fibrillation. Moreover, numerous
other forms of electrical stimulation therapy have been, and are being,
developed to treat various abnormalities of the heart.
For example, it is known that asystole (the absence of electrical
stimulation to the ventricles of the heart) may be treated by implanting a
pacer which periodically stimulates the ventricles with an electrical
pacing pulse. Moreover, sophisticated pacing techniques, including various
pacing modes (to be discussed in more detail below), have been developed.
Most automatic devices provide pulses to the atrium of the heart, but
practitioners are reluctant to automatically treat the ventricle of the
heart by pacing because of the dangers involved, for example, induced
fibrillation. Accordingly, it is considered desirable to develop a device
which has the capability of treating, by pacing modes and, if need be by
backup defibrillation, any induced arrhythmia or fibrillation which might
result from treatment of the ventricle of the heart.
It is presently known that electrical stimulation treatment modalities may
be primarily classified in accordance with the energy level utilized, as
follows:
______________________________________
Pulse Type Energy Range
______________________________________
Pacing equal to or less than 100
microjoules
Cardioverting or 1-100 joules
defibrillating
(internal)
______________________________________
Stated in simple terms, pacing pulses stimulate a very small volume of
heart tissue (approximately 1-10 mm.sup.3), and the impulse is then
contiguously conducted in a spreading fashion. Defibrillating pulses, on
the other hand, are of sufficient strength to simultaneously stimulate
all, or a critical mass, of the heart tissue, thus ameloriating the
dangerous disorganized patterns of cyclic self-stimulation associated with
ventricular fibrillation.
In the very recent past, combined pacing and cardioverting electrode
systems have been developed, such as are described in U.S. Pat. No.
4,030,509 noted above. Such systems allow the delivery of defibrillating
energy to either the atria or ventricles, and also allow for the delivery
of pacing pulses. A large number of possible electrical stimulation
options are thereby made possible from the combined electrodes.
Such combined pacing and defibrillating functions are quite effective in an
implanted device because some symptoms, such as the absence of R-waves,
could indicate an asystole (treatable by pacing) or life-threatening
ventricular fibrillation. It therefore would be desirable to have a
combined pacer-defibrillator that first could attempt pacing in the
presence of such symptons, and then, if the symptons persist, attempt
defibrillation.
A further copending patent application, Ser. No. 902,763 of Langer et al,
is directed toward the development of a data recording device, intended
for implantation along with an implantable automatic defibrillator. The
intended purpose is to record approximately 100 seconds of the heart's
electrogram before, during and after an episode of ventricular
fibrillation. At a later time, the stored information may be extracted to
provide a complete, permanent record of the ventricular fibrillation
episode, including the operation of the device during automatic
defibrillation. The use of this recording capability may be extended to
capture critical data for additional modes of electrical stimulation
therapy, and also to gain information which could lead to more effective
future electrical stimulation.
Pacers are increasingly becoming programmable, whereby parameters such as
pulse rate, pulse amplitude and R-wave sensitivity may be adjusted from an
external device in electromagnetic communication with the implanted pacer.
It would be highly desirable to implant a microprocessor within an
implanted pacer/cardioverter, for a communication link could thus be
established to enter data, such as a new program, changing the software
program (and, hence, operation) of the microprocessor. Moreover, the
presence of a microprocessor would allow the use of extensive logic and
analysis in the diagnosis and treatment of heart malfunctions with various
regimens of electrical heart stimulation. Thus, it is considered highly
desirable to develop an implantable heart stimulator capable of performing
more than one mode of electrical heart stimulation for a given
malfunction, and further provided with the capability of utilizing a
variety of parameters within any given mode of operation, and even further
with the capability of employing logic in a variety of fashions.
