U.S. patent number RE37,480 [Application Number 08/834,228] was granted by the patent office on 2001-12-25 for cardiac pacer which compensates for effects of abrupt changes in heart rate.
This patent grant is currently assigned to Medtronic, Inc.. Invention is credited to Stephen T. Denker.
United States Patent |
RE37,480 |
Denker |
December 25, 2001 |
Cardiac pacer which compensates for effects of abrupt changes in
heart rate
Abstract
Clinical studies of heart patients have demonstrated that
ventricular tachyarrhythmia often is preceded by a foreshortened
cardiac cycle length followed by a relatively long compensatory
pause, thus producing in an abrupt short-to-long cycle length
change. An implantable apparatus for preventing tachyarrhythmia
measures the cardiac cycle length and detects the occurrence of a
foreshortened cardiac cycle length more than a predefined amount
between consecutive cycles. When a normal heart beat does not occur
within a predefined period of time after such an abrupt change in
cycle length, the resulting compensatory pause is eliminated by a
cardiac pacer applying an appropriately timed electrical pulse to
produce a contraction of the heart. The apparatus also includes a
defibrillator to shock the heart in the event that the preventive
pacing is not effective.
Inventors: |
Denker; Stephen T. (Milwaukee,
WI) |
Assignee: |
Medtronic, Inc. (Minneapolis,
MN)
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Family
ID: |
23428985 |
Appl.
No.: |
08/834,228 |
Filed: |
April 15, 1997 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
363139 |
Dec 23, 1994 |
05545185 |
Aug 13, 1996 |
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Current U.S.
Class: |
607/14;
607/9 |
Current CPC
Class: |
A61N
1/39622 (20170801); A61N 1/3621 (20130101) |
Current International
Class: |
A61N
1/39 (20060101); A61N 1/362 (20060101); A61N
001/365 () |
Field of
Search: |
;607/14,4,9 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Stephen Denker et al. "Facilitation of macrorenetry within the
His-Purkinje system with aburpt changes in cycle length"
Circulation, Jan. 1984. .
Stephen Denker et al. "Divergence between refractoriness of
His-Purkinje system and ventricular muscle with aburpt changes in
cycle length" Circulation, Dec. 1983..
|
Primary Examiner: Schaetzle; Kennedy
Attorney, Agent or Firm: Duthler; Reed A. Patton; Harold R.
Wolde-Michael; Girma
Claims
I claim:
1. A method of preventing tachycardia comprising the steps of:
selecting threshold value for a change in heart cycle length from
one heart cycle to a subsequent heart cycle;
selecting a time period, which is longer than the subsequent heart
cycle;
measuring heart cycle lengths of an animal;
deriving a change in the heart cycle length from one heart cycle to
a subsequent heart cycle;
comparing the change in the heart cycle length to the threshold
value to detect when the heart cycle length decreases;
detecting when a new heart beat fails to occur within the time
period which commences after the subsequent heart cycle;
if a new heart beat fails to occur within the time period after the
change in heart cycle length exceeds the threshold value, commence
treating the animal with a cardiac pacing technique to prevent
tachycardia from occurring.
2. The method as recited in claim 1 wherein the step of deriving
derives a change in heart cycle length between two consecutive
heart cycles.
3. The method as recited in claim 1 wherein the cardiac pacing
technique comprises applying an electrical pulse to a heart of the
animal.
4. The method as recited in claim 1 wherein the cardiac pacing
technique comprises:
applying an electrical pulse to a heart of the animal; and
applying subsequent electrical pulses to the heart of the animal
whenever a heart beat fails to occur within the time period
following a preceding electrical pulse.
5. The method as recited in claim 1 wherein the cardiac pacing
technique comprises:
applying an electrical pulse to a heart of the animal; and
thereafter
a) increasing the time period to an increased time period;
b) applying a subsequent electrical pulse to the heart of the
animal if a heart beat fails to occur within the increased time
period.
6. The method recited in claim 5 wherein steps (a) and (b) are
repeated until at least one spontaneous heart beat occurs.
7. The method as recited in claim 1 further comprising applying
cardioversion/defibrillation to the animal, if after the a cardiac
pacing technique, the animal fails have a normal heart rate.
8. The method as recited in claim 2 wherein a determination is made
that the change in heart rate cycle length exceeds the threshold
value when the subsequent heart cycle has a length which is 80
percent or less of a length of the one heart cycle.
