Method and apparatus for discriminating among normal and pathological tachyarrhythmias

A method and apparatus for discriminating among the various normal and pathologic tachycardias. In response to the detection of tachycardia, far-field ventricular electrograms sensed using atrial electrodes and far-field atrial electrograms sensed using ventricular electrodes are analyzed in order to categorize the source and type of tachyarrhythmia detected. The detection method and apparatus may be employed in conjunction with dual chamber anti-bradycardia pacemakers to avoid pacemaker mediated tachycardias or may be employed to mediate the delivery of pacing energy or cardioversion/defibrillation shock energy to a malfunctioning heart, in the context of an implantable antitachycardia device.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to implanted medical devices and, more 
particularly, relates to a physiological waveform morphology 
discrimination method and apparatus for use in characterizing the origin 
of cardiac depolarizations and adjusting the operation of the medical 
device accordingly. 
2. Description of Prior Art 
Early automatic tachycardia detection systems for automatic implantable 
cardioverter/defibrillators relied upon the presence or absence of 
electrical and mechanical heart activity (such as intramyocardial 
pressure, blood pressure, impedance, stroke volume or heart movement) 
and/or the rate of the electrocardiogram. For example, the 1961 pamphlet 
by Dr. Fred Zacouto, Paris, France, entitled. "Traitement D'Urgence des 
Differents Type de Syncopes Cardiaques du Syndrome de Morgangni-Adams 
Stokes" (National Library of Medicine) describes an automatic pacemaker 
and defibrillator responsive to the presence or absence of the patient's 
blood pressure in conjunction with the rate of the patient's 
electrocardiogram. Later detection algorithms proposed by Satinsky, "Heart 
Monitor Automatically Activates Defibrillator," Medical Tribune, 9, No. 
9I:3, Nov. 11, 1968, and Schuder et al "Experimental Ventricular 
Defibrillation with an Automatic and Completely Implanted System," 
Transactions American Society for Artificial Internal Organs, 16:207, 
1970, automatically detected and triggered defibrillation when the 
amplitude of the R-wave of the electrocardiogram fell below a 
predetermined threshold over a predetermined period of time. The initial 
system proposed by Mirowski et al in U.S. Pat. No. Re. 27,757, which 
similarly relied upon the decrease in the amplitude of a pulsatile right 
ventricular pressure signal below a threshold over a predetermined period 
of time, was abandoned by Mirowski et al in favor of the rate and/or 
probability density function morphology discrimination as described in 
Mower et al, "Automatic Implantable Cardioverter Defibrillator Structural 
Characteristics," E, Vol. 7, November December 1984, Part 11, pp. 
1331-1334. Others have suggested the use of high rate plus acceleration of 
rate or "onset" (U.S. Pat. No. 4,384,585) with sustained high rate and 
rate stability (U.S. Pat. No. 4,523,595). 
Very generally, the systems that depend upon the aforementioned criteria 
are capable of discriminating tachyarrhythmia in greater or lesser degree 
from normal heart rhythm but have difficulty discriminating sinus or other 
supraventricular tachycardias from malignant, pathologic ventricular 
tachycardias, resulting in the delivery of inappropriate cardiac 
electrical stimulation therapies. 
A stated in the article "Automatic Tachycardia Recognition" by R. 
Arzbaecher et al (E, May-June 1984, pp. 541-547), antitachycardia 
pacemakers that were undergoing clinical studies prior to the publication 
of that article detected tachycardia by sensing a high rate in the chamber 
to be paced. The specific criteria to be met before pace termination was 
to be attempted involved a comparison of the detected rate to a preset 
threshold, such as 150 beats per minute (400 millisecond cycle length) for 
a pre-selected number of beats. As stated above, other researchers had 
suggested the rate of change of rate or suddenness of onset, rate 
stability and sustained high rate a additional criteria to distinguish 
sinus tachycardias from malignant tachycardias. Arzbaecher et al proposed 
in their article an algorithm implemented in a microprocessor based 
implantable device employing both atrial and ventricular rate detection 
via separate bipolar leads in order to detect the AA and VA, or VV and AV 
intervals (or "cycle lengths") against threshold intervals in order to 
distinguish pace-terminable and nonpace-terminable tachycardias. 
Arzbaecher et al introduced the concept of employing a single atrial extra 
stimulus to distinguish sinus tachycardia from 1:1 paroxysmal tachycardia 
in order to determine whether a ventricular response would be elicited. An 
atrial extra stimulus was delivered in late diastole (80 milliseconds 
premature), and the ventricular response, if appearing early as well, 
indicated that the patient was in sinus rhythm. However, in 
pace-terminable tachycardias, such as AV reentrant and ventricular with VA 
conduction tachycardia, the ventricular response would not occur early 
(indicating that the atrial and ventricular rhythms were disassociated) 
and the ventricular rhythm would be unperturbed. 
Other proposals for employing atrial and ventricular detection and interval 
comparison are set forth in The Third Decade of Cardiac Pacing: Advances 
in Technology in Clinical Applications, Part III, Chapter 1, "Necessity of 
Signal Processing in Tachycardia Detection" by Furman et al (edited by S. 
Barold and J. Mugica, Future Publications, 1982, pages 265-274) and in the 
Lehmann U.S. Pat. No. 4,860,749. In these cases also, atrial and 
ventricular rates or intervals are compared to one another in order to 
distinguish sinus and pathological tachycardias. 
Another approach to the detection of and discrimination between pathologic 
and sinus or normal tachycardias involves the comparison of current 
electrogram morphologies to a stored library of morphologies in the manner 
shown for example in the U.S Pat. No. 4,523,595. In such systems, the 
suspect electrograms are continuously digitized and compared against the 
reference digitized electrograms to find the closest fit and diagnose the 
suspect rhythm. 
The aforementioned discussion reflects the development in the art of the 
detection and discrimination of spontaneously occurring atrial and 
ventricular tachycardias. In the field of dual chamber atrial synchronous 
heart pacemakers, such as multiprogrammable DDD pacemakers, the generation 
of ventricular stimulation pulses ensues after an AV delay time following 
the detection of an atrial or P-wave signal. The ventricular stimulation 
rate varies within a relatively wide range from a programmable lower rate, 
such as 50-75 beats per minute, to a programmable upper rate, such as 
100-140 beats per minute. The ventricular stimulation rate may track the 
sensed atrial P-wave rate up to the upper rate limit whereupon 
synchronization may be lost periodically, the pacemaker exhibiting a 
pseudo-Wenckebach behavior, as described in Adams U.S. Pat. No. 4,059,116, 
for example. 
If retrograde conduction exists in the patient's heart, each ventricular 
stimulus may evoke a depolarization that is conducted back to the atrium, 
causing it to contract. The atrial sense amplifier may respond to the 
corresponding P-wave, and in turn, trigger the generation of a ventricular 
stimulus at or near the upper rate limit of the pacemaker. This behavior 
of the pacemaker is referred to as "pacemaker-mediated tachycardia" or 
PMT. In order to prevent PMT, most DDD pacemakers include a programmable 
post ventricular atrial refractory period that the physician may extend to 
cause the atrial sense amplifier to ignore the retrograde induced P-wave. 
