CA1192263A - Method and probe for sensing intracardiac signals - Google Patents

Abstract

METHOD AND PROBE FOR
SENSING INTRACARDIAC SIGNALS

Abstract of the Disclosure Near field, localized, intracardial electromagnetic events can be reliably sensed within a human heart by sensing the event within the human heart at two or more closely spaced points, generally lying in a plane approximately perpendicular to the local depolarization vector in the proximate heart tissue. The local electromagnetic event in the proximate heart tissue is sensed and discriminated from all other events within the heart, including events occurring at other locations within the same heart chamber, events occurring in adjacent heart chambers, events occurring in adjacent muscle or body tissue, and events occurring exterior to the body. Thus, the P wave within the atrium can be reliably and distinguishably sensed from the QRST
complex, the progression and sequence of heart events in the signal can be sensed at multiple locations wherein each event is reliably distinguished from the other. In addition, events immediately following a cardiac stimulating pulse can be reliably observed.

Description

2~;3 METHOD AND PROBE FOR SENSING INTRACARDIAC SIGNALS

The present lnvention relates to the fields of cardiology and methodologies for sensing intracardiac electrical signals; and more particularly relates to methods and appara~us for discriminatorily sensing a loca-lized endocardial electrogram subjacent to the sensingapparatus and distinguishing it from all unwantedl farfield electrical events.
The human heart is basically a pump through which blood is drawn into the atrium, or upper chambers, and then fills the ventricles, or lower chambers, through a con-necting valve. Blood is then pumped out of the ventricles to the vital organs. Each cycle of this pump is initiated by a series of electrical events which occur with a speci-fic order. It is the order and relationship of these electrical events which determines in part the efficiency of the heart's performance as a pump. The electrical cycle begins with spontaneous~y depolarizing cells of the sinoatrial node, ~:la9Zf~3 a small area of specialized tissue adjacent to th~ superior vena cava-high riyht atrial border. The depolarization front slowly spreads within the sin~s node and then through perinodal tissues to activate atrial myocardium. As the mass of the atrial myocardial tissues is activated, an electrical event is generated which, when detected on the bodyls surface, is called the "P wave". When detected from inside cardiac chambers, this same event is designated as the "Atrial electro-gram". Characteristics oE both the atrial electrogram and surface recorded P wave are a function of the mass of atrial myocardium activated and the specific activation sequence occurring within that human heart. It is this electrical activation sequence which results in contraction of atrial myocardium and at least partial filling of the ventricles~
The atri~l electrical excitation waVQ, however, woulc~ terminate at the fibrous boundary between the atria and ventricles were it not for the specific multicomponent atrial ventricular (AV) conducting system responsible for the transmission o electrical information across the fibrous boundary between atria and ventricles. The AV node is a small 1~2 cm. structure located in the floor of the right atrium midway between the ostium of the coronary sinus and the central fibrous body of the heart. This structure, composed o~ three separate classes of cells r acts as an electrical filter providing an area of slowed conduction between the atria and ventricles sufficient to allow appropriate ventricular filling. Electrical input to the AV node occurs in part via ordinary atrial myocard;um and in part via three specialized intra-atrial tracts responsible for conduction of electrical information between the area immediately adjacent to the sinoatrial node and the upper~ middle and lower regions of the atrio-ventricular node. Conduction within the AV

iL~L92ZG3 node is very responsive to autonomic influences, the effects of cardiovascular medications and coronary ar~ery disease.
As the electrical depolariæation front proceeds through the AV node, it enters the AV or His bundle which is responsible for transmission of the electrical impulses between the anatomic atria an~ ventricles. This structure actually tranverses the fibrous skeleton of the heart and proceeds for several centimeters down within the membranous interventricular septum. At the summit of the interventricular septum, the specialized ~V conduction system divides into two main branches, a right and left bundle; the left bundLe then trifurcating into specialized fibers responsible for ventricular septal activation; activation of the high lateral ventricular m~ocardium and a posterior division responsible for activation of the mass o~ the septum and left ventricular myocardium. The right bundle is responsible for right ventri-cular activation.
Althouqh by utilizing special techniques, electrical activLt:y can be detected from specialized areas of the cardiac conduction system, surface electrocardiography does not disclose arly discrete electrical events which occur during activa~ion of the AV nodes, ~V bundle, or any of the bundle branches. No electrical activity is discerned on the surface electrocardiogram ~ollowing the P wave un~il the mass o ventricular myocardium begins to be activated and a QRS
complex is inscribed. This complex ;s generally of signifi-cantly greater amplitude when recorded at the body surace than is the P wave because of the much larger mass of ventri-cular myocardium generating it. When recording from inside the cardiac chambers, this event is referred to as a ~Iventri~
c~lar electrogram". Ventricular myocardial repolarization events, whether detected on the body surface or inside the ventricle, are referred to as the T-wave.

The normal spontaneous diastolic depolar;æation of the sinus node ~ollowed by normal spread o electrical excitation through the atria and ventricle produces a normal car~iac rhythm with a frequency responsive to nervous system influences which alter both the rate of sinus node discharge and the rapidity Qf atrial and ventricular excitation.
A variety of disease states, however, may influence both the rhythm of the dominant natural cardiac pacemaker as well as the electrical excitation pattern of the human heart.
Abnormal rhythms`may be generated by intermittent failures of sinoatrial impulses to conduct out of the sinus node, by abnormalities of atrial myocardium resulting in rapid atrial arrythmias such as flutter o~ fibrillation, abnormal or absent AV nodal conduction which will result in the emergence of slow and inadequate intrinsic pacemakers from lower sites within the ventricle and abnormally slow heart rhythms;
by diseases in one or all of the bundle b~anches resulting in partial or complete AV block again with slow or inadequate heart rates; as well as by abnormalities in ventricular myocardium which may result in rapid tachycardias, ventricular fibrillation and death. The art of medicine has presently advanced to the state where many such defective electrical depolarization sequences may be symptomatically treated by electronically stimulat~ng the heart through external or implanted electronic devices generally considered as pacemakers.
The simplest and most frequently employed electronic device is a standard W I pacemaker. This device senses the ventricular electrogram and when the rate of depolarization falls below a preset level, stimuli are delivered to the ventricular myocardium to result in an electrical depolarization 2~3 r~ave and subsequent ventric~lar myocardial contraction.
Such an electrical escape or backup system may be implanted at the ventricular (W I~ or atrial (AAI) level to correct abnormalities of either AV conduction or sinatrial node function. Newer pacemaking systems employ electrodes at both the atrial and ventricular level to sense and pace at either or both sites in order to restore a more normal sequence of atrial and ventricular electrical ~xcitation.
Other implantable electrical devices have been used for the conversion of atrial and~or ventricular tachycardias as well as part of the input to an implantable electrical device designed for the termination of ventricular fibrillation.
The ability of any of these devices to function appropriately, however, is determined in no small measure by the adequacy of the electrical signals sensed from within either the atrium or ventricle~ since appropriate pacemaker output will be de~ermined only by adequate signal or sensing input. In general, most prior devices utilize the same electrodes used for myocardial stimulation for sensing myocar-dial depolarizatiGn in the absence of stimulation. In general, most systems have consisted of one electrode embedded in atrial or ventricular myocardium and a second electrode either in the form of a ring located proximal to the stimulating electrode along the body of the same catheter (bipolar sensing) t or the second electrode comprised o~ the casing of the pacemaker itself (unipolar sensing). Average signal amplitudes for detected P and QRS complexes using either unipolar or bipolar sensing systems have been well described in the cardiac literature ~or over ten years.
The necessity of implanted electrodes to provide a signal derived only from the myocardium with which it has been placed in contact has long been recognized. Problems G