Further referring to the employment of a microprocessor in an implantable
heart stimulator, it is to be recognized that various microprocessors
available today vary in both power consumption and speed, thus making
certain microprocessors (of lower power and speed) suitable for long-term
operations, while other microprocessors (of higher power and speed) are
more suitable for performance of sophisticated operations on a short-term
basis. Accordingly, it is considered highly desirable to provide an
implanted heart stimulator with a dual processor capability. It is also
desirable to provide the heart stimulator with both a high power, high
speed processor and a low power, low speed processor. This would
especially be advantageous in view of the further design criterion of
providing an implantable heart stimulator having multiple modes of
operation for performing various electrical heart stimulation techniques
(as discussed above), since some operations would be suitable for
performance by one processor, while others would be more suitable for
performance by the other microprocessor.
Finally, there are times, during operation, when it would be preferable for
a given processor to operate at a speed higher than its normal speed of
operation. Thus, it is considered highly desirable for an implantable,
microprocessor-based heart stimulator to have the built-in capability of
"gear shifting" so that the microprocessor operates temporarily at a
higher speed of operation.
SUMMARY OF INVENTION
According to the present invention, there is provided an implantable heart
stimulator and related method, and more particularly a highly versatile,
efficient, and externally programmable heart stimulator which forms an
integrated system for carrying out electrical heart stimulation techniques
(defibrillation, cardioversion, pacing, etc.) in response to the detection
of various heart disorders or arrhythmias. Such an implantable heart
stimulator is processor-controlled, and is preferably controlled by dual
processors, each of the two processors being specifically chosen, by
virtue of its design, for controlling a particular type of operation
(long-term versus short-term, simple versus sophisticated).
To be more specific, the present invention relates to an implantable heart
stimulator and related method capable of performing in a multiplicity of
operating modes, each of which can non-invasively be activated. In
addition, as will be explained below, various parameters for each mode are
externally programmable. The long-term operating modes, to be performed by
a simpler, slower, and less power-consuming processor, basically comprise:
(1) ventricular fixed rate pacing, (2) atrial fixed rate pacing, (3)
ventricular demand pacing, (4) bifocal pacing, and (5) automatic
defibrillation. A short description of each mode is appropriate.
In the ventricular fixed rate pacing mode, the parameters can be programmed
to various values. Such pacer parameters include pulse rate, rate limit,
pulse amplitude (milliamperes), and pulse width (milliseconds). In a
preferred embodiment of the present invention, an override capability
exists, allowing the attending physician to double pulse rates (up to an
appropriate maximum number of pulses per minute, e.g., 200 pulses per
minute) for the purposes of overdrive. By activating the override mode, a
burst of high rate pulses for a period of two to three seconds is issued.
After this burst, the override is deactivated automatically and the pacer
parameters return to original values.
In the atrial fixed rate pacing mode, the parameters can be programmed to
various values, the parameters including pulse rate, rate limit, pulse
amplitude and pulse width. In addition, an override capability exists,
allowing the attending physician to cause a burst of rapid atrial pacing.
In this mode, pulse rates can be increased by a factor of 10 over typical
pulse rates, (50, 55, . . . , 115, 120 pulses per minute). Such a burst
will last between 2 and 3 seconds, after which the pacer parameters return
to their original values.
In the ventricular demand pacing mode, parameters can be programmed to
various values; the parameters include pulse rate, rate limit, pulse
amplitude, pulse width, sensitivity, and refractory period.
In the bifocal pacing mode, the parameters can again be programmed to
various values, the parameters including pulse rate, rate limit, pulse
amplitude, pulse width, sensitivity, refractory and AV (atrioventricular)
delay.
Finally, automatic defibrillator operations can be performed in accordance
with conventional parameters, including pulse energy (joules), number of
pulses per sequence, and energy of each pulse. See, for example, U.S. Pat.
Nos. 3,952,750 and 4,030,509.
Short-term operating modes, to be performed by a sophisticated, high-speed
(and thus, high power-consuming) processor include: cardioversion,
automatic patient warning, and automatic ventricular tachycardia control
operations (including ventricular override pacing, rapid atrial pacing,
ventricular coupled pacing, and automatic cardioversion). In addition, the
more sophisticated and high-speed processor can perform, in a preferred
embodiment, four-function recording, such recording being long-term in
nature, but nevertheless performed by the high-speed processor, operating
in the direct memory access (DMA) mode. A brief discussion of each of the
latter operations is now appropriate.