However, lengthening the refractory time, in effect, reduces the upper 
rate limit and is, therefore, disadvantageous to the patient. Another 
proposal has been to provide timing windows for detecting the 
closely-coupled retrograde P-wave and to switch the paging mode of 
operation to a single chamber mode, such as VVI pacing. In U.S. Pat. No. 
4,802,483, circuitry is provided to improve the transition between VVI 
pacing and DDD pacing to avoid synchronization to atrial activity 
triggered by retrograde transition. In DDD pacemakers, it remains 
desirable to provide a reliable method and apparatus for distinguishing 
atrial activity triggered by retrograde conduction from normal physiologic 
atrial activity and to provide an appropriate response mode. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to distinguish among normal sinus 
tachycardia, pathologic supraventricular tachycardias, and ventricular 
tachycardias. 
It is a further object of the present invention to provide a method and 
apparatus for discriminating among the various normal and pathologic 
tachycardias and for providing appropriate therapies for the treatment 
thereof. 
It is the further object of the present invention to discriminate 
anterograde from retrograde atrial activation to avoid the induction of 
pacemaker mediated tachycardias by dual chamber pacemakers. 
The above objects of the present invention are achieved by a method and 
apparatus employing the near-field (i.e., closely-spaced) bipolar 
electrode sensed, atrial P-wave and ventricular R-wave electrograms to 
trigger storage in memory of concurrently occurring far-field, unipolarly 
sensed, atrial and ventricular electrograms, processing a certain number 
of the far-field electrograms, and comparing the electrograms against 
previously stored reference or control electrograms or other criteria to 
discriminate among various cardiac arrhythmias on the basis of their 
atrial and/or ventricular morphologies. 
These objects of the present invention are realized in a method and 
apparatus which provides for: continuous measurement of the atrial and 
ventricular cycle lengths of the patient's cardiac rhythm via bipolar 
detection electrodes situated in or on the atrium and ventricle of the 
patient's heart, respectively; comparison of the atrial and/or ventricular 
cycle lengths to respective atrial and ventricular tachycardia detection 
intervals; comparison of the atrial and ventricular cycle lengths to one 
another and, if they are equal, enabling of atrial and ventricular 
far-field sense amplifiers coupled to at least one of the ventricular or 
atrial electrodes and a common remote electrode to simultaneously detect 
the far-field atrial and ventricular electrograms; and examination of the 
characteristics of the far-field atrial and ventricular electrograms of a 
series of one or more such electrograms to determine if they reflect by 
their morphological characteristics a specific normal or pathologic 
rhythm. 
More particularly, the method and apparatus of the present invention 
provides for: enabling the atrial and ventricular far-field sense 
amplifiers continuously upon detection of tachycardia by the measurement 
of the cycle lengths; digitizing the far-field electrograms and applying 
the digitized signals to respective atrial and ventricular circulating 
buffers; detecting the corresponding near-field atrial and ventricular 
depolarizations and, after a pre-selected time delay, transferring the 
contents of the atrial and ventricular buffers into memory; continuing to 
collect and transfer a predetermined number of atrial and ventricular 
far-field electrograms in memory; and comparing certain characteristics of 
the stored far-field electrograms with characteristics of reference 
rhythms and classifying the detected tachyarrhythmia accordingly. 
Additionally the proposed invention provides for placing the system in a 
"learn" mode while the patient is in sinus tachycardia and/or an induced 
or spontaneously occurring pathological tachyarrhythmia, causing it to 
repeat the data collection operation described above in order to store in 
memory control or reference electrograms. In this fashion, digital 
representations of baseline normal and pathologic arrhythmia electrograms 
can be created. Suspect tachyarrhythmia electrograms are tested against 
the reference electrogram set by pairing the suspect electrogram with each 
of the reference electrograms and performing a least squares linear fit of 
the paired electrogram sample sets and evaluating the correlation 
coefficient of the computed linear regression. (Suspect 
Electrogram=A+B.Reference Electrogram.) The correlation coefficients for 
the curve fits are used as a programmable measure of morphology match. 
As an example, the physician may choose to have the device learn sinus 
tachycardia. Then, if a suspect rhythm correlates highly it is declared 
sinus and ignored. If it correlates poorly, it is declared pathologic and 
treated. Thus all non sinus tachycardias are likely to be detected and 
treated. Alternatively, the physician may have the device learn the 
specific waveform of the patient's primary clinical tachyarrhythmia. Then 
if the waveform of the suspect tachycardia correlates highly, it is 
declared to be the target rhythm and treatment is instituted. If the 
waveform correlates poorly, it is declared to be other than the target 
rhythm and is left untreated. If multiple arrhythmia reference 
electrograms have been established, the correlation coefficients can be 
used to institute different therapies for the suspect rhythm depending 
upon which reference electrogram provides the best correlation value. 
In the context of pacemaker mediated tachycardias, whenever the pacemaker 
attempts to pace the ventricles at elevated rates the far-field atrial 
electrogram can be evaluated against reference sinus rhythm far-field 
atrial electrograms in order to prevent pacing the ventricles at rapid 
rates in response to non sinus atrial tachyarrhythmias or retrograde 
atrial activation. This would then allow the physician to safely program 
short post-ventricular atrial refractory periods to permit better upper 
rate limit performance and to enhance arrhythmia detection processes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As stated hereinbefore, the present invention contemplates at least two 
embodiments which may be practiced in single or separate devices. Both 
embodiments encompass the concepts illustrated in FIGS. 1-7 and 9 in whole 
or in part. The first embodiment illustrated in particular with respect to 
FIGS. 4-7 involves the detection and discrimination of pathologic 
tachyarrhythmia and the treatment of same by a system of the type depicted 
and described in reference to FIG. 9. The second embodiment illustrated in 
particular with respect to FIG. 8 and employed in a portion of the system 
depicted in FIG. 9 is directed to the prevention and control of pacemaker 
mediated tachycardias due either to retrograde conduction or tracking of 
non-sinus atrial tachycardias in dual chamber, antibradycardia pacemakers. 
An understanding of the operational modes of the first embodiment of the 
present invention is facilitated by a brief discussion of the physiology 
of the heart and the theoretical mechanisms of cardiac tachyarrhythmias. 
The normal pumping action of the heart results from highly organized 
electrical activity in the cardiac tissue. Each natural spontaneous heart 
beat begins with an electrical discharge from the sino atrial node (S-A) 
located in the right atrium of the heart. This electrical impulse is 
conducted through tissues which result in the progressive depolarization 
of the atrial tissue causing it to contract. The contraction forces blood 
from the atrium through the heart valves into the ventricles. The 
electrical impulse from the atrium is communicated to the ventricles 
through the atrio-ventricular node (A-V), which is located on the septal 
wall dividing the right and left heart. The electrical signal is delayed 
in this conductive mode for approximately 0.15 seconds and is then 
transmitted through the His bundle and its branches to the Purkinje fibers 
which discharge ventricular muscle, causing the ventricles to contract in 
an organized fashion and pump blood throughout the body. In the healthy 
heart, this normal sinus rhythm may be repeated between 60 and 120 times 
per minute. In the diseased heart, however, a number of arrhythmias may 
occur which disrupt this normal activity. The type of arrhythmias are 
divided into two groups: tachyarrhythmias, which are generally 
characterized by heart rates faster than normal, and bradyarrhythmias, 
which are characterized by heart rates lower than normal. 