.~_ of electromagnetic interference external to the patient~
such as external magnetic or electrical fields and thGs~
produced by devices su~h as electric razors or microwave ovens, is n~w even recognized by the general public. Over recent years, however, an increasing awareness of the vulnera-bility o missensing cardiac electrical events due to the sensing of other electrical events occurring within the body has been increasingly recognized. See, for example, Symposium on Electromagnetic Interference, PACE, Vol. 5, Jan.-Feb. 1982. Particularly in unipolar electrode systems, the sensing of electrical signals from skeletal muscle adjacent to the pacemaker anode itself (myopotential sensing~ has been recognized as a frequent cause'o false inhibition of these pacemakers. Although standard bipolar ring electrod~s tend to reduce the incidence in which myopotential and EMI
sensing is seen, these sytems tQo are not immune to farfield, patient, or environmentally generated electrical phenomenon.
A ~ensing system ~immune" to EMI would be of great clinical value.
When either unipolar or bipolar contacting electrode systems are placed within the ventricle to sense ventricular myocardial activation (the ventricular electrogram), in general, adequate ventricular electrogram signals o~ 10 to 1~ mv. are seen and because of the relatively large mass of ventriculax m~ocardium in comparison to atrial myocardium, little, if any, electrical activity reflectiny atrial depolari-zation is sensed by this catheter~ Such is not the case, 'however, with electrograms derived ~rom a variety of locations within the atria. Here contactin~ electrical sensing`systems, whether unipolar or bipolar, have all suffered from the same flaw, and almost independently from their positioning within the right atrial appendage, against the lateral wall of the right atrium, or within the proximal: to distal coronary sinu~. Th~ problem would seem to be inher~nt in cardiac anatomy. That is, the atrial mass is small and, as a result, P wave amplitudes are also generally quite s~all, 1-2 mv even when sensed from within the atrium. Because the mass of ventricular myocardium so far exceeds that of atrial myocardium, ventricular depolarizations can be seen even from inside the atrium as discrete electrical events which, even ~hough occurring at a distance~ are indistinguishable from atrial electrical events in terms of both their signal amplitude and morphology.
This f]aw alone has virtually resulted in a very slow development and growth in the use of atrial iniibited (~AI) and P wave synchronous (VAT or VDD~ systemsO Its has hecome an almost insurmountable problem in the development of dual chambered demand pacemakers designed to sense at both the atrial and ventricular levels (DDD systems). The development of appropriate tachy-convertin~ pacemakers at both the ventricular and atrial level has also been severely hampered by the lack of signal discrimination between atrial and ventricular events sensed via standard pacemaking systems.
Each of these prior art pacemaking techniques have attempted to develop a reliable trigger signal or the stimulating pulse by sensing an electrogram by means of a catheter inserted through the veins into the heart.
In general, the signals sensed in one cardiac chamber, or their absence, have provided information required to pace that same cardiac chamher. It has long been the intent of cardiologists to have a pacemaking system wherein electrical activity in the upper portion of the heart, that is the atria, could he sensed and then the ventricle paced at a rate appropria~e to the atrial sensed rate. Most current systems employ two catheters; one placed in contact with the right atrial appendage, and a second in contact with the rlght ventricular apex. Thus, signals sensed at the atriu~ can be interpreted by the pulse generator and stimuli de]ivered to the ventricle at an appropriate rate.
Several previous art descriptions of single catheters which could be used to sense atrial activit~ and pace at the ventricular level exist. In generalr the sensing element in each case has heen electrodes similar to the stimulating electrodes; however, placed elsewhere along the shaft of the catheter. ~or example, one or two ring electrode placed at the atrial level to sense the atrial electrogram was shown b~ Thaler, "P Wave Con~rol, R Wave Inhibited Ventricular Stimulation Device", U.S. Patent 4,091,817. Other similar designs are ound within the patents by O'Neill, "Formable Cardiac Pace Lead and Method of Assembly and Attachment to a Body Organn, U.S. Patent 4,154,247; Berkovits, "Single Catheter for Atrial and Ventricular Stimulation", ~.S. Patent

3,825~015; or Bures, "Transvenous Coaxial Catheter", U.S.
Patent 3,865,118. In most instances, it was formerly believed essential to physically contact the inner surface of the atrium by the electrode, since the heart wave is generate~
within the m~ocardial tissue and contact, such as shown in Figure 5 by electrode 715 of O'Neill, or in Figure 1 by electrodes 18 and 19 of Bures, was considered essential.
In some prior art, atrial contact was not considered essential~ One prior device is described by Thaler, l1p Wave Control, R Wave Inhibited Ventricular Stimulation Device", U.S. Patent 4,091,817, whîch shows a pacemaker which senses the P wave from two circumferential ring electrodes El and E2, best shown in Figure 5, to generate subsequent ventricular stimUlation. Thaler is illustrative of the complex circuitry which the pr;or art has attempted to devise in order ~o distinguish the P wave from ventricular depolarization and subsequent wave elements in the ensuin~ heart wave complex.
The problem is a nontrivial one, since the P wave sensed from non-contacting ring electrodes is virtually identical to the following QRS complex. Similarly, there is no reliable way in which to spectroanalyze or otherwise discriminate by automated means the P wave rom its accompanying QRST
wave train. Other complex circuitry, used in connection with heart pa~ing, is also shown and discussed in connection with Berkovits; Lin, et al., "Variable P-R Interval Pacemaker", U.S. Patent ~060r090; and Funke, "Arrythmia Prevention Apparatus", U~S. Patent 3,937,226.
The problems of signal discrimination a~ both the atrial and ventricular level are compounded when one considers the effects of electrical stimulation via the pacemaker itself at either the atrial or ventricular level.
When one attempts to sense the myocardial depolarization evoked by either unipolar or bipolar 5 volt electrical artificial ~timulation via the same electrodes used for that stimulation, there is an electrical phenomenon which occurs at the electrode myocardial interface (known as the "after-potential")/ which is an event of several volts amplitude and of 200 to 500 msec. durationO This after-potential completely "swamps"
the evoked adjacent myocardial response so that without extremely complex circuitry, stimulated myocardial events cannot be sensed by the pacemaker itself. This has resulted in the recognition and acceptance in tbe prior art of what has been called a "sensing refractory period" following all stimulated or evoked depolarizations. Although "after potential" is operative at the ventricular level as a result ~,, '263 of ventricular s~imulation alone, at the atrial level, stimu-lation of either the atrium or the ventricle results in atrial electrogram obscuration for a significant period of time.
In any case, the prior art has been largely unsuc-cessful and frustrated in devising an acceptable P synchronous, VDD and DDD ~ype demand pacemaker b~ the inability of the prior art to successfully pick ~he P wave from the accompanying heart wave complex. This frustrati.on has been exacerbated ~y the fact that sensing electrodes and accompanying circui.try have been swamped by the strong stimulating pulse generated by the pacemaker itself, such that the pacemaker has been blind to whether or not a depolarization was succe~sfully stimulated.
A problem encountered with increasing frequency in complex dual--chambered pacing systems is that of abnormal retro~rade atrial activation occurring as the result either o~ spontaneous ventricular premature depolarizations, or ventricular pacing with retrograde or reverse conduction across the AV node. Retrograde conduction is a normal capacity of the AV node and is seen in 90~ of patients with sinus node dysfunction; therefore~ this problem is an extremely significant one for DDD pacing systems. Attempts to determine if retrograde conduction is responsible for any ~iven P
wave has relied on the use of extremely complex circuitry because of the inherent lack of ordinary electrical sensors to accurately analyze the direction in which myocardial activation i5 occurring.
This same dif~iculty is encountered in standard ventricular sensing systems ;f one attempts to utilize them to discriminate between normally conducted impulses and ,~