9. The method as recited in claim 1 wherein comparing the change in
the heart cycle length determines when the subsequent heart cycle
length is shorter than the one heart cycle.
10. The method as recited in claim 1 wherein the time period is at
least 1.2 times a length of the subsequent heart cycle.
11. The method as recited in claim 1 wherein the step of selecting
a time period comprises multiplying a length of the subsequent
heart cycle by a value that is greater than one.
12. The method as recited in claim 11 wherein the value is at least
1.20.
13. An apparatus for preventing tachycardia in a heart of an
animal, said apparatus comprising:
a cardiac pacer which responds to .[.a.]. control .[.signal.].
.Iadd.signals .Iaddend.by applying an electrical pulse to the
animal to contract the heart;
a mechanism that measures the cardiac cycle length;
a detector coupled to said mechanism to produce a first indication
when the cardiac cycle length decreases more than a predefined
amount from one cycle to a subsequent cycle;
a processor which responds to the first indication by producing a
second indication when a new heart beat fails to occur within a
predefined time period after the subsequent cycle, where the
predefined time period is longer than the subsequent cycle; and
an evaluator that responds to the second indication by sending the
control signals to the cardiac pacer to perform a cardiac pacing
technique to prevent tachycardia from occurring.
14. The apparatus as recited in claim 13 further comprising a
telemetry circuit for altering the predefined amount.
15. The apparatus as recited in claim 13 further comprising a
telemetry circuit for altering the predefined period of time.
16. The apparatus as recited in claim 13 further comprising a
defibrillator which is responsive to said evaluator for applying a
shock to the heart if a normal heart rhythm fails to occur after a
given number of electrical pulses are applied by said cardiac
pacer.
17. A method of preventing tachycardia comprising:
measuring a first length of a first heart cycle;
measuring a second length of a second heart cycle which occurs
after the first heart cycle;
determining when the second length is shorter than the first length
by at least a predefined amount;
deriving a time period as a function of the second length, where
the time period is longer than the second length;
detecting when a new heart beat fails to occur within the time
period following the second heart cycle and in response thereto
producing an indication of such occurrence; and
responding to the indication by commencing treatment of the animal
with a cardiac pacing technique to prevent tachycardia from
occurring.
18. The method as recited in claim 17 wherein the second heart
cycle consecutively follows the first heart cycle.
19. The method as recited in claim 17 wherein the cardiac pacing
technique comprises:
(a) applying an electrical pulse to a heart of the animal; and
thereafter
(b) increasing the time period to an increased time period;
(c) applying a subsequent electrical pulse to the heart of the
animal if a heart beat fails to occur within the increased time
period; and
(d) repeating steps (b) and (c) until at least one spontaneous
heart beat occurs. .Iadd.
20. A method of preventing tachycardia comprising the steps of:
selecting threshold value for a change in heart cycle length from
one heart cycle to a subsequent heart cycle;
detecting a patient's heartbeats;
measuring heart cycle length of the patient's heart;
deriving a change in the heart cycle length from a first measured
heart cycle to a subsequent measured heart cycle;
comparing the change in the heart cycle length to the threshold
value;
if the change in heart cycle length exceeds the threshold value,
treating the patient with a tachycardia preventing cardiac pacing
technique. .Iaddend..Iadd.
21. The method as recited in claim 20 wherein the step of deriving
derives a change in heart cycle length between two consecutive
heart cycles. .Iaddend..Iadd.
22. The method as recited in claim 20 wherein the step of treating
the patient with a tachycardia preventing cardiac pacing technique
comprises:
defining a time period following the subsequent measured heart
cycle which is longer than the subsequent measured heart cycle;
delivering a pacing pulse to the patients on expiration of the
defined time interval in an absence of a sensed heartbeat during
the defined interval. .Iaddend..Iadd.
23. The method as recited in claim 20 or claim 21 or claim 22
wherein the step of treating the patient with a tachycardia
preventing cardiac pacing technique comprises:
defining gradually increasing pause intervals and delivering pacing
pulses to the patient's heart on expirations of the pause intervals
absent sensed heartbeats during the pause intervals.
.Iaddend..Iadd.