The characterization and origin of a tachyarrhythmia is of practical 
significance since the success of drug treatment of such disorders depends 
to a great degree on the accurate determination goof their origin and 
cause. In contrast, when cardioversion is selected to treat these 
disorders, the characterization and origin of the arrhythmia is of less 
significance. For example, it has been shown that transthoracic DC 
electrical shock can successfully terminate many different types of 
tachyarrhythmias. See, for example, Cardioversion, B. Lown, Med. Ann. 
D.C., 38:543, 1969. However, in an implantable device where power source 
energy and patient tolerance to repeated cardioversion/defibrillation 
shocks are both limited, it is necessary to draw fine distinctions between 
types of tachyarrhythmias and to treat the detected tachyarrhythmias with 
the lowest energy, least painful electrical stimulation therapies. Thus, 
it is desirable to terminate tachyarrhythmias wherever possible by low 
energy painless pacing stimuli and, if necessary, increase the 
aggressiveness of the therapy if the arrhythmia is not pace terminable or 
accelerates to a nonpace terminable arrhythmia. 
Conversely, it is desirable to immediately discriminate the 
nonpace-terminable and life threatening ventricular fibrillation and 
unstable ventricular tachycardia, and immediately treat those arrhythmias 
with cardioversion/defibrillation shock therapies. 
Tachyarrhythmias may be characterized further by their location of origin. 
For example, the origin of supraventricular tachyarrhythmias is in the 
atria; and its maintenance involves the atria and sometimes ventricles. 
Ventricular tachyarrhythmias originate and are maintained within the 
ventricles and sometimes conduct to the atria by a retrograde conduction 
pathway. A separate group of tachyarrhythmias are called flutter or 
fibrillation. Flutter is generally characterized by rapid, organized heart 
activity and, when involving the ventricles, low cardiac output. 
Fibrillation is characterized by highly disorganized electrical activity 
that results in virtually no cardiac output when it involves the 
ventricles. In some patients there may be a progression from an organized 
tachycardia to fibrillation which will lead to death if the site of the 
fibrillation is the ventricles. In many patients, ventricular tachycardia 
precedes the onset of ventricular fibrillation; and if the former can be 
terminated, generally with small amounts of energies, the latter can be 
prevented. Some patients exhibit chronic atrial flutter or fibrillation 
which may be debilitating but does not cause death, and other patients 
exhibit occasional or paroxysmal attacks of ventricular tachycardias which 
require cardioversion. See, for example, "Cardiac Arrhythmias," in Current 
Diagnosis, W. B. Saunders Co., 1977, pp. 377-396, by Douglas P. Zipes, 
M.D. 
Ventricular tachycardias can be converted to sinus rhythm by the 
application of cardioversion shock or by the application of pacing energy 
electrical stimulation including rate adaptive or orthorhythmic 
stimulation as described first in Zacouto U.S. Pat. No. 3,857,399, 
overdrive stimulation, burst overdrive stimulation rate scanning or any of 
the other known pacing therapies as described, for example, in Fisher et 
al, "Implantable Pacers for Tachycardia Termination: Stimulation 
Techniques and Long-Term Efficacy", E, Vol. 9, November December 1986, 
Part II, pp. 1325-1333. As a general proposition, it is preferable to 
convert ventricular tachycardias, if possible, to sinus rhythm by 
application of lower energy stimulation in order to conserve energy of the 
power sources of the implantable device as well as to maintain patient 
comfort. Many patients cannot tolerate the pain associated with 
cardioversion or defibrillation shock therapies loading to dread of the 
implanted cardioverter/defibrillator. Thus, it is desirable to further 
distinguish pace terminable from nonpace terminable ventricular 
tachycardias and program the implanted device to first attempt to restore 
sinus rhythm through the application of programmed pacing energy therapies 
of one or more of the types described above. 
In this regard, certain ventricular tachycardias, referred to as stable 
ventricular tachycardias, are more likely to be terminable by pacing 
therapies than other ventricular tachycardias, referred to as unstable 
ventricular tachycardias. 
Turning now to FIG. 1, the relationship of the electrodes disposed in and 
around a patient's heart 10 for picking up the atrial and ventricular, 
unipolar (far-field) and bipolar (near field) electrograms (EGMs) is 
shown. The heart 10 includes an atrial chamber 12 and ventricular chamber 
14 within which or onto which atrial and ventricular leads 16 and 18 are 
implanted so as to dispose relatively closely-spaced bipolar electrode 
pairs 20, 22 and 24, 26 in order to pick up the heart's atrial and 
ventricular EGMs. The leads 16 and 18 are coupled to a pulse generator 
system 28 which is disposed outside the heart under the patient's skin in 
the normal fashion and which carries a remote indifferent electrode 30 on 
the pulse generator case. The implanted pulse generator 28 may incorporate 
the system elements depicted in the block diagram of FIG. 9 to be 
described in detail later. 
The bipolar electrode pairs 20, 22 and 24, 26 and their associated leads 16 
and 18 may take the form of conventional bipolar pacing leads wherein the 
inter-electrode spacing between each electrode of each pair is preferably 
less than 3.0 centimeters, optimally in the range of between 0.5 
centimeters and 1.0 centimeters. Such bipolar electrode spacings have been 
known in the art since bipolar pacing and electrogram sensing leads first 
became commonly used in the 1960's. 
When electrical signals are sensed across the electrodes 20, 22 and 24, 26 
constituting the bipolar electrode pairs, they are respectively referred 
to as the bipolar atrial and ventricular EGMs by virtue of the location of 
the electrodes in conjunction with the atrium and ventricle of the 
patient's heart. Normally, the bipolar electrode pair 20, 22 is employed 
to sense the atrial P-wave whereas the ventricular electrode pair 24, 26 
is employed to detect the ventricular depolarization waves (or QRS 
complex), commonly referred to as the R-wave. However, inasmuch as the 
P-waves and R-waves are conducted throughout the heart, and thereby pass 
by each electrode of each electrode pair, it is possible to detect the 
attenuated P-wave and R-wave signals by coupling suitable sense amplifiers 
across each electrode pair. However, normally, pacing systems ar designed 
to employ the atrial leads to detect the atrial electrogram or P-wave and 
ventricular leads to detect the ventricular electrogram or R-wave and to 
avoid detecting any other component of the EGM. In the context of the 
present invention, when reference is made to the bipolar atrial and 
ventricular EGMs it will be understood that the signals to be detected and 
emphasized are the P-wave and R-wave, respectively. The close spacing of 
the bipolar electrode pairs 20, 22 and 24, 26 is designed to optimize the 
detection of the P-wave and the R-wave, respectively, and to attenuate the 
detection of the R-wave and P-wave, respectively. 
FIG. 1 also illustrates the use in accordance with the present invention of 
a remote indifferent electrode 30 on the pulse generator 28 in conjunction 
with the proximal electrodes 20 and 24 (or the distal electrodes 22 and 
26) across which unipolar atrial and ventricular EGMs and the far-field 
ventricular and atrial EGMS, respectively, can be detected. In accordance 
with the present invention, it is desirable to detect in on chamber (such 
as the atrium) the far-field EGM representing the depolarization wave form 
of the other chamber of the heart (such as the ventricle). Thus, by 
connection to suitable sense amplifiers, the depolarization wave form 
signal appearing across the electrode pair comprising electrodes 20 and 
30, for example, can detect the far-field R-wave. Conversely, a suitable 
sense amplifier coupled across the electrodes 24 and 30 ma be employed to 
detect the far-field P-wave. These far-field EGMS derived in a fashion 
illustrated in FIG. 1 are preferably used in the present invention to 
discriminate pathologic tachyarrhythmia from normal sinus tachycardia 
and/or to distinguish a natural atrial tachycardia from a PMT (caused by 
the retrograde conduction of the pacemaker-triggered ventricular 
depolarization) and to provide appropriate therapies to the patient s 
heart to treat the detected tachyarrhythmia or to modify the operation of 
the dual chamber pacemaker to avoid PMT. 
