Method and apparatus for correcting abnormal cardiac activity by low energy shocks

The present invention relates to a method and apparatus for delivering two or more individual low-level pulses to correct certain cardiac arrhythmias such as high-rate ventricular tachycardia and ventricular fibrillation; at least the latter shocks are delivered in synchronism with a repeatable characteristic of the heart's electrical activity. Basically, the apparatus comprises an electrode sensor placed in the right ventricle of the heart or in the coronary sinus to act as a detector for the electrical activity of the heart. Connected to the electrode sensor is an arrhythmia detector chosen to detect such arrhythmias as ventricular tachycardia and ventricular fibrillation. Upon detection of one of these arrhythmias, the detector issues a signal which is interpreted by a programmable logic device to activate an energy storage device for delivering a first low-level shock to the heart through a shock delivering electrode. A slew rate detector is then activated by the logic device. The output of the slew rate detector is fed to a comparator to be compared with a continually updated reference signal supplied by the logic device. When the slew rate output equals or exceeds the value of the reference signal, the comparator issues a signal which activates the energy storage device to issue a second low-level shock to the heart through the shock delivering electrode. Additional shocks may be delivered in a manner similar to the delivery of the second shock under the control of the logic device.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method and apparatus for correcting 
abnormal cardiac activity by employing low energy shocks. More 
specifically, the present invention deals with the correction of 
arrhythmias such as high-rate ventricular tachycardia and ventricular 
fibrillation with multiple relatively low energy shocks issued in 
synchronism with a repeatable event in the arrhythmia. 
2. Description of the Prior Art 
Normal heart rhythm (sometimes called "sinus rhythm") originates in the 
sino-atrial (SA) node of the heart. Disorders of heart rhythm are called 
arrhythmias, and may arise in either the atria or the ventricles. For 
example, one type of arrhythmia manifests itself in the form of a rapid 
heart rate (known as "tachycardia") arising in the atria, which may be 
serious in patients with diseased hearts. 
A common treatment for serious tachycardia consists, basically, in passing 
a pulse of electrical energy through the heart, and momentarily stopping 
the heart completely, after which the heart is left to start spontaneously 
in sinus rhythm. The instrument utilized for such treatment is known as a 
cardioverter. This instrument is in most respects the same as the 
instrument known as the defibrillator, with the major difference being 
that the cardioverter typically delivers its shocks in synchrony with the 
cardiac activity so as to avoid the vulnerable period and hence lessen the 
possibility of worsening the rhythm being treated. Specifically, the 
delivery of electrical energy in the vulnerable period (e.g., on the 
T-wave) could induce ventricular fibrillation. 
Additionally, cardioverting pulses often are characterized by energy levels 
of approximately 25 to 100 joules externally, whereas the energy levels 
typically employed for ventricular defibrillation are in the range of 
100-400 joules for external application (or 20-50 joules for internal 
application). For purposes of comparison, pacing is accomplished by energy 
levels on the order of 0.04 millijoules or less. 
Ventricular fibrillation is an arrhythmia generally more difficult to break 
than tachycardia. As noted previously, the energies required to 
defibrillate generally are higher than those capable of cardioverting. 
And, there are several phenomena involved in ventricular defibrillation 
that are not adequately explained by the conventional theory that a 
defibrillating shock needs to be sufficiently strong to depolarize the 
vast majority (a critical mass) of the myocardium. For example, it has 
been observed that a heart not defibrillated by several shocks at a 
particular energy level suddenly responds to the same energy level and 
reverts to sinus rhythm. It further has been observed that at times a 
heart may not respond to high energy shocks, but effectively can be 
defibrillated if the energy is reduced to a lower level. 
There are obvious reasons for wanting to defibrillate at low energy levels, 
and hence there have been efforts made to reduce the energy levels in 
defibrillation. Some low energy defibrillation techniques have been 
described, but although at times effective, such techniques are not 
consistently so. For example, a double pulse low energy system is 
described by Jan Kugelberg in the article "Ventricular Defibrillation--A 
New Aspect" (Stockholm, Sweden: Acta Chirurgica Scandinavica, 1967). 
