retrograde activation of the his bundle in the human heart

Retrograde Activation of the His Bundle in the Human Heart

CESAR CASTILLO, MD

AGUSTIN CASTELLANOS, Jr., MD, FACC

Miami, Florida

The catheter technique of His bundle (HBE) recordings proved to be useful in the analysis of retrograde conduction in the human heart. Retrograde activation of the His bundle was more easily detected in bipolar HBE leads with interelectrode distances of 1 mm (or less) than in leads with an interelectrode distance of 10 mm.

A high bipolar atrial electrogram (BAE) simultaneously recorded with the HBE was essential to differentiate between superoinferior (sinoventricular) or inferosuperior (retrograde) activation of the atria. V-A delays occurred both above (nodal region) and below (Purkinje- bundle branch specialized tissues) the His bundle. The greatest area of delay was seen in the A-H (nodal) region. Ventricular echoes were observed during continuous and intermittent paired ventricular pacing. The atria appeared to be a necessary link in the production of echoes. The echoes, which passed through the His bundle during their forward and retrograde propagation, were best explained by a functional (longitudinal) intranodal dissociation. The reciprocating circuit was represented as follows: V -+ H + FCP + a + A + p + FCP + H + V. The study of His bundle recordings has enhanced our knowledge of cardiac electrophysiology.

The catheter technique of His bundle recordings has enhanced our knowledge of cardiac electrophysiology,l-7 and valuable information has been obtained regarding the nature of atrioventricular and intraventricular conduction disturbances.*-5 However, since less emphasis has been placed on ventriculoatrial conduction, this paper presents the various patterns of retrograde activation of the His bundle and discusses the probable underlying mechanisms.

Material and Methods

From the Section of Cardiology, Depart- ment of Medicine, University of Miami School of Medicine and the Veterans Ad- ministration Hospital, Miami, Fla. Manu- script received February 4, 1970, accepted April 6, 1970.

Address for reprints: Agustin Castel- lanos, Jr., MD, University of Miami School of Medicine, P.O. Box 875, Biscayne Annex, Miami, Fla. 33152.

The technique of His bundle recordings used in our laboratorys1° is essentially an extension of the original method introduced by Scherlag et a1.l in 1969. After obtaining appropriate consent from the patient, a tripolar or a bipolar catheter electrode was introduced through the femoral vein and positioned across the septal leaflet of the tricuspid valve. This cathet,er, used to record the His bundle electrogram (HBE) , was slowly withdrawn across the tricuspid valve until the His bundle deflection appeared on the oscilloscope screen. The interelectrode distance was 5 or 10 mm. To evaluate the reliability of the interelectrode distance in the analysis of retrograde conduction, 2 special catheters were used in 2 patients. One catheter had 4 poles (constructed by B. V. Berkovits, EE, American Optical Co., Boston, Mass.) with 3 electrodes separated by a distance of 0.5 mm and the fourth electrode placed 10 mm from the distal electrode (Fig. 1). The other catheter had 3 poles (Ele- cath Corp., N.J.) with a distance of 1 mm between the 2 distal electrodes and a distance of 11 mm between the proximal and the distal electrodes.

264 The American Journal of CARDIOLOGY

RETROGRADE ACTIVATION OF HIS BUNDLE

Two additional bipolar catheters were introduced per- cutaneously through an antecubital vein. One, placed high in the right atrium (BAE), was used to obtain a bipolar tracing from this site. The other catheter was used for atria1 or ventricular pacing, as needed. Fil- tered (40 to 2,000 cycles/set) bipolar leads were recorded simultaneously with the standard leads attach- ing the terminals from the catheters to an electrode distribution box, which was then connected to an Elec- tronics for Medicine recorder. Paper speed was 50 or 100 mm/set. Electric signals were also stored on mag- netic tape for future playback. During sinus rhythm, the onset of atria1 depolarization was measured from the beginning of the P wave in lead II. In these cases, inscription of P in the high right atria1 electrogram preceded that of the A spike in the His bundle electrogram, indicating superoinferior spread of activation. When retrograde (V-A) conduction occurred, the re- verse was observed, that is, the earliest moment of atria1 activity was recorded in the low right atrium, indicating reversed excitation of the atria. The onset of ventricular activity was recorded from lead II. When atria1 or ventricular pacing was performed the emis- sion of the spike was equated with the beginning of depolarization in the corresponding chamber. The fol- lowing intervals were measured whenever possible : P-R, P-H, H-R, R-P, R-H, P-H and P-R.