The block diagram of FIG. 2 depicts how the atrial and ventricular 
near-field and far-field EGMs picked up across the electrodes illustrated 
in FIG. 1 are processed in accordance with the routines illustrated in 
FIGS. 4-8. FIG. 3 depicts the EGMs as picked up from the electrodes 
illustrated in FIG. 1 and the storage of the far-field EGMs for morphology 
analysis. 
In FIG. 2, the bipolar atrial EGM detected across the bipolar electrode 
pair 20, 22 of FIG. 1 is amplified in sense amplifier 32 and applied to a 
multiplexer and A-D converter 34. Similarly, the unipolar ventricular 
electrogram detected across electrodes 24 and 30 of FIG. 1 is amplified by 
a sense amplifier 36 and applied to in further input of the multiplexer 
and A-D converter 34. In like fashion, the unipolar atrial EGM and bipolar 
ventricular EGMs are amplified in sense amplifiers 38 and 40, 
respectively, and applied to further inputs of the multiplexer and A D 
converter 34. 
The amplified and digitized bipolar atrial EGMs are applied by multiplexer 
and A-D converter 34 to the logic and memory block 36 which contains 
within it a detection logic block 38 for detecting atrial tachycardias in 
accordance with the algorithm illustrated in FIG. 4. In a similar fashion, 
the amplified and digitized bipolar ventricular EGMs are directed to the 
logic and memory block 36 by the multiplexer and A-D converter 34, and 
detection logic 40 detects the existence of a ventricular tachycardia. 
Atrial tachycardias are those tachycardias that originate in the atrium 
and are characterized by too fast an atrial rate of recurrence of P-waves, 
and ventricular tachycardias similarly are those which originate in the 
ventricle and are characterized by too high a rate of recurrence of 
R-waves. 
The amplified bipolar electrograms described above may alternatively not be 
digitized but instead applied to conventional analog pacemaker sensing 
circuits which detect repeatable characteristics of the P- and R-waves as 
is well known in the art. 
The amplified and digitized, unipolar ventricular and atrial EGMs are 
passed through the multiplexer and A-D converter 34 and applied to 
circular buffers 42 and 44, respectively. The multiplexer and A-D 
converter 34 is conventional in the art and accomplishes the sampling (at 
64-256 samples/second, for example), conversion of the sampled analog 
amplitude of the EGM wave form and the transfer of the sampled digitized 
data to buffers 42 and 44. The unipolar, or far-field, EGM data stored in 
buffers 42 and 44 is continuously updated as each successive stream of 
data entering the buffer replaces the data previously stored therein in a 
manner which is also conventional in the art. However, in the event that a 
tachycardia is detected by detection logic 38 and/or 40, the contents of 
the circular buffers 42 and/or 44 are frozen and transferred into memory 
after selectable delays 46 and 48, respectively. 
Turning now to FIG. 3, the wave forms depicted therein illustrate tracings 
of the atrial and ventricular EGMs or P and R-waves as they appear across 
the four electrode pairs employed to develop the four EGM signals applied 
to the sense amplifiers 32-40 of FIG. 2. At time t.sub.1, an atrial 
depolarization or P-wave originates in the atrium, and the resulting 
bipolar atrial EGM depicted in tracing A has the appearance normally 
associated with a bipolar P-wave. The signals illustrated in tracings B, C 
and D differ in each instance from the tracing of the P-wave illustrated 
in tracing A and from each other. Note that the P-wave is so attenuated by 
the closely spaced ventricular electrodes 24, 26 and sense amplifier 40 
that it is hardly apparent in tracing C. 
Similarly, at time t.sub.2, a ventricular depolarization or R-wave occurs, 
and the signal depicted in tracing C is reminiscent of the classic bipolar 
QRST complex, or R-wave. The signals as depicted in the tracings A, B and 
D differ substantially from the signal depicted in tracing C. 
Tracings A and B of the atrial EGM and C and D of the ventricular EGM are 
of particular interest to the present invention. The bipolar atrial and 
ventricular EGMs of tracings A and C may be easily processed to detect the 
rate of recurrence by measurement of the intervals between peak amplitudes 
or slew rates of successive P-waves and R-waves. By tracking the AA and VV 
intervals or cycle lengths (and optionally the suddenness of onset, rate 
stability and/or sustained high rate), a tachycardia can be detected. 
However, it becomes difficult to determine whether or not the tachycardia 
reflects a more or less normal response to the patient's emotional state 
or increased level of exercise or is pathologic in origin. Discrimination 
between stable and unstable ventricular tachycardia may be critical to the 
prescription of the appropriate therapy. In the context of a staged 
therapy device, where the appropriate therapies may range from less 
aggressive pacing stimuli to highly aggressive electroshock therapy, it is 
desirable to prevent the application of a more aggressive therapy to a 
tachyarrhythmia condition than is warranted in order to lessen the chance 
of accelerating the tachyarrhythmia from a benign to a dangerous 
condition, to avoid applying uncomfortable shocks to the patient and to 
preserve electrical energy in order to prolong the useful life of the 
device. In the context of the dual chamber pacemaker in a patient whose 
heart condition occasionally allows retrograde conduction, it is desirable 
to distinguish retrograde conducted atrial depolarizations from natural 
high rate atrial depolarizations again in order to avoid inducing an 
arrhythmia and making the patient uncomfortable by sustained pacing at the 
pacemaker's upper rate limit. 
In accordance with the present invention, once the rate discrimination 
criteria are satisfied, the far-field atrial and ventricular EGMs are 
focused on to provide the morphology discrimination In reference to FIGS. 
2 and 3, this is accomplished by thereafter transferring the contents of 
the circular buffers 42 and 44 into memory within block 36 each time a 
bipolar P-wave and R-wave is detected by detection logic blocks 38 and 40 
such that on each atrial and ventricular event, the far-field atrial and 
ventricular EGMs are stored for a predetermined number of events. Thus, in 
reference to FIG. 3, at time t.sub.1 the detection of the bipolar atrial 
EGM in tracing A causes the transfer of the contents of circular buffer 42 
into memory within block 36 for a predetermined time window established by 
delay 46. The time window is illustrated in tracing B as extending forward 
and backward in time from the peak of the P-wave illustrated in tracing A 
at time t.sub.1. 