Kugelberg discloses the use of two pulses separated by a predetermined 
time period, optimally 100 miliseconds. The theory is that the first pulse 
affects and synchronizes a small portion of the myocardium, while the 
second pulse then can more easily complete the job of reverting the 
arrhythmia. While the theory does appear to have some merit, and while 
experimentation has shown approximately 60% effectiveness with laboratory 
dogs at the optimal pulse interval of 100 milliseconds, the Kugelberg 
technique has not been accepted as a viable approach to defibrillation at 
low energies. 
Another low-energy theory previously advanced involves what is termed the 
"protective zone" theory. This theory is that a properly timed small shock 
can prevent desynchronization of myocardial depolarization which otherwise 
could lead to ventricular fibrillation when shocks are delivered in the 
vulnerable period. A timed second shock, according to this theory, 
resynchronizes the myocardial fibers which were disrupted by the first 
shock. But, again, this theory has not consistently been effective in 
practice. 
Defibrillation of the heart also has been effected by relatively low energy 
shocks delivered through an intravascular catheter. In this regard, it has 
been observed that catheter defibrillation in man often is effective using 
5-15 watt seconds. See commonly owned Mirowski et al U.S. Pat. No. 
3,942,536 , incorporated by reference herein. Still, there is benefit to 
be gained by further reducing energy levels. 
It is believed that the inability to obtain consistently favorable results 
at low energy levels with the prior art methods can be linked to the 
inherent random delivery of shocks in response to the malignant 
arrhythmias. There is thus a need for a method and apparatus which is 
effective in uniformly correcting certain cardiac arrhythmias with shocks 
having energy levels lower than those presently employed with the 
techniques of the prior art. The present invention is directed toward 
filling such need. 
SUMMARY OF THE INVENTION 
The present invention relates to a method and apparatus for delivering two 
or more individual low-level pulses, at least the latter of which are 
delivered in synchronism with a repeatable characteristic of the 
arrhythmia being treated, to correct certain cardiac arrhythmias such as 
high-rate ventricular tachycardia and even ventricular fibrillation. 
Basically, the apparatus comprises a sensitive detector of the heart's 
electrical activity, a pair of discharge electrodes, circuitry for 
recognizing treatable arrhythmias, and timing circuitry for effecting 
plural discharges in synchronism with a repeatable characteristic of the 
heart's electrical activity. 
In one embodiment, the detector is in the form of a high resolution ECG 
bipolar electrode sensor having two spaced electrodes; the electrode 
sensor is designed to reside in the right ventricle. The discharge, or 
shock delivering electrode, may be in the form of a bipolar catheter 
having two separated electrodes adapted to be placed, respectively, in the 
right ventricle and in the superior vena cava. Alternatively, the 
discharge electrodes can take the form of a unipolar catheter in the 
superior vena cava (which could reside on the sensor catheter) and a 
single patch electrode placed over the apex of the heart. It also is 
contemplated to use two epicardial patch electrodes positioned, 
respectively, at the apex and the base of the heart. Further, the same 
electrodes can serve both for delivering the low-level shocks and for 
monitoring ECG activity. 
Connected to the electrode sensor is an arrhythmia detector chosen to 
detect, for example, such arrhythmias as ventricular fibrillation, 
ostensibly a very disorganized cardiac rhythm, and ventricular 
tachycardia, a somewhat more organized cardiac rhythm by comparison. Upon 
detection of one of these arrhythmias, the detector issues a signal which 
is interpreted by a programmable logic device to activate an energy 
storage device for delivering a first low-level shock to the heart through 
the bipolar catheter, the epicardial electrodes, or a combination of the 
two. As used herein, a low-level shock is one within the range of 0.1-15 
joules. 