24. The method as recited in claim 20 or claim 21 or claim 22 the
comparing step comprises determining that the change in heart rate
cycle length exceeds the threshold only when the subsequent heart
cycle length is shorter than the one heart cycle.
.Iaddend..Iadd.
25. The method as recited in claim 20 or claim 21 or claim 22
wherein the comparing step comprises determining that the change in
heart rate cycle length exceeds the threshold value when the
subsequent measured heart cycle has a length which is 80 percent or
less of the length of the first measured heart cycle.
.Iaddend..Iadd.
26. The method as recited in claim 20 or claim 21 or claim 22
wherein the selecting step comprises selecting a threshold which is
at least a 20 percent decrease in cycle length. .Iaddend..Iadd.
27. A method of preventing tachycardia comprising:
measuring a first length of a first heart cycle of a patient's
heart;
measuring a second length of a second heart cycle the patient's
heart which occurs after the first heart cycle;
determining whether the second length is shorter than the first
length by at least a predefined amount; and
in response to the second length being shorter than the first
length by at least the predefined amount; commencing treatment of
the patient's heart with a cardiac pacing technique,
comprising:
(a) defining a time period commencing after the second heart cycle
which is greater than the second length;
(b) applying an electrical pacing pulse to the patient's heart on
expiration of the defined time period; and
(c) thereafter applying pacing pulses to the patient's heart at
gradually increasing intervals. .Iaddend..Iadd.
28. The method as recited in claim 27 wherein the step of measuring
the length of the second heart cycle comprises measuring the length
of the heart cycle which consecutively follows the first heart
cycle. .Iaddend..Iadd.
29. An apparatus for tachycardia, comprising:
means for selecting threshold value for a change in heart cycle
length from one heart cycle to a subsequent heart cycle;
means for detecting a patient's heartbeats;
means for measuring heart cycle length of the patient's heart;
means for deriving a change in the heart cycle length from a first
measured heart cycle to a subsequent measured heart cycle;
means for comparing the change in the heart cycle length to the
threshold value;
means responsive to the change in heart cycle length exceeding the
threshold value for treating the patient with a tachycardia
preventing cardiac pacing technique. .Iaddend..Iadd.
30. The apparatus as recited in claim 29 wherein the deriving means
comprises means for deriving a change in heart cycle length between
two consecutive heart cycles. .Iaddend..Iadd.
31. The apparatus as recited in claim 29 wherein the treating means
comprises:
means for defining a time period following the subsequent measured
heart cycle which is longer than the subsequent measured heart
cycle;
means for delivering a pacing pulse to the patient on expiration of
the defined time interval in an absence of a sensed heartbeat
during the defined interval. .Iaddend..Iadd.
32. The apparatus as recited in claim 29 or claim 30 or claim 31
wherein the treating means comprises:
means for defining gradually increasing pause intervals; and
means for delivering pacing pulses to the patient's heart on
expirations of the pause intervals absent sensed heartbeats during
the pause intervals. .Iaddend..Iadd.
33. The apparatus as recited in claim 29 or claim 30 or claim 31
wherein the comparing means comprises means for determining that
the change in heart rate cycle length exceeds the threshold only
when the subsequent heart cycle length is shorter than the first
heart cycle. .Iaddend..Iadd.
34. The apparatus as recited in claim 29 or claim 30 or claim 31
wherein the comparing means comprises means for determining that
the change in heart rate cycle length exceeds the threshold value
when the subsequent measured heart cycle has a length which is 80
percent or less of the length of the first measured heart cycle.
.Iaddend..Iadd.
35. The apparatus as recited in claim 29 or claim 30 or claim 31
wherein the selecting means comprises means for selecting a
threshold which is at least a 20 percent decrease in cycle length.
.Iaddend..Iadd.
36. An apparatus for preventing tachycardia comprising:
means for measuring a first length of a first heart cycle of a
patient's heart;
means for measuring a second length of a second heart cycle of the
patient's heart which occurs after the first heart cycle;
means for determining whether the second length is shorter than the
first length by at least a predefined amount;
means responsive to the second length being shorter than the first
length by at least the predefined amount for treating the patient's
heart with a tachycardia preventing cardiac pacing technique to
prevent tachycardia from occurring. .Iaddend..Iadd.
37. The apparatus as recited in claim 36 wherein the means for
measuring the length of the second heart cycle measures the length
of the heart cycle which consecutively follows the first heart
cycle. .Iaddend.