Similarly, the R-wave depicted in tracing C at time t.sub.2 is detected by 
detection logic 40 which triggers the storage of the digitized sampled 
far-field ventricular EGM in circular buffer 44. In like fashion, the 
delay 48 effects the storage of the sampled and digitized data of the 
far-field ventricular EGM for a time windrow preceding and following the 
peak of the bipolar R-wave. It will be understood that in reference to 
FIG. 3 that although the analog signals are depicted for ease of 
illustration, the signals actually within the circular buffers 42 and 44 
are digitized data bits representing the instantaneous amplitudes of the 
EGMs at each sampling point. Similarly, the digitized values of the 
bipolar atrial and ventricular EGMs are actually transferred by 
multiplexer and A-D converter 34 to detection logic blocks 38 and 40, 
respectively. Before leaving FIG. 3, it should be understood then that the 
large amplitude excursions of the signals detected in tracings B and D at 
times t.sub.2 and t.sub.1, respectively, are digitized and passed through 
circular buffers 42 and 44, respectively, but are not stored. The shapes 
of the far-field atrial and ventricular EGMs which are stored in memory 
are easier to employ in morphology analysis than any of the other signals 
depicted in the tracings of FIG. 3. The delay effected by the delay 
circuits 46 and 48, the number of stages in the buffers 42 and 44 and the 
sampling rate define the length of the windows depicted in tracings B and 
D of FIG. 3. 
Turning now to FIG. 4, the overall tachycardia detection algorithm for 
performing the analysis of tachyarrhythmias of the first embodiment of the 
present invention is depicted. The flow chart is similar to the flow chart 
depicted in the aforementioned Arzbaecher, et al article except that it 
goes to the morphology discrimination algorithms of FIGS. 5 and 6 in the 
event that the atrial and ventricular rates are both high. In connection 
with FIG. 4, the expression "rate," as opposed to interval or cycle 
length, is employed for convenience of description. 
In FIG. 4, at block 100, the algorithm continually measure the atrial and 
ventricular rates by actual measurement of the intervals between the 
bipolar atrial and ventricular EGMs or P-waves and R-waves. At each 
detection of a P-wave and R-wave, the preceding elapsed time or interval 
is measured and transformed into a rate of recurrence in beats/minute and 
the algorithm moves to decision block 102 to determine whether or not the 
onset algorithm is on or off. If the onset algorithm is on, the algorithm 
moves to decision block 104 where it is determined whether or not the rate 
reflects a sudden rate change in excess of either a fixed or percentage 
value of the average rate derived from a certain number of preceding 
intervals. If no sudden rate change is detected, the measurement of atrial 
or ventricular rates at block 100 is returned to. If the sudden rate 
change criteria is satisfied, or if the onset algorithm is "off", then the 
algorithm moves to the decision block 106 where a determination of whether 
or not the rate stability algorithm is on or off. If the rate stability 
algorithm is on, and if the rate is stable, as determined by decision 
block 108, the algorithm again doubles back to the start to measure 
successive atrial and ventricular rates at block 100. If the rate 
stability algorithm is not on or if the rate is stable, then the routine 
moves to decision block 110 to compare the atrial and ventricular rates 
against the rate threshold criteria that may be programmed into the 
device. It will be understood that in actual practice, the specific 
sequence of steps illustrated in FIG. 4 may be altered in that FIG. 4 as 
explained so far constitutes merely one illustration of the conventional 
criteria and analysis of atrial and ventricular rates to obtain some 
information as to whether or not the rates reflect normal sinus behavior 
or an arrhythmia. 
At decision block 110, the atrial and ventricular rates are checked against 
programmed rate criteria, in this case, 100 beats/minute. Thus, if the 
atrial and ventricular rates are below 100 beats/minute, the algorithm 
concludes that there is neither an atrial nor a ventricular tachycardia 
(whether or not the onset and stability criteria are satisfied or not) at 
block 112. No treatment is prescribed and the algorithm doubles back to 
start at block 100. 
However, if either the atrial or the ventricular or both rates are greater 
than 100 beats/minute, then the algorithm moves to decision block 114 to 
compare the atrial and ventricular rates to one another. If the atrial 
rate is greater than the ventricular rate, then it is concluded that an 
atrial tachycardia exists and that supraventricular tachycardia 
prescriptive therapies are to be applied at block 116. Similarly, if the 
ventricular rate exceeds the atrial rate, then the algorithm concludes 
that it is more likely than not that ventricular tachycardia exists and 
applies appropriate, programmed ventricular tachycardia prescriptive 
therapies at block 118. 
If, however, the atrial and ventricular rates are equal within a 
programmable range of deviation, the algorithm moves to block 120 to apply 
the morphology algorithm which encompasses the subroutines depicted in 
FIGS. 5 and 6. Depending upon the results of the morphology algorithm, the 
system of the present invention contemplates providing in block 122 
prescriptive therapies based on the morphology classification. 
Although not specifically illustrated in FIG. 4, it will be understood that 
the rate analysis and the morphology analysis contemplate the analysis of 
a programmed number of AA and VV intervals in order to satisfy the onset, 
stability and rate-related criteria and a further programmed number of 
digitized and stored far-field EGMs for morphology analysis to be 
described hereafter. 
Turning now to FIG. 5. the subroutine of the morphology algorithm far-field 
EGM data collection algorithm is depicted. This subroutine falls within 
block 120 of FIG. 4 and relates to the operation of this system depicted 
in FIG. 2 and the wave form tracings of FIG. 3. 
At decision block 150, the question is asked as to whether the morphology 
algorithm is programmed on or off. If off, the subroutine moves to block 
152 to by-pass the morphology algorithm analysis and return to block 122 
of FIG. 4 to apply those prescriptive therapies which may be programmed by 
the physician to meet the requirements of the specific patient. For 
example, in such a system, the physician may have the flexibility of 
prescribing an aggressive therapy when both the atrial and ventricular 
rates exceed a certain threshold rate and otherwise satisfy the onset and 
stability criteria or the physician may prescribe no therapy in the 
specific instance. In the context of the present invention, it will be 
presumed that the morphology algorithm is programmed on and the algorithm 
of the subroutine of FIG. 5 moves to block 154 where the sample far-field 
atrial and ventricular EGMs (of tracings B and D of FIG. 3) are being 
stored in the circular buffers 142 and 44 of FIG. 2. 
As explained in conjunction with FIGS. 2 and 3, when the morphology 
algorithm is programmed on and the rate and other criteria are met, the 
detection of the bipolar atrial EGM at time t.sub.1 is sensed and start 
the atrial delay timer 46. In FIG. 5 at decision block 156, the sensing of 
the bipolar atrial EGM starts the atrial delay timer in block 158. The 
atrial delay timer possesses a certain delay time of X milliseconds which 
in part defines the width of the window depicted in tracing B of FIG. 3. 
Similarly, in blocks 160 and 162, the bipolar or near-field ventricular EGM 
is sensed and starts the ventricular delay timer 48 which after Y 
milliseconds freezes the buffer and transfers its contents to memory 
within block 36 of FIG. 2. Both the atrial and ventricular delay timers 
signal the end of the delay periods at decision blocks 164 and 166, and 
stop storing the far-field atrial and ventricular data in buffers 46 and 
48 and transfer that data to memory within block 36 in blocks 168 and 170, 
respectively. After N atrial and ventricular far-field EGMs have been 
stored in memory within block 36, the decision block 172 moves to start 
the morphology analysis in block 174 in FIG. 7. The flow chart of the 
subroutine depicted in FIG. 5 may be rearranged to accomplish the 
described functions, and it will be understood that once the tachycardia 
detection algorithm of FIG. 4 is satisfied, a morphology algorithm 
far-field EGM data collection subroutine of FIG. 5 may be commenced but 
halted before completion if the patient's arrhythmia spontaneously 
terminates. 