Electrical stimuli were delivered by means of a spe- cially constructed pacemaker which could be used for continuous, paired or coupled pacing.l The pulses were slightly underdamped, 2.5 msec in duration. Current in- tensity was twice diastolic threshold. In 1 patient the ventricles were driven at a rate of lOO/min. Testing stimuli with progressive shorter couplings were delivered after every eighth driving beat. The tracings of several patients showing interesting features of retrograde His bundle excitation will be presented.

Results

Retrograde (V-A) Conduction of Ectopic Ventricular Beats

Case 1: The tracing in Figure 2 was obtained from a patient with coronary sinus rhythm. The P waves were negative in lead II, as well as in leads III and aVF (not shown). The P-R interval measured 140 msec. Values for the P-H and H-R intervals were 105 and 35 msec, respectively. The second QRS complex was pacemaker-induced. The corresponding P wave (which coincided with the stimulus artefact) did not contribute to ventricular depolarization. A His bundle deflection was not identified within this beat. The third QRS complex was a spontaneous ventricular extrasystole with a short coupling-in fact, it fell on the descending limb of the antecedent T wave. This premature ventricular beat showed retrograde conduction. In consequence, the resulting P-P- cycle was shorter than that of the basic interatrial cycle. A His bundle (H-) deflection appeared to be “sandwiched” between R and P-. Retro- grade activation of the His bundle must have been present, since H- was located too far away from the second P wave to have been induced by it.

With the technique used in our laboratory, retrograde activation of the atria or His bundle is not re-

VOLUME 27. MARCH 1971

Figure 1. Photograph of a recording catheter electrode distance of 0.5 mm.

with an inter-

corded as the mirror image of that present during antegrade sinoventricular conduction. Moreover, the large ventricular electrogram frequently obscures the smaller H deflection in the majority of cases in which they coincide. This is illustrated in the last beat in Fig- ure 2, which was also pacemaker-induced. Although there was retrograde conduction to the atria, the H deflection could not be identified. The corresponding R-P- interval was shorter than that of the previous beat. In this case, retrograde conduction appeared to be

Figure 2. Case 1. Intermittent ventricular pacing in a patient with “coronary sinus rhythm.” Retrograde activation of the His bundle (H-) and of the atria (P) occurred after the third QRS complex. Bipolar leads recording the His bundle electrogram are labeled HBE. The interelectrode distance was 10 mm on the top HBE tracing and 5 mm on the bottom one. In this figure conventional ladder diagrams were modified to accommodate the His bundle deflection. A = atria; H = His bundle; V = ventricles. The “coronary sinus” is represented as lying between the atria and the His bundle. Paper speed 50 mm/set.

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CASTILLO ET AL.

A-V

EX. DlSSOCliiTlON

Figure 3. Case 2. Ventricular extrasystole (Ex.) with retrograde activation of the His bundle and of the atria leading to a short period of A-V dissociation. Paper speed 50 mm/set.

Pi d P-

H \ \ / * ,

I I I

“I Figure 5. Case 3. Retrograde activation of the His bundle (H-) and of the atria (P) in the third cycle. The interelectrode distance was 10 mm. Paper speed in Figures 5 to 10 100 mm/set. BAE = bipolar atrial electrogram, that is, a bipolar lead recording from the high right atrium.

P’ Figure 4. Case 2. Enlarged photograph of the ventricular extrasystole (presented in Fig. 2) which showed retrograde activation of the His bundle and atria. Paper speed 50 mm/ sec.