Description
BACKGROUND OF THE INVENTION
The present invention relates to implantable medical devices which
deliver energy to cardiac tissue in an attempt to restore a normal
sinus rhythm to a patient.
Tachycardia refers to any fast, abnormal rhythm of the heart which
may be amenable to treatment by electrical discharges. One form of
tachycardia is referred to as ventricular tachyarrhythmia (VTA). A
common therapy for treating VTA is to implant a cardiac
pacer/defibrillator in the patient, Cardiac pacers traditionally
have been used to detect a slow heart rate and in response
discharge electrical energy into the heart tissue at a faster pace
which increases the heart rate. Pacing technology also can respond
to the detection of arapid heart rate by producing rapid pacing
which terminates the tachycardia and thereby causing the heart rate
to return to normal. This present cardiac pacing technology is not
specifically designed to reduce the occurrence of ventricular
tachyarrhythmia, but rather to terminate the condition after it
occurs.
However, rapid pacing techniques can accelerate and worsen the
arrhythmias in some instances. As a consequence, cardiac pacers
that treat rapid heart rates do so in conjunction with an
implantable cardioverter defibrillator (ICD). The cardioverter
defibrillator detects rapid ventricular tachyarrhythmias that do
not respond to rapid pacing and employs
cardioversion/defibrillation to terminate the arrhythmia.
Clinical studies have demonstrated that abrupt short to long
changes in the ventricular cycle length often preceded and possibly
precipitated ventricular tachyarrhythmia. The ventricular cycle
length, i.e. the period between ventricular contractions, normally
remains relatively constant and varies only gradually, even upon
the commencement of strenuous exercise. However, occasionally a
premature ventricular contraction occurs in the form of a spurious
pulse from a muscle cell which disrupts the normal electrical pulse
pattern in the heart. Because the heart tissue often does not
recover from an early beat in time to conduct the next regular
electrical pulse, the subsequent normal heartbeat does not occur.
Thus, the heart undergoes very rapid beat followed by a
significantly longer compensatory pause before a subsequent beat
occurs. As a result, the heart is subjected to a very fast heart
rate which quickly changes to a very slow rate. Such rapid rate
change significantly intensifies dispersion of refractoriness in
patients who already have other causes of increased dispersion of
refractoriness, such as damaged ventricular myocardium from a
myocardial infarction or cardiomyopathy. A premature ventricular
contraction may facilitate tachyarrhythmia in these patients.
SUMMARY OF THE INVENTION
A general object of the present invention is to provide an
antitachycardia therapy device which detects a premature heartbeat
followed by a resulting long pause and responds with corrective
action to restore a normal heart rate by preventing the pause. This
decreases dispersion and thereby lowers the incidence of VTA.
Another object is to electrically stimulate the heart to shorten
the compensatory pause and thus reduce the severity of the
short-to-long change in cardiac cycle length.
A further object of the present invention is to provide such
functionality in an implantable cardiac pacer/defibrillator.
Yet another object is to enable the treatment initiation criteria
to be programmable so that the operation of the antitachycardia
therapy device may be configured for each particular patient.
These objects are satisfied by an apparatus that detects the
occurrence of a very short cardiac cycle length followed by a long
compensatory pause and responds by pacing the heart for one or
several beats until a relatively constant rate is restored. The
apparatus measures the cardiac cycle lengths and detects
significantly premature heart beats by sensing abrupt changes heart
cycles. When a heart beat fails to occur within a predefined time
period after a significantly premature heart beat, cardiac pacing
is initiated. For example, a series of electrical pulses are
applied to produce contractions of the heart, until a normal heart
beat occurs between the pulses.
In the preferred embodiment of this apparatus, a mechanism is
provided for a physician to select a threshold value for the
cardiac cycle length change, and the predefined time period before
initiating the cardiac pacing technique.