A slew rate detector, which is operatively connected to the sensing 
electrodes, is activated by the logic device. The output of the slew rate 
detector is fed to a comparator to be compared with a reference signal, 
indicative of a predetermined slew rate, supplied by the logic device. 
When the actual slew rate output exceeds the value of the reference 
signal, the comparator issues a signal, which after being delayed for a 
time determined by or programmed into the logic device, activates the 
energy storage device to issue a second low-energy shock to the heart 
through the shock delivering electrodes. Additional shocks may be 
delivered in a manner similar to the delivery of the second shock under 
the control of the logic device. 
In this way, the first shock is delivered with or without regard to 
synchronization to the arrhythmia being detected. The primary purpose of 
the first shock, with or without synchronization, is to "coarsen" or alter 
the configuration of the arrhythmia, primarily by lowering its frequency 
or by increasing its amplitude. It is theorized, following the work of 
Kugelberg, that a small portion of the myocardium is organized by the 
first shock, thus reducing the random electrical activity in the 
myocardium, and reducing the frequency of the resulting arrhythmic ECG 
waveform. Further, it is noted through careful analysis of the ECG, that 
although some arrhythmias, even ventricular fibrillation, appear totally 
random, there are periodic, predictable, segments of the ECG; this is 
particularly the case after the waveform is coarsened by a first shock. 
The second and subsequent low-level shocks, through the slew rate detector, 
are synchronized to a particular repeatable characteristic of the 
arrhythmia being treated. As an example, for ventricular tachycardia, the 
repeatable characteristic may be the leading edge of the R-wave found 
within the arrhythmia. For ventricular flutter and certain ventricular 
tachycardias, where there sometimes are no recognizable R-waves, 
synchronization may be on polarization waves; for ventricular 
fibrillation, where there likely are no recognizable R-waves, 
synchronization may be on the peak of a maximum rise time ECG spike. 
Specifically, the detected ECG is analyzed by the slew rate detector and by 
the logic circuitry. It presently is contemplated that the analysis be by 
tracking the maximum instantaneous slew rate and by storing the peak slew 
rate (i.e., the waveform segment exhibiting the fastest rate of change, in 
this case, rise time or leading edge of the ECG signal). The logic 
circuitry then effects delivery of a first shock, preferably on the ECG 
segment showing the highest slew rate, but possibly without regard to 
synchronization. If shocking in synchrony, the stored peak slew rate is 
proportioned by pre-programming or by the logic circuitry, and is fed to 
the "reference" input of a comparator, the other input of which receives 
actual slew rate signals. 
After the first shock, and within a preprogrammed time interval, the device 
performs by continually updating the signal representing the highest slew 
rate segment stored in the preceding time cycle. However, if the ECG is 
showing a steady decrease in slew rate readings (wherein no segment is of 
a comparable slew rate to that maximum previously stored), the device 
updates itself by storing a new peak slew rate. Preferably, updating 
occurs if the new slew rate differs by a predetermined amount from that 
previously stored. The device similarly updates itself if the slew rate is 
steadily rising. After appropriate updating, the above-described cycle is 
repeated until the arrhythmia is broken and the heart returns to normal 
sinus rhythm. 
Certain patients exhibit more manageable arrhythmias, thus obviating the 
need for the first unsynchronized shock. In this case, as noted above, it 
is envisaged that the first shock and all subsequent shocks be 
synchronized to a particular repeatable characteristic of the arrhythmia 
being treated. 
Thus it is a primary object of the present invention to provide a method 
and apparatus for correcting certain cardiac arrhythmias through the 
delivery of multiple, properly timed, low-level shocks. 
It is a further object of the present invention to provide a method and 
apparatus wherein the delivery of the low-level shocks is synchronized 
with a repetitive event occurring in the abnormal cardiac activity. 