those generated by abn~rmal foci within the ventricles themselves, that is, to distingulsh normal anterograde conduction from ventricular premature d~polarizations. This latter capacity becomes extremely important when considering the problem of tachycardia converting pacemakers. Thus far, because of the lack of an adequate sensor, only the rate with which ventricular events are occurring can be used to discriminate wanted ~rom unwanted tachycardias. Normal sinus rhythm, atrial tachycardias with 1:1 conduction, atrial flutter with variable conduction, and as well ventricular tach~cardias may all occur at rates in excess of any preselected rate for ventricular tachycardia conversion. Thus, when rate alone is used to determine the pres~nce oE ventricular tachy-cardi~, significant diagnostic errors may and have resulted.
Therefore, clearly what is needed is a method and apparatus whereby:
1. The local electrogram from the tissue subadjacent to the pacing catheter may be sensed discriminatorily from all farfield electrical events whether generated by other areas of the same chamber, delivered electrical stimuli, or depolarization of the opposite chamber.
2. The local electrogram is rendered insensitive to efects of EMI and myopotential sensing.
3. Evoked depolarization may be sensed without interference by the polarization effects at the electrode myocardial interface ~afterpotential sensing).

4. ~he electrogram derived from atrial myocardium may be recorded at significantly higher amplitudes and with clear cut discrimination from ventricular electrical events despite the inherently larger magnitude o the electrical forces producing ventricular events.

a~

5. The local myoc~rAial depolarizations recorded from multiple sites may be used to discriminate normal from abnormal activation pat~erns.
The development of such a discrete and specific electrical sensor when applied to problems confronted in the treatment of electrical disorders of the heartbeat wo~ld result in the development of a generation of electrical devices which for the first time are accurate and reliable in the therapy of cardiac arrythmias.

The present invention is ~n improvement in a me~hod for sensing heart electrical activity comprising the steps of discriminating a wanted local cardiac electrical event from a complex series of waves, pacemaker stimuli and/or extra cardiac electrical events by sensing the desired electr~-gram via one or more pairs of points lying in a place approxi-mately perpendicular to the depolarization vector in heart tissue proximate to the pair of points at which the local excitation wave is sensed. The signals sensed at this pair or pairs of points are then electrically "compared" to derive a differential signal specific to the desired local electrogram, and are substantially independent of all of the other elements of the complex heart wave or extraneous electrical events by virtue of the fact that they are "farfield" electrical events. By this methodology, any local cardiac electrical event may be reliably and unambiguously sensed and selected from any complex heart wave or non-local electrical events, even if those non~local events generate signals of substantially greater magnitude~
Discriminatin9 the desired local electrogram is particularly characteri~ed by not having the paired sensing --X~--electrodes necessarily in contact with myocardi~m (heart tissue). In other words, the senslng electrodes are free-floating within the heart cavity. It is also particularly characterized by having electrodes dedicated to electrogram sensing which are not utilized in myocardial stimulation, thus allo~ing sensing of evoked (stimulated) electrical events in a manner unobscured by polarization effects at the electrode-myocardial interface (afterpotential sensing).
Specifically, the step of discriminating the intra-atrial electrogram (P wave) from all other depolarization events, such as those occurring at the ventricular level, is particularly characterized by not having the sensing electrodes in contact with the hear~ tissue. Whether positioned in the high lateral right atrium or right atrial appendage, the sensing electrodes are free-floating within the heart cavity.
In a second embodiment of the present invention, the step of discriminating the P wave also includes sensing the P wave signals at more than two points in the plane which is perpendicular to the depolari~ation vector in proximate heart tissue In one embodiment, the cardiac signal is sensefl at three or four points in this plane, which are equally spaced about a circle lying in the plane from which sensing two or more differential signals are derived.
In addition, the normal atrial or ventricular activation sequence, that is, activation occurring from the normal spread of electrical activity in the heart, may be distinguished ~rom abno`rmal activation se~uences, such as produced by tachyarrhythimias, single premature depolariza-tions or retrograde conduction, by utilizing two or more sensors in various parts of the atrium or ventricle~ The characteristics of the sensor in each area would be identical _,~

to those previously described, with the addition that logic w~uld be applied to the sensed signals indicative of the activa~iOn sequence of each derived electrogram so tha~
normal and abn~rmal acti~ation patterns could be reliably discriminated.
These and other embodiments of the present invention can best be understood by viewing the following figures, which are illustrative of the method of the present ;nvention, in light of the following Detailed Description of the Preferred Embodiments.

~ igure 1 is a graph showing the time relationship between a conventional surface electrocardiogram and atrial and ventricular electrograms derived from the right atrial appendage and right ventricular apex~ in comparison to-electro-grams recorded from sensing probes of the present invention.
Fiqure 2 is a graph showing the t;me relationship between a conventional surface electrocardiogram of a normal heartbeat, a standard atrial appendage electrogram, and the high right and lower right atrial electrogram deri~ed from the sensing probe of the present invention. This is contrasted with the time relationship between a retrograde heartbeat, illustrated by conventional surface electr~cardiogram~
as compared to a conventional atrial electrocardioyram and high right and lower right electrograms sensed by a probe of the present invention.
Figure 3 is a diaqramatic, ~ragmentary view of a catheter including the sensor of the present invention adja~ent to myocardial tissue.