Figure 6. Case 4. Retrograde activation of the His bundle (H-) after a closely paired ventricular beat. The P wave follow- ing this H- deflection was not retrograde, but sinus in origin. Note that the onset of atrial activity occurred in the BAE, indicating superoinferior spread of activation throughout the atria. The interelectrode distance of the HBE was 10 mm. St = pacing stimulus.

266 The American Journral of CARDIOLOGY


directly proportionate to the prematurity of the QRS complex and perhaps to the length of the preceding R-R cycle.

Case 2: Figure 3 starts with 2 beats of sinoatrial origin. The corresponding P-R, P-H and H-R intervals were 160, 100 and 60 msec, respectively. The second QRS complex was followed ‘by a ventricular extrasystole with a short coupling. The premature contraction had retrograde conduction to the atria. Hence, the P-P- interval was shorter than the basic interatrial cycle. The ventricular stimulus reached the atria through the His bundle deflection buried inside the bizarre ventricular complex. (This beat is amplified in Figure 4.) There was a slight postextrasystolic sino- atria1 depression so that the pause was terminated by an escape, which in turn initiated a short run of A-V dissociation. The escape beat was preceded by an H deflection showing an H-R conduction time similar to that of sinus beats. The positive P wave in lead II occurred after the H deflection, indicating that they were unrelated. Similarly, the last P-H interval was also too short to suggest atrioventricular conduction. In other words, in the last 2 beats the His bundle had been activated before the atria1 impulse had time to reach the bundle. Since the corresponding QRS complexes were similar to those present during A-V conduction, they fulfilled the criteria of Damato and Lau3 for His bundle rhvthms.

was recorded in the atria1 electrogram (BAE) earlier than in the His bundle electrogram. The forward H deflection was located between the P wave and the QRS complex. Note that it was smaller than the ventricular complexes in the HBE lead. The second (pacemaker- induced) beat appeared in late diastole. It was preceded by a sinus P wave which was distorted (in the HBE lead) by the stimulus artefact. Activation of the His bundle could not be seen since the deflection was buried in the QRS complex. Finally, the tracing ended with an artificial beat closely paired to the previous beat. Retro- grade activation of the His bundle (H-) occurred after the second iatrogenic extrasystole. The R-H- interval measured 200 msec, indicating that there was a significant infra-His bundle (retrograde) delay. This deflection was not an atria1 extrasystole since it had no counterpart in the BAE. The P wave that followed the H- deflection was not retrograde. On the contrary, it was similar to the 2 preceding P waves (of sinoatrial origin), and was therefore inscribed first in the high bipolar lead from the right atrium (BAE). A-V conduction did not occur because of the refractoriness cre- ated by the retrograde activation of the His bundle. Similarly, retrograde atria1 conduction was not seen, probably because the atria had been depolarized by the sinus impulse.

Ventricular Echoes

Case 3: The tracings presented in Figure 5 were obtained from a patient with complete left bundle branch block and a P-R interval of 220 msec (measured from lead II). The duration of the P-H interval of the first (sinus) beat was 140 msec. The H-R interval was prolonged to 80 msec. (The upper limit of normal in our laboratory is 55 msec.) Therefore, this patient had (forward) delay below the His bundle. Since the left branch was presumably “completely” blocked, the infra- His bundle delay probably occurred in the right branch. Therefore, the diagnosis of bilateral bundle branch block (complete in the left, incomplete in the right) was made.2

An echo beat occurs when the timing of a retrograde P wave is appropriate to allow reexeitation of the ventricles through the His bundle. Although appearing more frequently during paired ventricular stimulation, echoes are not rare during continuous pacing at slow rates.

Case 4: Figure 7 was obtained from the same patient whose tracings are presented in Figure 6. The first 3 QRS complexes were pacemaker-induced. The first P wave was of sinoatrial origin. A His bundle

During sinus rhythm (first 2 beats) the onset of atria1 activity was recorded in the high bipolar atria1 (BAE) lead. The third (pacemaker-induced) QRS complex was followed by a negative P wave with V-A conduction time of 140 msec. The P-P- interval was shorter than the regular P-P cycle. In addition, the P- wave was inscribed in the HBE lead ‘earlier than in the lead from the high right atrium (BAE). This indicates retrograde activation of the atria. The H- deflection resulting from retrograde activation of the His bundle appeared 90 msec after the stimulus artefact and 65 msec ahead of the negative P- wave. This H- deflection, although buried inside the QRS complex, could be identified because of its large size. The last QRS complex (also pacemaker-induced) was not propagated to the atria. The block occurred below the His bundle since an H- deflection was not observed within the ventricular complex.