Still another object of the present invention is to provide a method and 
apparatus wherein the delivery of low-level defibrillating shocks is 
synchronized with a portion of the ECG exhibiting maximum slew rate within 
a predetermined sampling period. 
The above and other objects that will hereinafter appear, and the nature of 
the invention, will be more clearly understood by reference to the 
following description, the appended claims, and the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A preferred embodiment of the device for correcting abnormal cardiac 
activity in the form of a generally disorganized arrhythmia through the 
deliverance of multiple low-level shocks is designated at 10 in FIG. 1. 
The device is implantable in its preferred form. A conventional sensing 
electrode 12, which acts as a sensing probe, has a pair of electrodes 
positioned in or about the heart in a position suitable for monitoring the 
electrical activity (ECG) associated with the heart. For purposes of the 
present invention, it is essential that the sensing electrode 12 be able 
to develop precise ECG data so that the electrical logic circuitry can 
recognize and respond to the repeatable electrical activity that might 
otherwise go undetected. 
An example of one sensing electrode, embodying extracardial base and apex 
electrodes, is disclosed in Heilman U.S. Pat. No. 4,270,549, incorporated 
by reference herein. Another example of a known sensing electrode, in the 
form of an intravascular catheter, can be seen in previously referenced 
U.S. Pat. No. 3,942,536. It also is contemplated that the sensing 
electrode 12 take the form of a small, high resolution ECG bipolar sensor 
having two closely spaced electrodes designed to reside in the right 
ventricle of the heart. 
The electrical activity monitored by the detector 12 is amplified by 
amplifier 14 and then is fed to an arrhythmia detector 18 and in parallel 
to a slew rate detector 20 through appropriate leads. 
The arrhythmia detector 18 is designed to detect certain arrhythmias such 
as ventricular fibrillation, a highly disorganized cardiac rhythm, or 
ventricular tachycardia, a more organized cardiac rhythm by comparison. 
One such arrhythmia detector is disclosed in Langer et al U.S. Pat. No. 
4,184,493, incorporated by reference herein. Upon detection of one of 
these arrhythmias, the arrhythmia detector communicates with a 
programmable logic device 22 via data lines 24. A programmable logic 
device capable of performing the basic activities to be described, is 
discussed in copending U.S. patent application Ser. No. 263,910, filed May 
15, 1981, incorporated by reference herein and now U.S. Pat. No. 
4,393,877. FIG. 3 is a flow diagram showing the basic steps carried out by 
a preferred embodiment of the device for correcting abnormal cardiac 
activity in conjunction with the programmable logic device 22. The logic 
device 22 interprets the data from the arrhythmia detector and issues an 
appropriate signal on lines 26 in order to activate an energy storage 
device 28 to deliver a first low-level shock to a shock delivering 
electrode 30 of known design. Examples of known shock delivering 
electrodes are the subject matter of previously referenced U.S. Pat. Nos. 
4,270,549 and 3,942,536. It also is contemplated that the sensing 
electrodes and the shock delivering electrodes be combined partially or 
completely. The energy storage device, which typically is a capacitor, is 
recharged through a conventional charging circuit 21 under the control of 
the logic device 22. 
The previous description relates to the delivery of the first of a series 
of shocks. The first shock may be delivered without regard to 
synchronization, and serves to convert the highly disorganized arrhythmia 
into a more organized arrhythmia, "coarsening" the arrhythmia. When 
coarsened, the electrical activity representing the arrhythmia is altered, 
primarily in terms of its frequency and amplitude, placing it in a more 
manageable form, so that electrical patterns more easily are recognized. 
For example, ventricular fibrillation, a highly disorganized arrhythmia, 
may be coarsened into a ventricular tachycardia or a ventricular flutter, 
more organized arrhythmias exhibiting more recognizable repeatable 
characteristics. In like manner, a low amplitude ventricular tachycardia 
pattern may be changed to a high amplitude pattern, which typically is 
easier to break. 