Figure 4 is ~ cross sectional vlew taken through lines 4-4 of Figure 3.
Figure 5 is a side elevat;onal view taken through lines 5-5 of Figure 3.
Figure 6 is a diagramatic view of the right atrial and ventricle of a human hear~ having a catheter oE the present invention inserted therein.
F;gu~e 6a is a diagramatic enlargement of the circled portion o~ Figure 6, denoted by reference numerals 6a.
Figure 6b is a diagramatic enlargement of the circled portion of Figure 6 generally referenced by numeral 6b.
Figure 7 is a diagra~atic, fragmentary vie~ of a ve~tricular sensing and ventricular stimulating probe in the apex of the ventricle.
Figure 8 is a fragmentary diagramatic view of a "J"-shaped a~rial sensing and atrial stimulating probe disposed in tlle right atrial appendage.
Figure 9 is a diagramatic sectional view showing a sin~le filament catheter having a plurality of sensing electrodes placed within the atrium.
Figure 10 is a diagramatic ragmentary view of a portion of the catheter, shown in enlarged scale, wherein two electrodes are provided for sensing.
Figure ll is a diagramatic view of a catheter in enlarged scale showing an embodiment wherein four electrodes are utilized.
Figure 12 is a fragmentary diagramatic view of a portion of catheter showing an embodiment wherein three electrodes are utilized.
Figure 13 is a flow diagram outlining the methodology of the present invention.

.~ -The present invention includes a metho~ whereby localizea cardiac electrical activity is sensed in such a manner that a local cardiac signal is discriminatorily sensed from all other unwanted electrical signals, regardlesE
of whether they are generated in other areas of the same cardiac chamber, in other cardiac chambers, or occur as electrical events outside the heart, such as signals generated in skeletal muscle or extrinsic elec~rical "noise'~
The electromagnetic field mechanism upon which the methodology of the present invention is based is not well un~erstood. However, by sensing the heart wave at two or more points substantially lying in a plane perpendi-cular to the direction of the local depolarization vector in proximate or adjacent myocardial tissue; and then differen-tially amplifying these two or more sensed signals, it has been found that a high amplitude local electrogram is produced with an extremely high signal-to-noise ratio. For example, the existence of the much higher amplitude Q~S complex, the following T wavel or the massive stimulating pulse generated by an associated pacemaker, has been found to have no effect upon the efficacy of the present methodology to reliably sense the.mu~h smaller P wave in the he~rt and thereby reliably discriminate the P wave.from all other natural or artifically introdu~ed electrical activity~
Figure 1 illustrates at line 10 a conventional surface or skin electrocardiogram (EKG) of a normal heartbeat showing the P wave 1~ which is the surface representation of the atrial myocardial depolarization, followed by the QRS complex 14, which similarly reflects ventricular myocardial ,,~g7 depolarization; and then the T wave 16~ which reflects ventri-cular repolari~ation. Line 18 is a conventional atrial EKG such as is typically measured by prior art ring electrode probes, whether ~floating" in the atriurn or con-~acting the right atrial appendage. Conventional atrial EKG's typically have a bipolar P wave response, generally denoted by referenced numeral 20, followed by a somewhat smaller amplitude bipolar ~RS response, generally denoted by reference numeral 22.
The wave shape of QRS response 22 is generally the same as P wave response 20 but tends to be smootber and lower in ampli~ude. ~lowever, even within a normal individual, one heartwave complex may dif er significantly rom that measured in another individual or that measured in the same individual at another time. Slight variations in the intervals between responses of various component from those shown in line 18 in the atrial EKG, including varlations in relative amplitudes of each component response, are to be expected in an abnormal heart. ~he variations in the waveform and amplitude of P wave response 20 and QRS response 22 are such that it becomes very difficult, even for a human observer, to perfectly distinguish between the two in all circumstances.
It is even more dif~icult for electronic circuitry and log;c to reliably distinguish between P wave response 20 and QRS
response 22 than it is for a human observer, and this frustration has generally led to a limitation in the development of present day cardiac pacemakers.
Line 24 is a conventional ventricular electrogram derived from a standard contacting electrode system showing the relatively large response generally denoted by reference numeral 26 to ventricular activation corresponding to the QRS and T waves identi~ied in the surface electrocardiogram of line 10. The ventricular EKG has no discernible response ~3 corresponding to the P wave 12.
Line 28 is an electrogram taken in the right atrial appendage from an orthogonal pro~e used and devised according to the present inventionO The sensed signal is a response generally denoted by reference numeral 30 indicative ~f a P wave and has the shape of a sharp, discrete spike of 2-10 mv and 15-40 msec. duration with virtually no other response to any other electrical heart event. Line 32 is a ventricular electrogram sensed according to the present invention showing a large, discrete QRS response generally denoted by reference numeral 34, with a very small T wave response generally denoted by reference numeral 36 with no other discernible responses to any other heart event, including the atrial P wave.
Lines 10, 18, and 24, in comparison with lines 28 and 32, dramatically illustrate the localized and discrete responses obtained in intracardiac electrograms derived according to the present invention as compared to those which can be sensed by prior art apparatus and methods.
Not only is the signal-to-noise ratio substa~tially better in the electrograms shown in lines 28 and 32 of the present invention, but the duration of the response is indicative of a localized or near field heart event only. A probe placed, for example, in the right atrial appendage produces a response as illustrated on line 28, and is completely unafEected by any heart activi-ty in the ventricle, in stark contrast with a conventional atrial electrogram as illustrated by line 18.
Figure 2 ~urther illustrates that the present invention senses only near field events in the heart and provides a sharp depolarization spike in response thereto with an exkremely high signal to-noise ratio~ Surface EKG

=~r J~

10 has beel~ reproduced as the first line 16 of Figure 2 and a conventional atrial electr~gram as line 18 of Figure 2. This is compared against electro~rams sensed by a catheter having multiple orthogonal sensing electrodes, i~e., an electrogram from a sensing electrode in the high right atrium illustrated at line 38 and the electrogram from a sensing electrode located in the lo~er right atrium at line 40.
During normal antegrade conduction, a P wave response ~2, sensed in the high right atrium, i5 clearly indicated at line 38 as originating at a time during the beginning phases of the P waveO However, the P wave as sensed by the electrodes in the lower right atrium at line 40 shows a P wave response 44 appearing at a later time as compared to response 42.
This interval is indicative of the normal propogation delay within the heart from the upper to lower portions of the atrium. In each case, electrograms taken at the high or lower right atrium regions have virtually no response to the massive ventricular signal which is generated in the subadjacent, ventricular heart chamber.
With continued reference to Figure 2, a conventional sur~ace electrogram of a retrogradely propogating heartwave is i:Llustrated at line 46, wherein the P wave response 12' is inverted, and follows the QRS complex. The same sensing electrodes as used in Figure 1, whose output is graphed on lines 38 and 40, produce an electrogram as shown in line 48 from the high right atrium and in the lower right atrium as shown in line 50. The high right atrial response is, as before, a sharp P wave response 52, and in the case of the lower rlght atrium, a sharp P wave response 54. The wave forms of EKGs 48 and 50 are essentially identical to that shown in EKGs 38 and 40, except that their time sequence ~2 ~