In a preferred embodiment, the cardioversion mode is activated only by
reception of an external command signal (such as detection of placement of
a magnet on the surface of the skin adjacent to an implanted reed switch
and the transmission of a word over the data channel). The output produced
in the cardioversion mode is synchronized with the next R-wave following
receipt of the command signal. Preferably, only one pulse per command is
issued, and the pulse energies are non-invasively selected from among
certain predetermined values (for example, 2, 5, 10, 15, 20, 25, 30 or 35
joules).
The automatic ventricular tachycardia control mode of operation is the most
complex of all modes of operation implemented by the system of the present
invention. This mode of operation can be implemented under program
control, the implantable heart stimulator being pre-programmed and enabled
by the physician. However, there exists also the capability of
reprogramming the implantable heart stimulator, in correspondence to the
results of the treatment thus far, and then enabling the operation of the
heart stimulator so as to treat the patient further in accordance with the
reprogrammed procedure. In this mode, any combination and/or sequence of
the following sub-modes can non-invasively be selected (programmed) by the
attending physician: ventricular overdrive pacing, ventricular coupled
pacing, automatic cardioversion, and rapid atrial pacing. Any or all of
these can be selected, so that, if the first response is not effective in
controlling ventricular tachycardia, the next response is activated. That
is, initially, a list associated with the various modes can be developed;
then, the doctor can revise the list depending on the patient's reaction
to treatment. A more detailed discussion of each of these sub-modes of
operation now follows.
When ventricular tachycardia is detected, a two- to three-second burst of
ventricular overdrive pacing is issued. The rate of overdrive pacing is
pre-programmed at 10, 15, 20 or 25% above the sensed ventricular
tachycardia rate. The number of bursts is pre-programmed at 1, 2, 3, or 4
before automatically proceeding to the next response mode. There is,
typically, a five-second delay between bursts.
In the ventricular coupled pacing sub-mode, N ventricular pulses existing
above a given rate cause a ventricular pacing pulse to be placed at a
given time (expressed in percentage of the R-R interval) following the Nth
ventricular pulse. In addition, if the tachycardia continues, a search
mode comes into effect for which, after each Nth pulse, the coupling
interval decreases by a given amount of time, until the final coupling
interval is reached. This procedure can be repeated a number of times
(preferably, up to four) before proceeding to the next response mode.
Various parameters for this sub-mode of operation include the number of
precursor pulses, the tachycardia rate (pulses per minute), the initial
coupling interval (percentage of R-R interval), the coupling decrement
(percentage), the final coupling interval (percentage), and the number of
response cycles.
In the automatic cardioversion sub-mode of operation, when ventricular
tachycardia is detected, an output pulse synchronized with an R-wave is
issued. Up to four such pulses may be issued with any combination of
pre-programmed energies (for example, 2, 5, 10 or 15 joules) before
proceeding to the next response mode. There is, typically, a 5 second
delay between each cardioversion pulse.
Finally, in the rapid atrial pacing sub-mode of operation, a two- to
three-second burst of rapid atrial pacing is issued at a pre-programmed
rate. The number of bursts before proceeding to the next response mode are
pre-programmed at 1, 2, 3 or 4. There is typically, a five-second delay
between each burst.
As stated previously, four-function recording constitutes a mode of
operation which, although long-term in nature, is performed by the
short-term processor, operating in the DMA mode. Typical information to be
recorded for various events includes times and dates of episodes (such as
fibrillation episodes or defibrillation pulses in the absence of
fibrillation), ten seconds of precursor ECG, capacitor charge times (for
example, for each fibrillation pulse), ninety seconds of post-pulse
(post-fibrillation pulse) ECG, and various other data, as required by the
attending physician, such capability being readily available merely by
externally pre-programming the implanted heart stimulator device.