This technique responds, to a ratio of two consecutive cycle
lengths exceeding a defined magnitude as a precursor of ventricular
tachyarrhythmia, by initiating pacing treatment. This is in
contrast to conventional cardiac pacing which merely is responsive
the heart rate slowing to below a set threshold level. Furthermore,
conventional cardiac pacing responds by applying stimulating
electrical pulses at a constant rate to produce heart beats at a
constant cycle length and does not address preventing the onset of
tachyarrhythmia.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of an antitachycardia system which
includes a cardiac pacer and a defibrillator;
FIG. 2A depicts several heartbeats having a normal rhythm;
FIG. 2B depicts several heartbeats which include a premature
beat;
FIG. 2C depicts heartbeats which occur in a condition similar to
FIG. 2B, but where the heart is treated with a device according to
the present invention; and
FIG. 3 represents a flowchart of a software routine which is
programmed into a cardiac pacer/defibrillator to perform
antitachycardia therapy according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1, an arrhythmia control system 10 is designed to
be implantable and includes a pulse module 11 of a conventional
hardware design. The pulse module 11 comprises a cardiac pacer 15,
a microcomputer 16, a defibrillator 17 and a power supply 18.
Cardiac leads 12 connect the cardiac pacer 15 to the patient's
heart 14 for the detection of analog signals representing cardiac
electrical activity and for the delivery of pacing pulses to the
heart. The cardiac pacer 15 comprises pacing circuit 35 which
includes a pacing pulse generator 36, sensing circuit 37, and
telemetry circuit 38. In addition, there is a controller 39 which
includes an interface to microcomputer 16.
The microcomputer 16 responds to signals received from cardiac
pacer 15 as well as from defibrillator 17 by performing operations
which generate different control and data output signals for both
the cardiac pacer and the defibrillator. The defibrillator 17
produces a high voltage to charge its capacitors and then
discharges them in response to control signals from microcomputer
16. Defibrillator electrode leads 19 transfer the energy of a
defibrillator charge from the implanted pulse module to the heart
14.
As more fully described below, the microcomputer 16 is connected to
an external memory 20 by an address and data bus 22 for the storage
of data. Microcomputer 16 and cardiac pacer 15 are connected by a
communication bus 25, a sense line 26, a pace control line 27, a
sensitivity control bus 28, and a pacing energy control line 29.
The microcomputer 16 is connected to defibrillator 17 by a charge
level line 30, a charge control bus 31, a shock control bus 32, and
a dump control bus 34.
In operation, a conventional sensing circuit 37 detects analog
signals from the heart 14 and converts the detected signals to
digital signals. Furthermore, sensing circuit 37 receives an input
sensing control signal (which determines the sensitivity of the
detection circuits in the sensing circuit) via a sense control bus
41 from controller 39. A change in this sensitivity affects the
voltage deviation required at the sensing electrode for a sensed
event to be registered.
Pacing circuit 35 also receives inputs from controller 39 including
a pace control signal and a pacing energy control signal by way of
pacing control bus 42 which carries the signals that arrive at the
controller over pace control line 27 and pacing energy control bus
29. The pace control signal determines the type of pacing to occur
while the magnitude of the pulse energy is determined by the pacing
energy control signal. Pacing circuit 35 causes pulse generator 36
to generate the pacing pulse 44 which is delivered to the patient's
heart 14 by means of cardiac leads 12.
Telemetry circuit 38 provides a bi-directional signal interface
between the controller 39 of the cardiac pacer 15 and a
conventional external programmer (not shown). The signals are sent
between the telemetry circuit 38 and the external programmer by
inductive or RF coupling thereby enabling reprogramming and
recovery of data from the pulse module 11 after implantation. This
interface forwards commands from the programmer to the controller
39 allowing operating parameters of the cardiac pacer 15 to be
altered. Communications bus 25 carries other commands from the
external programmer to microcomputer 16 to configure operation of
defibrillator 17. As in previous pacing devices operational and
physiological data stored in the memory can be sent to the external
programmer via the telemetry circuit 38 so that the cardiologist
can monitor the patient and performance of the arrhythmia control
system 10.
Appropriate telemetry commands also cause the telemetry circuit 38
to transmit cardiac function information and other data from the
pulse module !1 to the external programmer. Stored data is read out
by microcomputer 16 and sent via communications bus 25, through
controller 39 in cardiac pacer 15 and into telemetry circuit 38 for
transmission to the external programmer.