Once the requisite number of atrial and/or ventricular far-field EGMs are 
stored in memory in block 36, the morphology analysis subroutine of FIG. 7 
is commenced. There are many known approaches to pattern recognition and 
morphology analysis to classify or categorize unknown or suspect wave 
forms. Physicians are trained in the recognition of arrhythmias and the 
classifications of specific arrhythmias and discrimination of those 
arrhythmias from normal or sinus EGMs by study and training and through 
experience learn to be able to classify EGMs by visualizing a suspect EGM 
and mentally comparing it against memorized patterns. In working up 
individual patients, physicians conduct electrophysiologic studies to 
obtain patient EGMs under a variety of conditions, both natural and 
induced, in order to diagnose a patient's specific arrhythmia, the 
causation of that arrhythmia, and the response to applied therapies. In 
medical device technology, the attempt is made to perform these same 
functions on a machine basis. Thus, it is known, for example, to work up a 
patient to store a number of EGMs during normal sinus rhythm at rest and 
under exercise, as well as induced arrhythmias, to store those reference 
EGMs in memory and to conduct a comparison between the reference library 
and suspect EGM samples as taught, for example, in the aforementioned 
Zibell U.S. Pat. No. 4,523,595. Many different techniques may be employed 
to conduct that analysis but in the present invention, it is preferred to 
employ linear regression techniques of the type described, for example, in 
the book by Sanford Weisberg, Applied Linear Regression, Wiley Series in 
Probability and Mathematical Statistics, John Wiley & Sons, 1985. 
In the context of the present invention, it is contemplated that the 
library of stored morphologies against which the suspect EGM samples are 
to be compared is collected in the same fashion as described in reference 
to FIGS. 1, 2, 3, and 5, and stored in referenced RAM memory registers 
within logic and memory block 36. Thus, the device itself is contemplated 
to be employed to create the library of reference EGM against which those 
suspect EGMs which satisfy the detection criteria of FIG. 4 (or FIG. 8 to 
be described) are compared. 
If the suspect EGM were to exactly match one of the reference EGMs in the 
library of stored reference EGMs, then one would expect that at each 
sampled point of the two EGMs, the digitized amplitude values would be 
identical and the difference between the two values would be zero 
(assuming the polarities are also identical and no baseline shift). Such a 
situation is seldom if ever realized. Linear regression techniques, and in 
particular, least squares estimation, facilitate the sampled data 
comparison between the suspect EGM and the library of reference EGMs to 
realize an aggregate number ranging from zero (absolutely no matching 
sample point values) and 1.0 (full matching sample point values), referred 
to as the correlation coefficient. In practice an intermediate number 
(such as 0.9) may be employed as a sufficient reference regression 
correlation coefficient value to declare the suspect EGM as matched to a 
reference EGM for classification and therapy purposes. Linear regression 
analysis consists of a collection of techniques used to explore 
relationships between variables and is especially useful for assessing 
fits between sample and reference data of the type involved in the present 
invention. High quality software for regression calculations is available 
to conduct linear regression fitting of suspect and reference data. 
The linear regression technique employing reference and suspect EGM samples 
may be illustrated by a scatter plot such as that depicted in FIG. 6. In 
FIG. 6, the X axis is labeled the reference EGM and the Y axis is labeled 
the suspect EGM. Both axes are marked off in millivolts ranging from -3 mV 
to 4 mV. The millivolt scales are arbitrarily not identical in order to 
fit the drawing into an A-4 sized drawing sheet. Normally, the X axis and 
Y axis millivolt scales would be identical but no harm is caused by making 
one different from the other. The scatter plot illustration of FIG. 6 is 
not in fact constructed by the system depicted in FIG. 2 nor is it 
realized in the morphology algorithm arrhythmia classification subroutine 
of FIG. 7. It is presented merely to illustrate the concept of employing 
least squares to arrive at a correlation coefficient for the suspect EGM 
compared against the reference EGM as described in the aforementioned 
Weisberg text. 
In reference to FIG. 6, the suspect EGM sample point values are plotted as 
the small squares against the vertical axis. If the suspect EGM were 
identical to the reference EGM, the suspect EGM sample point values would 
all fall on the straight line labeled Z. For purposes of illustration, a 
straight line has been drawn as one would to approximate the distribution 
of sample point values. Real data will almost never fall exactly on a 
straight line. The differences between the values of the real data and the 
straight line values in the aggregate reflect a degree of correlation or 
lack or correlation between the suspect EGM and the reference EGM sample 
point values. 
Employing the X values as the suspect EGM values, and the Y values as the 
reference EGM data sampling point values, the equations for arriving at 
the correlation coefficient value, r, may be expressed as: 
##EQU1## 
As stated hereinbefore, the system and algorithms of the present invention 
do not actually construct the scatter plot illustrated in FIG. 6. Instead, 
the calculations necessary to arrive at the correlation coefficient is 
simultaneously performed using the above equations by the software, and 
those correlation coefficients are further processed in a fashion depicted 
in the flow chart of FIG. 7. Linear regression techniques advantageously 
eliminate base line shift o scaling distortions introduced into the 
electrode system of FIG. 1 as the electrodes maturate in the patient's 
body. 
Turning now to FIG. 7, the morphology algorithm arrhythmia classification 
subroutine which follows from block 174 of FIG. 5 is illustrated. At block 
200, the N atrial and ventricular far-field EGMs are averaged by 
mathematic averaging of the digitized amplitude values of each sample 
point. Then at block 202, the suspect atrial and ventricular EGM average 
values (which are characterized in the algorithm as a pair of EGMs) are 
correlated against n pairs of reference rhythm EGM values to arrive at 
respective n pairs of regression correlation coefficients for each 
comparison. Thus, for example, the averaged suspect atrial and ventricular 
far-field EGM pair is compared against the normal sinus rhythm atrial and 
ventricular far-field EGM pair and a specific regression correlation 
coefficients (A and V) ranging from 0 to 1.0, are calculated for the 
suspect atrial EGM and for the suspect ventricular EGM. 
At block 204, the regression correlation coefficients for the atrial and 
ventricular correlations are added together and the largest value or 
result is determined. In block 206, that largest result is used to 
identify the reference rhythm with the highest sum of atrial and 
ventricular regression correlation coefficients and at block 208, the sum 
of the atrial and ventricular regression correlation coefficients for 
sinus rhythm is also identified. These values will be somewhere between 0 
and 2.0 and the steps set forth in blocks 206 and 208 may be reversed in 
sequence or done in parallel. In any case, the value derived in block 208 
is compared in decision block 210 against a reference value (designated 
2R) and if the sinus rhythm summed regression correlation coefficients 
exceed 2R, a match is declared between the suspect rhythm and sinus rhythm 
at block 212. If sinus rhythm is declared, the program exits the 
subroutine of FIG. 6, and declines to apply any therapy even though the 
rate onset and stability criteria have been satisfied in FIG. 4 as 
evidence of a tachyarrhythmia. 
Thus, a significant value of the concept of the present invention is to 
eliminate the delivery of inappropriate therapies in the event that the 
morphology discrimination of the far-field EGMs evidences a high 
correlation or fit to normal sinus rhythm. Of course, in the system 
contemplated, the EGMs may be stored for later retrieval and evaluation by 
the physician to determine if the programmed correlation coefficient 
threshold value R is appropriate or not. 