~ 3~ ~ 3 with respect to each other is reversed. The high right atrial P wave response 52 i5 clearly later in time that ~he lower right atrial response 54, ~this retrograde atrial depolarization results in an inversion of the P wave 12').
However, a conventional atrial electrogram, such as shown at line 53, corresponding to a retrograde heartwave, is virtually indistinguishable from the conventional atrial electrogram corresponding to a normal heart wave as shown in line 18. Thus, by using multiple sensing si~es according to the present invention, for the first time the propogation sequence of events within a specific cardiac chamber can be accurately detected, and then used to discriminate normal from abnormal heartbeats.
Figure 3 illustrates the implementation of the method of the present invention in a human heart. A sectional fragmentary view of a heart generally denoted by re~erence numeral 56 is diagramatically illustrated in Figure 3, showing a single filamentary catheter 58, which is inserted through one o~ the major veins, according to conventional medical practice, into heart 56. Figure 3 represents a generalized location in the heart and is ~o at least be read as including any location within the atrium or ventricle. Catheter 58 is provided with at least two electrodes 60 arranged to lie in a plane which i5 approximately perpendiculax to t~e depolarization vector ~2 in adjacent myocardial tissue 64.
Depolarization vector 62 mathematically symboli~es the direction of the advancing heart wave in myocardial tissue 64, and is perpendicular to the wave~ront at each point. The sensed slgnals received at electrodes 60 are then transmitted by conventional flexible conductive leads included within ca~heter 58 to a pacemaker circuit (not shown). A conventional pacemaker circuit could be used, although it is expected that the invention will spawn many heretofore unrealizable pacemakers.

F;gure 4 is a d;agramatic section ~aken through lines 4-4 oE Figure 31 illustrating the proximity of electrodes 60 with;n catheter 58 adjacent to myocardial tissue 6~.
It has been determined according to the present invention that electrodes 60, regardless of their angular orientation within the plane illustrated in Figure 4, will sense only electrical activity in the immediate proximity of electrodes 60. Electrodes 60 will not produce a response to heart activity occurring in other chambers of the heart, in muscle tissue which may be exterior hut adjacent to the heart, or outside electromagnetic interference. It should also be noted tha~ in Figure 4 that catheter 58 and electrode 60 in particular are not, and do not neeZ to touch interior wall 68 of heart 56 in order to produce useful signals as is erroneously believed in the prior art.
Figure 5 is a side elevational section taken through lines 5-S of Figure 3, and illustrates the disposition of electrodes 60 of catheter 58 in a plane symbolized by dotted line 68 which is generally and approximately perpendicular to the local depolarization vector 62 in the adjacent myocar-dial tissue 64.
Figure 6 illustrates in a diagramatic sectional view of heart 5~ the insertion of catheter 58 within the fluid filled chambers of the heart, namely right atrium 70 and right ventricle 72. Sensing electrodes 60 in the atrial sensing and ventricular stimulating embodiment, shown in Figure 6, are located in the upper right atrium in region 6a, shown in Figure 6a in enlarged scale. Catheter 58 continues through atrium 70, through tricuspid valve 74, and extends down to the apex 76 of heart 56. A stimulating tip 78 is provided at the end o catheter 58 for implantat;on within ventricular apex 76, as shown in enlarged scale in Figure 6b, illustrating the region of heart 56 generally designated as 6b in Figure 6. A stimulating tip 78 is provided at the end of catheter 58 for direct contact with myocardial tissue of ventricular apex 75. The large voltage, stimulating pulse is delivered by a conventional pacemalcer to tip 78 to initiate the required ventricular contraction according to established medical principles. Stimulating tip 78 may be any conventional tip which may include any fixation device (not shown) which facilitate the implantation and anchoring of tip 78 within adjacent myocardial tissue of ventricular apex 76.
Figure 7 illustrates another embodiment of the present invention wherein catheter 58 is shown as including sensing electrodes 80 included wi~hin ventricle 72 as illus-trated in a diagramatic and fragmentary view in enlarged scale o~ ventricle 72. Electrodes 80 are placed proximate to ventricular myocardial tissue 82 in the ventricular wall oE heart 56. As before, signals sensed by electrodes 80 are co~lpled through conventional flexible leads to a conven-tional pacemaker circuit for subsequent processing and genera~
tion of a stimulating pulse delivered to tip 78~
When sensing is practiced according to prior art methods, the application of a large stimulating pulse to tip electrode 78 completely swamps the sensing circuits attached to the stimulating means placed within the ventricle.
Therefore, heart activit~ which might be initiated by the stimulating pulse delivered to tip 78 cannot be reliahly sensed by any method known to ~he prior art. However, when electrical heart activity is sensed at the location o~ electrodes 80, which are located in a plane which is approximately perpendicular to the proximate depolarization vector ;n adjacent myocardial tissue 82, and the sensed signals are differenced in a differential amplifier, a discrete local;.zed electrogram response 34 such as shown in line 32 of Figure 1 is reliably produced which is not af~ected by the stimulating pulse applied to tip electrode 78, even in that application shown in ~igure 7, where sensin~ electrodes ~0 are within one to three centimeters of the application site of the stimulating pulse.
Therefore, the method of the presen~ invention gives rise to the first true demand pacemaker~ In other words, the stimulating pulse may be generated solely in response to whether or not a previous ventricular stimulating pulse created a heart contraction.
Figure 8 illustrates a third embodiment o~ the present invention wherein cardiac stimulation is provided in the right atrial appendage generally denoted by reference numeral 86, illustrated in fragmentary diagramatic view in enlarged scale. Catheter 88 is formed in the shape of a "J" to allow atrial sensing in the high lateral atrial wall or in right atrial appendage 86 by means of electrodes 90. Again, electrodes 90 generally lie in a plane approximately perpendicular to the proximate depolarization vector in adjacent myocardial tissue (not shown)~ The end of catheter 88 is provided with a stimulating tip 92, similar to the conventional stimulating tip 78 shown and described in connec-tion with Figure 7. Atrial appendage 86 as illustrated in Figure 8 is close to the right ventricular outflow tract generally denoted by reference numeral 94. Notwithstanding electrical activity which might be associated with the ventricle or outflow tract 94, electrodes 90 within atrial appendage 86 sense only the nearfield and proximate heart activity ., }