Referring still to FIG. 1, the microcomputer 16 includes a
microprocessor, timers, I/O circuits, random access memory (RAM)
and read only memory (ROM). The internal RAM acts as a scratch pad
memory and active memory during execution of software programs and
routines stored in internal ROM. This software includes system
supervisory programs and detection algorithms, as well as programs
for storing in external memory 20 data concerning the functioning
of module 11 and the electrogram provided by cardiac leads 12. The
interval hardware timers implement some timing functions required
by microcomputer 16 without resorting to software, thus reducing
computational loads on and power dissipation by the
microprocessor.
Microcomputer 16 receives various status and/or control signals
from cardiac pacer 15 and defibrillator 17. During normal pacing
operations, a sense signal on sense line 26 from the cardiac pacer
15 is used by microcomputer 16 to perform operations such as
arrhythmia detection. The microcomputer 16 produces output signals
such as a pace signal on pace control line 27 which determines the
type of pacing to occur. Other cardiac pacer control output signals
generated by microcomputer 16 include an energy signal on pacing
energy control bus 29 which determines the magnitude of the pulse
energy, and a sensitivity signal on control bus 28, which
determines the sensitivity setting of the sensing circuit 37.
The microcomputer 16 provides the defibrillator 17 with a shock
signal on shock control line 32 which instructs that a shock is to
be delivered to the patient, a charge signal on charge control bus
31 which determines the voltage level of the shock, and a dump
signal on dump control line 34 which indicates that a capacitor
charge is to be dumped to an internal load within defibrillator 17.
Charged voltage level line 30 provides a digital signal
representative of charge voltage from an analog to digital
converter within defibrillator 17, thus providing a feedback loop
which assures that a shock of a proper energy level is delivered by
defibrillator 17.
In addition to being programmed to perform conventional heart
pacing and defibrillation functions, additional routines are stored
within the internal ROM of the microcomputer 16 which carry out the
novel tachycardia prevention technique of the present invention.
This technique is graphically depicted in FIGS. 2A-2C which
represent several heartbeats during different conditions of the
patient. Specifically, FIG. 2A depicts a normal heart rate with
each beat 50 occurring at relatively constant cardiac cycle lengths
(CL). As noted previously, even when an individual commences
strenuous exercises, the heart rate and therefore the cardiac cycle
length changes in a relatively gradual manner. Thus, a sudden,
significant change in cardiac cycle length indicates an abnormal
condition.
The heartbeat pattern depicted in FIG. 2B commences with a normal
heartbeat 51 followed by a premature heartbeat 52 occurring before
the point at which a normal heartbeat, indicated by dashed line 53
would occur. Because the conductive tissue of the heart does not
recover immediately following the premature beat 52, when the next
normal beat 53 is to occur, the electrical conduction is impeded
and the normal heartbeat 53 does not occur. Thereafter, a
relatively long pause denoted by interval 54 occurs, thus
presenting the heart with a relatively short cycle length between
beats 51 and 52 followed by a much longer quiet period 54. Thus,
the heart is subjected in effect to a very rapid heart rate
followed by a significantly slower heart rate. As discussed
previously, this abrupt, rapid to slow heart rate transition often
precedes tachycardia. As a consequence, the heart beats occurring
after the premature beat 52 are very rapid, representing VTA.
The novel software routine added to the programming of the
microcomputer 16 detects the heart rate going from rapid to slow
within one beat and initiates a pacing technique which restores the
normal heart rhythm and avoids the onset of tachycardia.
When the conventional software within the pulse module 11 detects a
heartbeat, one of the routines stored within the ROM of
microcomputer 16 that is called is depicted by the flowchart of
FIG. 3. This routine commences at step 60 where microcomputer 16
determines the new cardiac cycle length (CL.sub.NEW) by reading one
of the internal timers of the microcomputer that is used to measure
the interval between heartbeats. Then at step 62, the value of
variable DELTA is computed by dividing the value of CL.sub.NEW with
the value of the previously measured cardiac cycle length,
CL.sub.OLD. The value of DELTA represents the change in cardiac
cycle length from one ventricular period to the next period. Next,
the microcomputer 16 determines whether the newly computed value of
DELTA is below a threshold value X which indicates prematurity. The
value of X can be varied from patient to patient and is programmed
by the cardiologist via an external programmer and telemetry
circuitry 38 when the pulse module 11 is implanted or anytime
thereafter. For example, DELTA may have a value of 0.80. If DELTA
exceeds the threshold value, the new beat was not sufficiently
premature to require intervention by the arrhythmia control system
10, and the program execution branches to step 66 where the value
of CL.sub.OLD is set to the new cycle length value CL.sub.NEW
before returning to the main system program of the cardiac pacer
15.