Returning to FIG. 6. in the event that the sum of the regression 
correlation coefficients of the suspect EGM are less than 2R, the 
algorithm moves to block 212, which merely declares that the analyzed EGMs 
do not reflect sinus rhythm and moves to decision block 214, which employs 
the largest summed correlation coefficients derived in block 206 and 
compares it against a further (or the same) value 2R. If the largest 
summed correlation coefficients exceed 2R, then, at block 216, the suspect 
EGM is declared to represent the arrhythmia of the reference EGM against 
which it most closely correlates. The program reverts to block 122 of FIG. 
4 to apply the therapy prescribed for the matched arrhythmia. 
However, if the largest summed correlation coefficients identified in block 
206 fail to exceed 2R, then at block 218, the arrhythmia is declared to be 
indeterminate, although probably the reference rhythm with the largest 
paired coefficients. If that reference rhythm happens to be fibrillation 
or if the physician programs the device to apply the most aggressive 
therapy in the event that the arrhythmia cannot be determined then the 
program exits block 218 and the system applies the prescribed 
defibrillation therapy in block 122. The system admits the possibility of 
applying a succession of therapies from lesser to greater degrees of 
aggressiveness in conjunction with a determination in block 218 of an 
indeterminate condition. Alternatively, the physician may prescribe staged 
therapies for recurrences of arrhythmias matched in block 216 until the 
arrhythmia is terminated and store the successful therapy for initial use 
at the next episode. 
If a stable ventricular tachycardia is diagnosed, then the appropriate 
therapies to be delivered to the patient's heart by the device depicted in 
FIG. 9 are low energy, high rate pacing stimuli. However, if ventricular 
fibrillation or unstable ventricular tachycardia is detected, the 
appropriate therapy to be delivered is a medium energy synchronized 
cardioversion shock followed by further, higher energy synchronized or 
unsynchronized shocks if the initial shock is insufficient to break the 
tachycardia. 
It will be understood that the system of the present invention contemplates 
data storage and retrieval of the correlation coefficient values, the 
number and times of occurrences of the episodes, and other data associated 
with the detection of the arrhythmia and delivery of the therapies for 
subsequent interrogation and read-out as is known to be conventional in 
the art. 
Turning now to the second embodiment of the invention, FIG. 8 sets forth 
the flow chart for the algorithm for avoiding the tracking of retrograde 
and pathologic P-waves in a dual chamber antibradycardia pacemaker. In 
such pacemakers which employ atrial and ventricular pacing and sensing 
leads in a pulse generator of the type depicted in FIG. 1, it is desirable 
to allow the pacemaker to physiologically track the spontaneous atrial 
events and provide synchronized stimulation in the ventricle when needed 
as long as the spontaneous atrial rate ranges between a programmable lower 
rate limit of perhaps 60 beats/minute and a programmable upper rate limit 
of perhaps 150 beats/minute. However, as stated hereinbefore, it is 
undesirable that the pacemaker track rapidly recurring P-waves that are 
either pathologic in origin or are created by the retrograde conduction of 
a ventricular depolarization, either natural or stimulated, via the AV 
mode or an accessory pathway of the heart muscle. Where such conditions 
occur and the pacemaker is continuously triggered to stimulate the 
ventricle at elevated rates in the absence of an exercise induced need, 
the patient is rendered uncomfortable, his hemodynamic performance is 
impaired, and a slight possibility of acceleration of the arrhythmia 
exists. In the past, various schemes have been developed to avoid this 
pacemaker-mediated tachycardia condition but they tend to compromise the 
performance of the device inasmuch as the conditions usually cannot 
distinguish between a pacemaker mediated tachycardia and a sinus 
tachycardia. In accordance with the present invention, it is contemplated 
that the system depicted in FIGS. 1 and 2 in conjunction with the wave 
shapes of FIG. 3 may be employed to discriminate sinus rhythm P-waves from 
pathologic or retrograde conducted P-waves by morphology analysis of the 
far-field atrial EGM. 
It should be understood that pacemaker mediated tachycardias can be 
triggered by sensed P-waves that recur at rates that are less than the 
upper rate limit of the dual chamber pacemaker. In the context of the 
present invention it is desirable to discriminate normal and pathologic 
P-waves at atrial rates less than the upper rate limit so that the 
physician may program the device to respond appropriately to a sinus 
rhythm during exercise to provide cardiac output suitable to the patients' 
needs. For example, the physician may program the device to operate 
synchronously to an upper rate limit of, perhaps, 175 bpm and separately 
program the device to test the far-field EGM morphology at a suspect rate 
of, perhaps, 100 bpm. In such a case, when the atrial rate exceeds 100 
bpm, it is considered suspect and the far-field electrogram is examined, 
at least periodically, to determine if it is sinus or pathologic in 
origin. If pathologic, the atrial tracking function is disabled until the 
spontaneous rate falls below the suspect rate. In a DDD pacemaker, for 
example, the pacing mode would switch to single chamber VVI pacing (with 
continued atrial sensing) until normal atrial sinus rhythm resumes. 
In FIG. 8 at block 300, the program is intialized by setting the rapid 
atrial count to 0 and thereafter measuring the atrial rate at block 302. 
At block 304, the measured atrial rate is compared against an upper rate 
limit of 100 bpm, for example, and as long as it is less than or equal to 
100 bpm, the rapid atrial counter is reset again to 0 in block 306 and the 
pacemaker tracks normally in block 308. A resetting of the rapid atrial 
counter to 0 in block 306 may occur in the event that the rapid atrial 
counter has not yet reached its threshold count as described below. 
In the event that the instantaneous atrial rate is determined to exceed the 
suspect rate of 100 bpm, the rapid atrial counter is incremented in block 
310. If a succession of measured atrial rates exceed 100 bpm, the count of 
the rapid atrial counter will be incremented in block 310 and when it 
reaches N" rapid atrial counts, the morphology algorithm is applied to 
examine a series of the far-field morphologies of the atrial EGM or 
P-wave. Thus, in decision block 312, until the count of the rapid atrial 
counter exceeds N", the algorithm reverts back to block 302 via block 308 
to measure the atrial rate. If at any time before the count end is 
reached, the atrial rate falls below or equal to 100, then the counter is 
reset at block 306 to 0. However, if the count reaches N", then in block 
314, the counter is reset but the program moves to apply the morphology 
algorithm in block 316. At block 318, the morphology is correlated in the 
fashion described hereinbefore to a stored sinus atrial electrogram 
obtained from the patient during a post pacemaker implant evaluation and 
stored in RAM memory. If the morphology correlates to the stored sinus 
P-wave, the tracking of the P-waves is declared to be appropriate. 
However, if the suspect morphology fails to correlate to the stored 
reference sinus P-wave morphology, then atrial tracking may be disabled in 
block 320 until the measured atrial rate again falls below the reference 
of 100 beats/minute in block 322, whereupon atrial tracking may be 
restored in block 324. 
The algorithm depicted in FIG. 8 thus represents a simple algorithm for 
conducting a morphology correlation to discriminate high rate sinus atrial 
depolarizations from retrograde and pathologic P-waves. It will be 
understood that the morphology correlation may be more sophisticated to 
compare the atrial morphology against a library of reference P-wave EGM 
values representing pathologic or retrograde conducted P-waves if they can 
be induced and stored during a post pacemaker implant workup of the 
patient by the physician. In such circumstances, a more sophisticated 
correlation may be conducted although for most purposes, it would be 
unnecessary to do so. In most instances, the wave shape of P-waves during 
sinus atrial tachycardia are highly regular and reproduceable. 