i3 to produce the high signal-~o-noise output signal as illustrated at line 28 in Figure 1.
The present invention allows the implementation of a fourth embodiment as shown in Figure 9, where sensing of local myocardial depolarization from multiple sites may be achieved. Electrograms are recorded from the lower right atrial region 98 by means oE electrodes 100 and near the atrial floor 102 by means of electro~es 104. As beEore, the high right atrial signals are sensed by electrodes 60.
Each o the electrodes 60, 100 r and 104 lie in a plane which is generally perpendicular to the proximate depolarization vector in the adjacent myocardial tissue. Each of the elec~
trodes 60, 100 and 104 will produce discrete, spike shaped respollse indicative o~ the proximate, local heart activity oE the ~ype as shown in lines 38, 40, 48 and 50 of Figure 2. Each of the electrodes 60, 100 and 104 are coupled through corresponding flexible leads within catheter 106 to a pacemaker circuit (not shown~ for subsequent processing and appropriate generation of a s~imulating pulse. The use of multiple sensing electrodes as illustra~ed in Figure 9 gives rise to cardiac processing which will be based on the sequential progression of heart activity within the heart, and not merely its absence or presence.
Thus, it is evident that the methodology and probe of the present invention can be used in any location in or near the heart structure in a wide variety of probe shapes~
For example, although sensing has been shown both in the atrial appendage, various locations within the atrium, and in the ventricle, it is also possible that a probe could be disposed in the coronary sinus. Again, only local or near field events in the coronary sinus would be received by the probe, and these event could be reliably monitored without interference from nearby lar~e amplitude signal sources.

Figures 10-12 illustrate various embodiments of the probe in diagramatic fragmentary view enlarged scale.
For example~ the probe illustrated in en].arged scale in Figure 10 has been symbolically used in each of the prior Figures and is comprised of two opposing electrode plates 108 and 110. Each electrode plate has a corresponding flexible wire lead 112 and 114, respectively. Electrodes 108 and 110 are circumferentially disposed about catheter 116 on the surface OL an imaginary cylinder 118. The surface of cylinder 118 may actually lie just below the physical surface 120 of catheter 116. The skin of catheter 116, which may be an outer protective and insulating sheeting, is cut away to expose electrodes 108 and 110 to.allow direct contact with the surrounding blood.
. Electrodes 108 and 110 are thus coupled through leads 112 and 114 respectively to a differential amplifier 122, .schematically shown in Figure 10, whose output is the trigger pulse shown and described in Figures 1 and 2. Although leads 112 and 114 have been diagramatically shown as straight wires, in a practical case, the leads are comprised of multi-strancled coil wires with an extremely high fatigue tolerance, whicb is exploited to accomodate the ~lexing expected within a human heart. In the case illustrated in Figure 10 where only two points, electrodes 108 and 110, are used for sens.ing proximate depolarization vector 150, two signals are used as the input to conventional differential amplifier 122.
The output of differential amplifier 122, the spike shaped response, is coupled to processing and pulse generation circuitry 124, which may be of conventional design, included within the subcutaneously placed pacemaker schematically and generally designated by reference numeral 126. Processing ~7--and ].ogic circui~ 124 responsively genera'Les a stimulating pulse accord.ing to principles well known to the art based upon the trigger pulse provided by differential amplifier 122. The stimulating pulse is then coupled thr~ugh a conven-tional flexible lead 128, led back through catheter 116 ~not shown) to the stimulating electrode tip.
Figùre 11 diaseramatically illustrates a fragment of catheter 130, which is provlded with four sensing electrodes 132-138. As in the case of electrocees 108 and 110 of Figure 10, electrodes 132-138 each lie on the surface of an imaginary cylinder 140, defined within catheter 130. In practice, the supporting cylindrical surface/ here described as an imaginary cylinder 140, may actually.be underlying sheating of catheter 130~ or a nonconducting cylindrical ring servins as a form for support of electrodes 132-1380 In the case where four electrodes are used a~ shown in Figure 11, true ortho~30nally sensed signals can be derived. For example, electrodes 132-138 are e~ually spaced about the circumference of catheter 130, thereby being separated by 90 degrees from each other. Electrode 134 is paired with electrode 138 and electrode 136 paired with electrode 132~ Each of the pairs are led through their corresponding flexible leads to an associated differential amplifier. For example, elec-trodes 134 and 138 provide the input signals to-differentj.al amplifier 142, while electrodes 136 and 132 provide the input to differential amplifier 144. The outputs o~ differen~
tial amplifiers 142 and 144 are then provided as input sisenals for processing and pulse generation circuitry 146, s.imilar to circuitry 124, included within subcutaneous pacemaker 148. The outputs of the two differential ampli~iers then comprise an "X" and an "Y" signal, which can then be subse~
quently processed to obtain an absolute magnitude signal regardles~ of the an~ular orientation of catheter 130 within the plane which is perpendicular to the local depolarization vectorl symbolically shown as arrow lS0 in Figures 10-12.
In the e~bodiment of Figure 12, electrodes 152-156 are equally spaced apart on the surEace of an imaginary cylinder 158 defined w;thin catheter 160. In this embodiment, where three electrodes are used, each of the electrodes may be spaced apart by 120 degrees, although it as been determined that sensing is also possible if two of the elec-trodes, for example electrodes 152 and 156, are diametrically opposed on the opposite ends o~ a diameter, while electrode 154 is spaced halfway therebetween, separated by 90 degrees from each oE the electrodes 152 and 156. In any case, elec-trodes 152-156 can be associated in any logical manner to form electrode pairs Erom which to derive a pseudo-orthogonal signal. For example, electrode 156 can arbitrarily be chosen as the co~non electrode and a first signal developed between electrodes 152 and 156 in differential amp].ifier 162. A
second signal can similarly be developed across electrodes 156 and 154 and provided as inputs to differential amplifier 164. The outputs oE differential amplifiers 16Z and 164 thus comprise a pseudo-"X and "Y" signal which can be subse-quently processed by circuit 146 in the manner described in connection with Figure 11.
Figures 10-12 illustrate that the electrodes are clearly two-dimensional, even though they have been discussed as if they were points in connection with the sensing environ-ments in applications described in connection with Figures 1-9. Clearly, point sensing is a mathematical concept which is never realized in practice~ However, the electrodes are sufficiently small such that point sensing .is approached.
The catheter diameter may be in the range of 1 to 4 millimeters, JD ~7~

and it has been experimentally determined that the optimum area for each electrode is approximately 1 o 4 square milli-meters. It has been found that the signal amplitude is degraded if the area of each electrode is increased to, for example, 10 square millimeters.
Viewing Figures 10-12 generally, arrow 150 symboli-cally represents the direction of the depolari~ation vector in adjacent myocardial tissue proximate to the senslng elec-trodes of each of the catheters. The angular orientation of the electrodes with respect to the depolarization vector is immaterial as long as the electrodes are close to each other, namely within one centimeter or less of each other, and as long as the geometric center of each electrode generally lies in a plane substantially perpendicular to depolarization vector 150. It appears that the maximum sensing occurs when the plane of the surface oE each electrode is approximately parallel to an imaginary plane in which the depolarization vector would lieO Small variations from this ideal do not substantially degrade the operability or effectiveness of the present invention.
Furthermore, in each of the embodiments de~cribed in connection with Figures 1-9, the sensing electrodes have been disposed within the heart so as not to be ;n contact with the adjacent myocardial tissue. This is exactly contrary to the prior art belief that it is necessary to make electrode contact with the tissue in order to obtain a reliable and use~ul signal. This was generally true in the case of providing a stimulating pulse to the heart, but proves to be erroneous with respect to sensing signals. In fact, the effectiveness of sensing a local electrical heart event from proximate myocardial tissue is enhanced if no contact is made. Chronic contact leads to fibrosis, which results in total or partial !