The logic device issues a predetermined reference signal on lines 32 for 
input into one port of a comparator 34. The value of the reference signal 
represents a percentage of the peak slew rate sensed by a peak follower 35 
during a preceding timing cycle (which preferably is the slew rate of the 
ECG segment on which the first shock was issued). For start-up, a 
predetermined slew rate initially may be incorporated into the system 
through the programmable nature of the logic device, or an actual slew 
rate may be detected by the peak follower 35. The reference signal 
represents a slew rate, on or above which low-level shock therapy is 
indicated, and continually is updated by the actual slew rate detected by 
peak follower 35, adjusted by a fixed pre-programmed percent or by other 
parameters fed into the programmable logic device 22. As an example, the 
reference signal on line 32 could be 85% of the last peak slew rate 
detected by peak follower 35. 
The slew rate detector 20, under the activation of the logic device 22, now 
issues an output signal on lines 36 which is fed to the other port of the 
comparator 34. When the slew rate has a value equal to or greater than 
that of the reference signal, the comparator issues a signal on line 38 
which passes through a delay 40. The period of delay is determined by the 
logic device, and is set to zero if it is desired to shock on the peak of 
the ECG segment. After the predetermined delay, a control signal passes on 
line 42 to activate switch 50 and to pass the energy stored in the energy 
storage device 28 through the shock delivering electrode 30 to the heart. 
The deliverance of additional shocks is accomplished in a manner similar 
to that of the second shock and is carried out until the heart returns to 
normal sinus rhythm. 
The second and subsequent shocks are synchronized to an electrical activity 
of the heart associated with a repeatable characteristic of the cardiac 
arrhythmia. As an example, for ventricular tachycardia, the repeatable 
characteristic may be the leading edge of the R-wave occurring within the 
arrhythmia (or the zero-crossing of the differentiated ECG signal 
corresponding to the peak rise time of the R-wave). In this regard, and as 
described above, it is contemplated that the logic device 22 include 
timing circuitry to issue discharge commands to the energy storage device 
28 so that discharge into the heart through electrode 30 takes place at 
the desired time relative to the ECG data presented by detector 18 on line 
24. Synchronized low-level shocks permit the utilization of lower energy 
levels to correct certain cardiac arrhythmias. In this way, certain 
potential dangers associated with high-level shocks, such as tissue damage 
and the induction of ventricular fibrillation, are minimized. 
As noted previously, even ventricular fibrillation, regarded as an 
arrhythmia whose electrical activity is random, has been found to display 
certain electrical regularity. Therefore, it sometimes may be possible to 
issue even the first shock in synchronization with a repeatable 
characteristic of the ECG. Even when not possible to so synchronize the 
first shock in VF, second and subsequent shocks typically can be 
synchronized due to the ECG having been coarsened by the previous 
shock(s). 
Typically, the first and subsequent shocks (for the preferred implantable 
device) are within the energy range of from about 0.1 joule to about 15 
joules. Subsequent shocks typically will be of energies equal to or less 
than that of the first shock. This is because the more organized 
arrhythmia (seen after the first shock) should convert to normal sinus 
rhythm at lower energies than is necessary to convert the previously less 
organized arrhythmia into the more organized arrhythmia. 
Although it is possible that some patients will respond to the first 
unsynchronized (or synchronized) shock and return to normal sinus rhythm, 
it is expected that in most cases, multiple low-level shocks will be 
needed. The major purpose of the first shock is to set up a more 
organized, manageable arrhythmia exhibiting a repeatable characteristic so 
that the subsequent synchronized shocks can further organize or eliminate 
the arrhythmia entirely. In the preferred embodiment, the detectable event 
on which the synchronized shocks are issued is the peak of the wave most 
closely resembling an R-wave. 