Consequently, it would be only necessary to fit the suspect P-wave against 
the reference P-wave sample point values and to provide for a correlation 
value of between 0.8 and 1.0 to declare a match and allow the pacemaker to 
track the atrial rate. 
In addition, other modes of corrective action than mode switching or 
disabling the atrial tracking may be contemplated including lengthening 
the post ventricular atrial refractory period (PVARP). In any case, the 
system would continue to track the atrial rate and analyze the far-field 
P-waves in accordance with FIG 8. Data storage and retrieval of such 
episodes may be provided for in order to analyze the arrhythmia and system 
response. 
Before leaving the description of this second embodiment, it should be 
pointed out its principles can be applied to either unipolar or bipolar 
atrial synchronous pacing systems along with the remote indifferent 
electrode. The electrode and pulse generator system depicted in FIGS. 1 
and 2 illustrates the bipolar electrode version, although it should be 
understood that the sense amplifier 38, delay 48, and buffer 44, are not 
necessary since only the far-field atrial EGM is of interest. To produce 
the bipolar version only the atrial and ventricular tip electrodes 22 and 
26 are employed and the atrial rate and ventricular synchronous 
stimulation are derived from the unipolar P-wave sensed between the tip 
electrode 22 and indifferent electrode 30. Again, the far-field atrial EGM 
for morphology analysis is derived between ventricular tip electrode 26 
and indifferent electrode 30. 
Reference is now made to FIG. 9 which depicts a block diagram of the major 
components of automatic implantable device for detecting and treating 
brady and tachyarrhythmias. It is contemplated that such a device would be 
implemented in analog and digital microcircuits under the control of a 
central microprocessor/memory block 41 powered by high (for cardioversion 
and defibrillation) and low (for the remaining circuitry on pacing 
therapies) power sources in block 412. The high power pulse generator 
block 414 would include the cardioversion/defibrillation pulse generator 
circuitry coupled by output terminals to two or more 
cardioversion/defibrillation electrodes to apply synchronized 
cardioversion or unsynchronized defibrillation shocks to the electrodes 
situated in or about the heart in a manner well known in the art. 
It is contemplated that the implantable device depicted in FIG. 9 would 
function under the control of a resident operating program or software 
retained in memory within the microprocessor/memory block 410 and would be 
programmable by a external programmer/receiver (not illustrated in FIG. 9) 
communicating with the implanted device by radio frequency energy received 
or transmitted by antenna 416 under the control of the programming and 
data transmission block 418 and reed switch 420 which is responsive to an 
external magnet. The programming and data transmitting block 418 would be 
capable of receiving programming instructions and directing them to the 
memory within microprocessor/memory block 410 as well as transmitting data 
stored within the memory block 410 as well as an electrogram representing 
the patient's atrial and ventricular activity in a manner well known in 
the pacing art. 
The timing of all processing functions, including the determination of 
atrial and ventricular cycle lengths, is controlled by system clocks 
within microprocessor/memory 410 driven by crystal oscillator 422 in a 
manner well known in the prior art of implantable digital pacemakers. 
The cardiac signal processing blocks of FIG. 9 include the 
isolation/protection or interface block 424 which operates to direct 
atrial and ventricular pacing stimuli from the pacing pulse generator 
block 426 to respective atrial and ventricular output terminals which in 
turn are coupled through the pacing leads to the bipolar pacing electrodes 
situated in or near the atrium and ventricle of the heart as shown in FIG. 
1, respectively. In addition, the interface 424 (when unblanked) couples 
the near-field and far-field atrial and ventricular electrograms to the 
sense amplifier block 428. Interface 424 is blanked or prevented from 
passing any signals picked up on the atrial and ventricular pacing/sensing 
electrodes to the sense amplifier block 428 during short blanking 
intervals following the delivery of an atrial or ventricular pacing 
stimulus in a fashion well known in the pacing art. 
The indifferent plate electrode of FIG. 1 is coupled to the interface 
circuit 424 which is used in conjunction with the bipolar pacing/sensing 
electrodes to provide far field, unipolar signals to the sense amplifier 
428 in the manner described hereinbefore. The plate electrode may be one 
of the cardioversion/defibrillation electrodes, the case of the pulse 
generator or a separate electrode on or attached to one of the lead 
bodies. 
Furthermore, the interface 424 disconnects or shorts out the pacing/sensing 
electrodes during the delivery and for a short period after the delivery 
of a cardioversion/defibrillation shock by application of a control signal 
to the interface 424 by the cardioversion/defibrillation pulse generator 
block 414. 
The P-wave and R-wave signals transmitted through the interface 424 to the 
sense amplifiers 428 are amplified and shaped to generate the near-field 
and far-field atrial and ventricular signals AS and VS, respectively, 
which are conducted to microprocessor/memory 410 in order to derive the 
atrial and ventricular cycle lengths, the AV delay interval, and other 
intervals and rates described hereinbefore to perform the inventive 
functions of the device. A further signal from a physiologic sensor 432 
representative of cardiac or patient activity is also applied to the 
microprocessor/memory 410 in order to control the bradyarrhythmia pacing 
rate in the DDDR or other rate responsive mode of operation and to augment 
detection of tachyarrhythmias. 
The microprocessor/memory 410 responds to atrial and ventricular AS and VS 
signals by generating appropriate atrial and ventricular refractory and 
blanking intervals which are in turn applied to the sense amplifier block 
428 during certain windows of time following each respective AS and VS 
signal in a fashion well known in the pacing art. 
It is contemplated that the system depicted in 435 FIG. 9 may be programmed 
to operate in any of the known bradycardia single or dual chamber pacing 
modes. The signal from the physiologic sensor 432 may be employed to 
modify the atrial and ventricular escape intervals to allow for a certain 
range of atrial and ventricular pacing depending upon the level of the 
patient's activity in a fashion well known in the bradycardia pacing art. 
Suffice it to say, that atrial and ventricular escape intervals 
established in memory are compared against the atrial and ventricular 
cycle lengths encountered in the patient and, if a bradycardia condition 
exists, the microprocessor/memory 410 applies atrial and ventricular pace 
trigger signals AT and VT through analog rate limiter block 430 to the 
pacing pulse generator 426 which responds by developing the respective A 
pace and V pace signals. Analog rate limiter 430 operates to limit atrial 
and ventricular pacing rates to a safe high rate and effect an appropriate 
upper rate behavior in the event that the spontaneous atrial rate exceeds 
the programmed upper rate limit as is described above in relation to the 
second embodiment of the invention. 
Although presently preferred embodiments of the invention have been 
described, it will be apparent from that description to those skilled in 
the field to which the invention pertains, that variations of the present 
embodiments may be implemented without departing from the principles of 
the invention. Further, as technological advances are made, for example, 
in developing practical small size, low-cost high voltage components, 
similar to the advances in the semiconductor field, the principles of the 
invention may be applied directly to a "universal" implantable device for 
performing an all-purpose cardiac treatment function. 
Accordingly, it is intended that the invention be limited not by the 
structural or functional elements of the described embodiment, but only as 
set out in the appended claims.