~0 ;3 covering of the contacting portion of the catheter by body tissue. Fibrosis is found to qenerally attenuate the si~nal, which can be sensecl from the elec~rodes thus covered. Although it is imperEec~ly understood, it appears that the local electrical heart event can be more effectively sensed if the electrodes are no~ in direct physical contact with the adjacent myocardial tissue. Thus, no provision is included within the methodology of the present invention, contrary to prior art teachings, ~or touching or affixing the electrodes to adjacent tissue. In fact, the electrodes described in connection with each of the embodiments shown in Figures 10-12, are slightly recessed within the respective catheter~
such ~hat even if the catheter should make physical contact with the adjacent tissue, the electrodes themselves would not be physically contacted.
Figure 13 is a flow diagram generally summari~ing the practice of the method discussed in connection with ~he embodiments described in Figures 2-9 with the probes as ill~lstrated in Figures 10-1~. The flow diagram in Figure 13 illustrates a single heart cycle.
A cardiac wave is sensed at step 16Z to derive a local electrogram as described above. In the case of the catheter of Figure 10, a single signal is generated as the output of diferential amplifier 122, while the embodi-ments o~ Figures 11 and 12 each generate a multipolar signal from the corresponding plurality of differential amplifiers.
In an~ case, a plurality of pairs of signals are sensed in step 166, with each associated pair being differenced at step 168 to produce one or more trigger or response pulses~
In the illustrated e~bodiment, differencing has been described as effected by an analog differential amplifer. However, any equivalent means could also be employed, such as digitizing the sensed signals and taking their ari~hmetic average at step 166.

-~r The trigger signals derived at step 166 are then processed at step 168 in any one of an arbitr~rily large number of different ways according to well understood design principles and according to principles t`nat can be expected to be developed, once a reliable sensed signal is available as in the present invention.
One way in which the signals may be processed at step 16~ allows variable amplitude pacing to be realized.
The voltage of a train of stimulating pulses can be varied depending upon the success of the just prior pulse, or the success percentage of a group of prior pulses in stimulating a responsive heart contraction. Thus, if the stimulatin~
pulse fails to cause the heart muscle to depolarize because it is of insuEficient strength, this failure will be sensed and the lack of a response or trigger pulse at the output of the differential amplifier coupled to the sensing electrodes will be noted at the processing circuitry at step 168. The next subsequent pulse can then be generated at a predetermined increment in stimulus strength. An iteration can continue until successful pulse output has been achieved, at which point the pacing pulse output can be held stable until ~he heart conditions once again change. This can be expected, for example, in conditions where the responsivity of myocardial tissue i5 altered by drugs or ischemia.
Thus, the magnitude of the current of the stimulating pulse can be increased depending upon the success of the prior pulse or any group of prior pulses in creating a heart contraction or any other predetermined pattern of heart activity. The criterion upon which the temporal spacing of stimulating pulses or the magnitude oE pulses may be conditioned, can be determined in a large variety of ways according to well understood medical principles. The manner . ; ~_ i3 in which infor~ation is processed by the sensing method described herein is not lim;ted or restricted in any way by the sensing method.
Step 170 may include decision logic which may result in a decision to inhibit the generation of a cardiac stimulating pulse, thereby resetting the pacemaker to its initial condition at step 17~. On the other hand, a decision can be made, based upon objective criteria, to generate or trigger a cardiac stimulation, thereby entering step 174, wherein the appropriate s~imulus is generated and delivered.
Thereafter, the pacemaker may reset and return to its ini~ial-ized state at step 176.
~ s a result of the processing at step 170, a decision is made according to processing rules whether or not to generate a stimulating pulse, and îf so, what type of stimula-ting pulseO Not only the timing and amplitude o~ the pulse may be varîed, as described above, but the location of the application of the pulse may also be selected. For example, two st;mulating electrodes may be provided on the same or different catheter to provide a selection of points at which stimulus May be delivered to the adjacent heart tissue.
In addition, bifurca.ed or multit;pped catheters can allow the delivery of an appropriate stimulus to any one of a multiple number of heart locations, for exa~ple, the ventricular apex on one hand, and the atrial appendage on the other.
Similarly, selective stimulation in the atrium and ventricle could be easily prac~iced depending upon the heart activity which has been observed according to the present method at any point or points within the heart.
Thus, Figure 13 illustrates the flexibility and power of the sensing methodology of the present invention.

By being able to reliably and discriminatorily pick the local electr;cal hear~ ~ave out from the welter of electronic signals, both natural and artificial, which are present in the l-uman heart, the heart may be paced for the first time in virtually an unlimited and arbitrary fashion. The capacity of the method to sense a local heart wave event at any locat;on within the heart substantially increases the ability to detect heretofore unmonitored conditions and patterns within the heart and to selectively respond thereto.
It must be understood that the various embodiments of the method of the present invention described above are set forth only for the purposes oF illustration~ and are not to be taken as limiting the scope of the present invention.
~an~ alterations and modifications can be made by those having ordinary skill in the art without departing from the spirit of the present invention as set forth in the following claims~

Claims (36)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An improvement in a method for sensing heart activity corresponding to a depolarization vector in a heart comprising the steps of:
discriminating a local cardiac signal from other intracardiac signals by sensing said local cardiac signal at at least a pair of points lying in a plane generally perpendicular to said depolarization vector in proximate heart tissue, and comparing signals sensed at said pair of points in said plane to derive a difference signal indicative of said local cardiac signal, whereby said local cardiac signal may be reliably sensed and selected from said other intracardiac signals.

2. The improvement of claim 1 where said step of discriminating said local cardiac signal by sensing car-diac activity at said pair of points in said plane includes the step of sensing said cardiac activity at a location within said plane proximate to said heart tissue wherein neither one of said pair of points is in contact with said heart tissue.

3. The improvement of claim 1 where said step of discriminating said local cardiac signal includes sensing said heart activity at more than two points in said plane.

4. The improvement of claim 3 where said step of discriminating includes sensing said heart activity at three points.

5. The improvement of claim 4 wherein said heart activity is sensed at said three points equally spaced about a circle lying in said plane.

6. The improvement of claim 1 where said step of discriminating said local cardiac signal includes sensing said heart activity in the proximity of ventricular tissue.

7. The improvement of claim 1 further comprising the step of generating a responsive reaction to said discriminatorily sensed local cardiac signal in response to said difference signal.

8. The improvement of claim 7 wherein said reaction includes the step of generating a ventricular pacing signal.

9. The improvement of claim 8 where the step of generating said ventricular pacing signal is contingent upon verification of the existence of said discriminatorily sensed local cardiac signal, said ventricular pacing signal being generated if said discriminatorily sensed local car-diac signal is absent.