The slew rate detector 20 is employed to synchronize the second and 
subsquent low-level shocks. If there is a detected slew rate in the timing 
period that is equal to or higher than the highest rate measured in the 
previous timing cycle, say, for example, one volt per second, then a shock 
can be delivered on that particular portion of the waveform, preferably at 
its peak. Through the programmable logic device 22, the time between 
pulses can be varied from about 10 milliseconds to about 5 seconds, with a 
time selection of from about 80 milliseconds to about 1.5 seconds being 
preferred. It is to be understood that the actual time between pulses is 
not necessarily continuous. The range is actually broken up into a series 
of discrete time intervals, typically less than a millisecond in duration. 
For a preferred embodiment, the discrete time intervals are those within 
which the peak slew rate is sought. This could occur a short time after 
the last shock, or could occur two or more timing cycles later if the 
circuit recognizes the need to update its stored data. In this regard, it 
is theorized that the effectiveness of any such subsequent shock depends 
on the energy level, the precision of the synchronization with the 
particular ECG segment, and the overall elapsed time between it and the 
first shock. 
As an example of the device operation, FIG. 2 illustrates an intra-cardiac 
tracing showing a normal ECG degenerating into ventricular fibrillation, 
coarsening after a first low-level shock, coarsening further after a 
second shock, and then returning to normal sinus rhythm shortly after the 
issuance of a third low-level synchronized shock. 
Turning to a related topic, the concept of synchronization of the waveform 
in ventricular tachycardia already is well accepted. However, 
synchronization with regard to ventricular fibrillation has never been 
considered a valid concept because of the rapidity of the waveform and its 
apparent chaotic nature. Nevertheless, it has been found that with a 
bipolar sensing electrode having two closely spaced electrodes located in 
the right ventricle, the ECG waveform, even in ventricular fibrillation, 
can be specified with sufficient clarity so that observation can be made 
as to when the wave front passes the bipole, is approaching the bipole, or 
is receding from the bipole. In addition, the relative distance of the 
wave front from the bipolar electrode also can be estimated. Thus, with 
appropriate circuitry as the type described herein, it is possible to 
synchronize pulses to the fibrillatory waveform and to deliver the pulses 
in a certain relationship to it. When the relationship of the wave front 
to the defibrillatory electrodes is favorable, a somewhat greater amount 
of myocardium can be depolarized, and the waveform coarsened, even if only 
slightly. Repeated similarly timed shocks would successively capture more 
and more myocardium until, finally, complete defibrillation is achieved by 
the succession of shocks, each one ineffective by itself for this purpose. 
It is likely that the shocks could be delivered either to a small area of 
the myocardium via a catheter, or globally via epicardial electrodes 
surrounding the heart. Since the critical timing depends ultimately on the 
particular geometry and time relationships of the wave front to the 
sensing electrodes 12, it is not surprising that any fixed interpulse 
timing, as for example the Kugelberg pulses or the protective zone 
phenomenon, do not consistently work. On the other hand, the timing 
relationships of the pulses with respect to the altering innate properties 
of the wavefront as it is actually depolarizing the heart enables the 
approach described herein to be effective. 
Although it is believed that the approach outlined above can be effective 
for most arrhythmias, it is desirable to provide a back-up approach should 
the low-level shocks prove ineffectual. Previous implantable 
defibrillators deliver larger amounts of energy (in the 35-50 joule range, 
for example), through the shock delivering electrodes, to eliminate 
arrhythmic conditions. See U.S. Pat. No. Re. 27,652 for an example of such 
an implantable automatic defibrillator. It is desirable to incorporate the 
provision for such higher energy shocks, and in this regard, it is 
contemplated that the charging circuit 21, under the control of the logic 
device 22 and solid state switch 50, have the capability of charging the 
energy storage device 28 with sufficient energy to deliver shocks of up to 
approximately 30-35 joules. 
Although the present invention has been shown and described in terms of a 
specific preferred embodiment, it will be appreciated by those skilled in 
the art that changes or modifications are possible which do not depart 
from the inventive concepts described and taught herein. Such changes and 
modifications are deemed to fall within the perview of these inventive 
concepts.