A plastic warning mode of operation constitutes a further short-term mode
of operation performed by the more sophisticated of the two processors. In
accordance with this mode of operation, a patient warning pulse burst is
issued upon detection of ventricular fibrillation. The parameters of this
signal are programmable to optimize patient detection, such parameters
including burst duration (preferably, rather short), burst amplitude,
pulse width, and pulse rate. It is also considered desirable to program
the implantable heart stimulator to include a service request pulse burst
activated by device fault detection, loss of pacer capture or sensing, low
battery voltage, and other similar conditions. The service request signal
must be distinguishable from the warning signal, and could, for example,
be constituted by two short bursts of a few seconds duration, occurring
within ten seconds, and repeated once every hour with amplitude, width and
rate programmable, as stated above.
Thus, the implantable heart stimulator and method of the present invention
provide the capability of external programming so that various operations,
or sequences of operations, can be performed in accordance with various
parameters which are capable of being externally set at the discretion of
the attending physician.
In general, the implantable heart stimulator according to the present
invention comprises an input stage for receiving various status and sensor
inputs, a controller section (preferably implemented by a microprocessor)
for selectively performing any one of various operations of various types,
an output stage responsive to signals provided by the controller for
activating the various electrical heart stimulation devices (as well as
for activating a patient warning system), and a data input/output channel
for receiving and providing, to the controller, data inputs thereto, and
for receiving from the controller and providing as an output various data
outputs (for example, data to be displayed). In a preferred embodiment,
the controller comprises a first controller for selectively performing any
one of various operations of a given type, a second controller for
selectively performing any one of a plurality of operations of a different
type, and an interface for providing exchange of information and control
signals between the two controllers.
The implantable heart stimulator and method involve the determination of a
condition, from a plurality of conditions, afflicting a patient, the
choosing of at least one mode of treatment for treating the condition, and
the execution of the steps of each mode or modes of treatment. In a
preferred embodiment, the steps or functions just described are
repetitively and continuously implemented by the stimulator of the present
invention.
Finally, in one embodiment of the implantable heart stimulator and method,
a sensing system is provided for sensing the absence of a natural R-wave,
as a result of which pacing is performed, and then sensing the presence or
absence of a forced R-wave (as would result from successful pacing), the
system taking no action in the presence of a forced R-wave, or performing
defibrillation in the absence of a forced R-wave.
Therefore, it is an object of the present invention to provide an
implantable heart stimulator and method, and more particularly a
multi-mode implantable heart stimulator and method capable of
accomplishing various types of electrical heart stimulation in response to
detection of the occurrence of various heart disorders or arrhythmias.
It is a further object of the present invention to provide a highly
versatile implantable heart stimulator capable of performing
defibrillation, cardioversion and pacing.
It is a further object of the present invention to provide an implantable
heart stimulator which is microprocessor-controlled, and, further, which
is externally programmable with respect to various operations, or
sequences of operation, to be performed, and various parameters in
accordance with which such operations are to be performed.
It is a further object of the present invention to provide an implantable
heart stimulator which is not only microprocessor-controlled, but which is
controlled by a plurality of processors (for example, in a preferred
embodiment, two processors), each processor being specially selected, by
virtue of its design, for performing operations of a given type.
It is a further object of the present invention to provide an implantable
heart stimulator controlled by dual processors, one processor being
specially selected and designed for the performance of long-term
operations, simple in type, while consuming relatively low power, the
other processor being selected for the performance of short-term,
sophisticated operations, even though consuming relatively high power.
It is a further object of the present invention to provide an implantable
heart stimulator controlled by at least one processor which is specially
designed for normal operation at a given speed, and which can selectively
be actuated to a higher processing speed for the performance of
specialized operations requiring high speed of performance.
It is a further object of the present invention to provide an implantable
heart stimulator having a data recording device capable of being implanted
along with the implantable heart stimulator.