Case 4: The records in Figures 6 and 7 were obtained from a patient with complete right bundle branch block and a P-R interval of 185 msec. The P-H interval measured 50 msee. The first beat in Figure 6 is of sinoatrial origin. In consequence, atria1 activation

BAE

Figure 7. Case 4. His bundle electrogram during a ventricular echo (third and fourth cycles). The major area of both retrograde and forward delay occurred in the A-H region. The interelectrode distance of the HBE was 10 mm.

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Figure 8. Case 5. His bundle electrogram during a ventricular echo (last 2 cycles). The major area of retrograde delay occurred in the A-H region. HBE = His bundle electrogram with an interelectrode distance of 10 mm. CHBE x close His bundle electrogram (bipolar lead with an interelectrode interval of 0.5 mm, Fig. 1). Whereas the forward H deflection was seen in both HBE and CHBE, the retrograde (H-) deflection was recorded in the latter and not in the former. Numbers at the A-H and H-V level in the diagram are expressed in msec and represent duration of propagation through the corresponding regions.

deflection was not observed within the first 2 ventricular complexes. However, retrograde activation of the His bundle (H- ) appeared at the end of the third QRS complex. This H- deflection was followed 260 ms’ec later by a retrograde P- wave (negative in lead II). Atria1 activity appeared first in the HBE lead, as ex- pected. The retrograde P wave was properly timed to reenter the A-V node. Thereafter, it traversed the His bundle in an antegrade fashion, finally reexciting the ventricles. An echo was thus produced. Values for the corresponding interval (in msec) were as follows: R-H- 110, H-P- 260, P--H 220 and H-R 50. The greatest area of delay for both retrograde and forward conduction occurred at the A-H region. The reciprocating beat engaged the His bundle in a forward as well as in a retrograde fashion. Variations in size of the P wave in the HBE lead were due to slight movement of the recording catheters.

It should be stressed that the R-P- interval was much longer than the P--R interval, as had been postulated by previous investigators’“-l” who did not use His bundle recordings.

Case 5: Figure 8 was obtained from a patient with complete right bundle branch block and abnormal left axis deviation due to block in the anterosuperior subdi- vision of the left branch. The P-R interval was prolonged (225 msec) . In the first (sinus) beat the P-H interval measured 155 msec. The H-R interval was prolonged (70 msec) . This indicated significant infra- His bundle delay. Since both the right branch and the anterosuperior division of the left branch were “completely” blocked, the infra-His bundle delay must have occurred in the inferoposterior division. Hence, this patient had trifascicular block, complete in 2 fascicles and incomplete in the remaining fascicle.16s17

In the close His bundle electrogram (CHBE), that is, in the recording obtained with an interelectrode dis-

Figure 9. Case 6. Retrograde activation of the His- bundle and of the atria during ventricular pacing (St). Note that the retrograde H deflection (second and third cycles) was seen better in the HBE lead with an interelectrode distance of 1 mm (top) than in the lead with a distance of 11 mm (bottom). Arrows show direction of atrial activation.