10. The improvement of claim 9 where in said step of generating, said ventricular pacing signal is generated at a rate determined by a computed measure of a plurality of prior discriminatorily sensed local cardiac signal.

11. The improvement of claim 1 wherein said step of discriminating said local cardiac signal includes sensing said heart activity proximate to atrial heart tissue.

12. The improvement of claim 11 where said step of sensing said heart activity includes sensing said heart activity proximate to the high lateral area of the atrium.

13. The improvement of claim 11 wherein said step of sensing said heart activity includes sensing said heart activity in the proximity of the atrial appendage.

14. The improvement of claim 11 where said step of sensing said heart activity includes sensing said heart activity in the proximity of the coronary sinus.

15. The improvement of claim 11 where the step of sensing said heart activity includes sensing said heart activity in the proximity to the floor of the atrium.

16. The improvement of claim 1 further comprising the step of selectively generating a response to said discriminatorily sensed local cardiac signal in response to said difference signal.

17. The improvement of claim 16 wherein said response includes generating a ventricular pacing signal.

18. A method for discriminating a local cardiac signal corresponding to a depolarization vector in a heart from intracardial signals within said heart, comprising steps of:
sensing heart activity in said heart at at least two closely positioned locations, each of said locations lying near a plane generally perpendicular to said depo-larization vector in myocardial tissue proximate to said locations, and differencing signals sensed at at least two loca-tions to derive a trigger pulse indicative of said local cardiac signal, whereby said local cardiac signal may be reliably sensed and selected from said heart wave regardless of selection of said locations within said heart.

19. The method of claim 18 where said step of sensing said local cardiac signal includes sensing said local cardiac signal at three locations lying near said plane, said three locations associated with each other to form two pairs of locations, one of said three locations serving as a common location for purposes of said association.

20. The method of claim 19 where said step of sensing said local cardiac signal includes sensing said local cardiac signal at three equally spaced locations.

21. The method of claim 18 where said step of sensing said local cardiac signal at said locations inclu-des disposing an electrode at each said location to generally lie in a plane parallel to said depolarization vector in said proximate myocardial tissue, and sensing said local cardiac signal at each said location by means of said electrode.

22. The method of claim 18 where said step of sensing said local cardiac signal includes sensing at each location by means of an electrode having a convoluted surface.

23. The method of claim 22 wherein said step of sensing said local cardiac signal by means of said electrode includes sensing said local cardiac signal by an electrode having a geometric extent circumscribed by a pla-nar geometric surface in the range of 1 to 4 square milli-meters, and wherein each said location is separated from each of said other locations by no more than 10 millimeters.

24. A method for discriminatorily sensing a local cardiac signal corresponding to a depolarization vec-tor in a heart from a heart wave complex and extracardial electromagnetic noise, comprising the steps of:
sensing local electrical activity in said heart at a plurality of closely spaced points lying near a plane generally perpendicular to said depolarization vector defined in proximate myocardial tissue;
differencing said local electrical activity sensed at said plurality of points to obtain at least one trigger pulse indicative of said local cardiac signal;
determining whether a stimulating pulse should be generated in response to said at least one trigger pulse;
and generating said stimulating pulse for coupling said stimulating pulse to a heart to stimulate contraction thereof, whereby demand pacing of said heart may be effected.

25. The method of claim 24 where said step of sensing said electrical activity includes sensing at said plurality of points with a corresponding plurality of electrodes located at said plurality of points, wherein none of said electrodes is in physical contact with said proximate myocardial tissue.

26. The method of claim 24 wherein said step of determining includes determining whether to generate a sti-mulating pulse depending upon the presence or absence of a P wave following a prior stimulating pulse.

27. The method of claim 24 or 26 where said step of determining includes determining whether to change the magnitude of a stimulating pulse, depending upon whether or not said steps of sensing and differencing indicates whether or not a specified local cardiac signal was present in said heart wave complex at the location sensed at a time subsequent to the occurrence of the just prior stimulating pulse, the magnitude of said stimulating pulse being increased when said steps are indicative of said specified local cardiac signal being absent and the magnitude of said stimulating pulse being decreased when said steps are indi-cative of said specified local cardiac signal being present.

28. The method of claim 24 where said step of determining includes determining the temporal spacing bet-ween consecutive stimulating pulses depending on whether said steps of sensing and differencing are indicative of the presence or absence of a specified local cardiac signal.

29. The method of claim 24 where said step of sensing includes sensing said electromagnetic activity at a plurality of locations, each location being sensed at a plurality of closely spaced points lying near a plane generally perpendicular to the depolarization vector defined in proximate myocardial tissue.

30. The method of claim 29 where said step of differencing includes differencing said electromagnetic activity at each said location as sensed at said corresponding plurality of points to generate a corresponding plurality of trigger signals.

31. The method of claim 29 or 30 where said step of determining includes detecting a discriminatorily sensed local and sequenced pattern of heart activity from said plurality of trigger signals.

32. An electrode system for sensing intracardiac signals, comprising:
a catheter in the form of a single non-diverging filament for insertion into the heart through the vascular system;
sensing electrode means on said catheter and spaced from the distal end of said catheter in said heart, said sensing electrode means positioned adjacent an inter-nal wall of said heart and for sensing signals varying as a function of a local intracardiac wave and comprising at least first and second electrodes mounted in said catheter wall equidistant from said distal end and insulated from each other to provide a bipolar signal.

33. The electrode system of claim 32 wherein said sensing electrode means comprises first, second, third and fourth electrodes mounted in the wall of said catheter equidistant from said distal end and substantially equally spaced from each other, said first and third electrodes disposed oppositely from each other to form a first set, and said second and fourth electrodes disposed oppositely from each other to form a second set, such first and second sets of electrodes sensing orthogonal components of said local intracardiac wave.

34. The electrode system of claim 32 wherein said sensing electrode means produces signals varying as a function of the cardiac P-wave, and is positioned adjacent to the atrial wall.

35. An apparatus for sensing heart activity corresponding to a depolarization vector in a heart, comprising:
first means for discriminating a local cardiac signal from other intracardiac signals by sensing said local cardiac signal at at least a pair of points lying in the plane generally perpendicular to said depolarization vector in proximate heart tissue; and second means for comparing signals sensed at said pair of points in said plane to derive a difference signal indicative of said local cardiac signal, whereby said local cardiac signal may be reliably sensed and selected from said other intracardiac signals.

36. A probe for sensing heart activity corresponding to a depolarization vector in a heart, comprising:
a single, nondiverging filamentary catheter; and means for discriminating a local cardiac signal from other intracardiac signals by sensing said local car-diac signal at at least a pair of points lying in the plane generally perpendicular to said depolarization vector in proximate heart tissue, whereby said local cardiac signal may be reliably sensed and selected from said other intracardiac signals.