It is a further object of the present invention to provide an implantable
heart stimulator and method, wherein absence of a natural R-wave is
sensed, as a result of which pacing is performed, followed by sensing of
the presence or absence of a forced R-wave, with no further action being
taken in the presence of a forced R-wave, and defibrillation being
performed in the absence of a forced R-wave.
The above and other objects that will hereinafter appear, and the nature of
the invention, will more clearly be understood by reference to the
following description, the appended claims, and the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of the implantable heart stimulator of the
present invention.
FIGS. 2A, B and C are diagrams relating to the input stage 12 of FIG. 1.
FIGS. 3A and 3B are diagrams relating to the controller 14 of FIG. 1.
FIG. 4 is a block diagram of the interface 16 and controller 18 of FIG. 1.
FIG. 5 is a block diagram of the output stage 22 of FIG. 1.
FIG. 6 is a flow chart of a typical program exemplifying the types of
operations performed by the controller 14 of FIG. 1.
DETAILED DESCRIPTION
The present invention will now be more fully described with reference to
various figures of the drawings, FIG. 1 of which is a block diagram of the
implantable heart stimulator of the present invention.
As seen in FIG. 1, the implantable heart stimulator 10 comprises: an input
stage 12 for receiving various sensor and status inputs, that is to say,
an impedance sensor input derived from electrodes (not shown) connected to
the heart, an electrocardiogram (ECG) input derived from conventional ECG
detection and amplifier circuitry (not shown), and an external command
signal ("magnet in place") alerting the implantable heart stimulator to
the fact that an external command is being received by virtue of placement
of a magnet in proximity to the skin, and thus in proximity to (for
example) a reed switch (not shown) located just under the skin's surface;
a controller (A) 14 which, in response to various signals and inputs
received from the input stage 12, as well as from an interface 16 and
controller (B) 18 (to be discussed below), performs various operations so
as to generate differrent control and data outputs to both an interface 16
and an output stage 22; an interface 16 providing a conduit through which
various data, status and control signals pass to and from input stage 12,
controller 14 and controller 18; a second controller (B) 18 responsive to
various data and control inputs received from input stage 12, interface
16, and a data input/output channel 20, for performing various operations
to provide control and data outputs to the input stage 12, the controller
14, the interface 16, the data input/output channel 20, and the output
stage 22; a data input/output channel 20 forming a conduit through which
both data in and data out pass on their way to or from various elements
(visibly, controller 14 and controller 18) of the implantable heart
stimulator 10; and an output stage 22 responsive to various control
signals from controller 14 and controller 18 for not only actuating
conventional defibrillation, cardioverting and pacing devices connected
thereto, but also for actuating a patient warning system (to be discussed
below).
In accordance with a preferred embodiment of the present invention,
controller 14 and controller 18 are specially selected, by virtue of their
design, to perform certain, respective operations for which they are
particularly suited. This feature of the present invention, including the
precise breakdown of functions between controllers 14 and 18, will be
discussed in more detail below.
FIGS. 2A, 2B and 2C are diagrams relating to the input stage 12 of FIG. 1.
As seen therein, input stage 12 comprises amplifier and signal
conditioning circuitry 30, converter 32, a dedicated cardiac state
evaluation circuit 34, and an input selector 36.
In operation, amplifier and signal conditioning circuitry 30 receives an
ECG input provided by conventional ECG detection circuitry, and performs
amplification and signal conditioning (filtering) to provide an analog
output. Furthermore, amplifier and signal conditioning circuitry 30
receives an input CONTROL WORD from the controller 18 (FIG. 1), which
control input sets the corner frequency for differentiating the ECG input,
and also sets the maximum gain which the amplifier can have (thus, setting
the sensitivity for the ECG input).
The amplifier and signal conditioning circuitry 30 and the converter 32 of
FIG. 2A are shown in more detail in FIG. 2B. As seen therein, the
amplifier and signal conditioning circuitry 30 comprises filtering
capacitor 298 and a differentiation circuit made up of amplifier 300 and
resistor 302. The converter 32 comprises absolute value circuit 304, an RC
circuit made up of resist | | |