However, if at step 64 the value of DELTA is found to be below the
threshold value X, a determination is made by the microcomputer 16
that a significantly premature heartbeat has occurred. For example,
as shown in FIG. 2C, after a regular heartbeat 55, a premature
heartbeat 56 occurs producing a value for DELTA which is less than
the threshold value. In response to this occurrence, the software
routine branches from step 64 to step 67 where the microcomputer 16
derives a value for a variable designated PAUSE which determines
how long to wait for a regular heart beat to occur before
initiating corrective treatment. The value of PAUSE is computed by
multiplying the new cycle length CL.sub.NEW by Y, where Y is a
value programmed by the cardiologist and may be varied from patient
to patient. Typically, Y has a value greater than one (e.g. 1.2) so
that the apparatus will wait for at least as long as the new cycle
length. At step 68, an internal timer of the microcomputer is
initialized with the value of PAUSE.
The microcomputer 16 checks for the occurrence of a spontaneous
heartbeat at step 70. A spontaneous heartbeat is a heartbeat
produced by the heart, as opposed to a heartbeat produced by an
electrical pulse from the cardiac pacer 15. Should another
spontaneous heartbeat occur before the timer elapses, that is a
heartbeat occurs before the expiration of the interval defined by
the variable PAUSE, the execution by the microcomputer 16 returns
to the main system program of the pulse module 11. Thus, if the
heart resumes a normal rhythmic pattern, the treatment provided by
the software routine in FIG. 3 is aborted. There may be several
rapid heart beats before a compensatory pause occurs, in which case
the software routine will terminate after each beat until the
compensatory pause is detected.
However if a normal spontaneous heartbeat has not occurred, the
value of the timer is checked by the microcomputer 16 at step 72 to
determine whether it has elapsed, as occurs at the end of the
interval defined by PAUSE. If the timer has not elapsed, the
program execution returns to step 70. This loop of checking for a
spontaneous heartbeat or the elapse of the timer continues until
one of those two events takes place.
If the heart does not return to normal rhythm following a premature
beat, that is a normal spontaneous heartbeat does not occur within
the PAUSE interval, an indication that the defined short to long
cycle length sequence has occurred is stored in memory 22 at step
74. Then at step 76, a determination is made whether
antitachycardia prevention pacing has been enabled. The arrhythmia
control system 10 may first be configured simply to detect and
record the occurrence of each short to long cycle length sequence.
This information can be read from the system so that a cardiologist
can determine whether that sequence precedes tachycardia in this
patient and thus whether the patient will benefit from the
antitachycardia prevention pacing. If such benefit would be
derived, the cardiologist then can enable system 10 to stimulate
the heart following subsequent short to long cycle length sequence
occurrences.
When the pulsing is found enabled at step 76, microcomputer 16
commands the cardiac pacer 15 to stimulate the heart with a pulse
of electricity at step 78. For example, if a another spontaneous
heartbeat would not occur until interval 54 shown in FIG. 2B and
that interval is greater than the PAUSE time, the cardiac pacer 15
will apply an electrical pulse to the heart through cardiac leads
12 in a conventional manner to produce a heartbeat 57 as shown in
FIG. 2C.
Then the variable PAUSE is increased by multiplying its present
value by Y at step 80. Thereafter execution of the software routine
by microcomputer 16 returns to step 68 where the timer is
re-initialized with the new value of PAUSE. If a normal, or
spontaneous, heartbeat does not occur before the timer elapses
again, another pulse of electricity is applied to the heart to
stimulate a heartbeat 58. Thus upon each loop through the pulsing
routine the waiting period determined by PAUSE is lengthened, which
also increases the interval between electrical pulses applied by
the cardiac pacer 15 to the heart. This process continues to apply
pulses to generate heartbeats until a normal spontaneous heartbeat
59 occurs during a PAUSE interval.
When a spontaneous heartbeat occurs, the program execution returns
to the main software program for the pulse module 11.
Alternatively, several normal spontaneous heartbeats may be
required to occur before the antitachyarrhythmia routine depicted
in FIG. 3 returns to the normal operation of the pulse module, thus
ensuring that the heart has returned to a normal rhythm.
* * * * *