tance of 0.5 mm (Fig. l), the H deflection in the HBE coincided with that of the CHBE during forward (A-V) conduction. The second, third and fourth QRS complexes were pacemaker-induced. Retrograde activation of the His bundle (H-) was detected in the CHBE in all the paced beats, but not in the HBE (R-H interval of 75 msec). Retrograde conduction from the second and third H deflection was greatly delayed in the A-H region, so that the atria could not be activated in a retrograde manner before the sinus node discharged. However, a slight sinus arrhythmia allowed the H- deflection within the last pure pacemaker beat (fourth QRS complex) to be conducted in a retrograde manner to the atria. The corresponding P wave (inverted in lead II and initially inscribed in the HBE lead) was able to reenter the A-V node and traverse the His bundle in a forward fashion, finally exciting the ventricles totally (or in part). Note that the forward H deflection in the HBE coincides with that of CHBE. Values for the corresponding intervals (in msec) were as follows: R-H- 75, H--P- 420, P--H 125 and H-R 70. The reciprocating beat engaged the His bundle during both its forward and retrograde propagation. As in Figure 7, the greatest areas of delay during both retrograde and forward conduction occurred at the A-H region. The R-P- interval was also longer than the P--R interval.

Case 6: The recordings shown in Figure 9 are presented for further evaluation of the conventional 10 mm bipolar His bundle lead. They were obtained from a 53 year old patient with angina pectoris and possible old inferior wall myocardial infarction.

The bottom HBE lead was recorded with an interelectrode distance of 11 mm. In the lead immediately above, this distance was only 1 mm. During sinus rhythm the P-R, P-H and H-R intervals measured 160, 120 and 40 msec, respectively. The H deflection was



sandwiched between the atria1 and ventricular deflections in both HBE leads. The P waves were hardly visible in the 1 mm lead. The last 2 QRS complexes showing abnormal left axis deviation were induced by pacing from the apex of the right ventricle. Both were followed by retrograde P waves (R-P intervals of 180 msec) which were inscribed in the 10 mm HBE lead earlier than in the lead recording from the high right atrium (BAE). Retrograde activation of the His bundle with an R-H interval of 50 msec was seen in the 1 mm HBE lead but was inconsistent in the 10 mm lead. That these deflections were retrograde (and not antegrade) is suggested by the fact that the first deflection was not preceded by a P wave falling in the preceding R-R cycle. It obviously could not have been related to the first (sinus) P wave. The retrograde H deflection was inscribed before the local ventricular electrogram in the close bipolar lead. This occurred because thd impulse originating in the stimulated site (in the right ventricular apex) propagated to the inflow tract (where the recording catheter was located) more rapidly through the conduction system than through the or- dinary muscle fibers.

Case 7: The tracings presented in Figure 10 were obtained from a 45 year old man with a clinical <iag- nosis of alcoholic cardiomyopathy. The conventiotal 12 lead electrocardiogram showed no evidence of intraventricular block. During sinus rhythm the P-R interval measured 200 msec. In this patient the H-R interval was normal (45 msec) .

Figure 10 shows the effect of intermittent ventricular pacing at close VI-V2 intervals (250 msec>.14 The driving rate was lOO/min (R-R cycle of 600 msec) (first 2 QRS complexes). There was persistent retrograde conduction, so that the onset of atria1 activity was recorded in the HBE. The basic R-P interval of the first 2 beats was 270 msec. The third (paired) QRS complex appeared very prematurely. In consequence, it had a different configuration from that of the basic intervals. This is a characteristic of stimulation during the relative refractory period. Its R-H- interval measured 190 msec. In addition, the third (premature) stimulus artefact fell in the vulnerable period of the antecedent beat and produced repetitive firing (fourth and fifth ventricular complexes). The R-H- interval of the fourth QRS complex was longer (240 msec) than the previous R-H- (190 msec) (reversed infra-His bundle Wenckebach phenomenon). However, retrograde activation of the atria did not occur, probably because of the appearance of a sinus P wave. The latter was inscribed first in the lead from the upper part of the right atrium (BAE). Finally, the last beat at the end of the strip shows normal sinus rhythm and intraventricular conduction. Atria1 activity was also recorded earlier in the BAE than in the HBE.

The diagram shows that the last ectopic QRS complex was not a ventricular echo with aberrant eonduc- tion. This occurrence would have required the deflection site to be located below the His bundle, an unlikely possibility. This assumption is in keeping with the work of others12-15.1s-20 who postulated that ventricular echoes are due to intranodal (that is, supra-His bundle) dissociation.

Discussion

Retrograde His bundle conduction: His bundle (HBE) recordings have been most useful in the unraveling of complex disorders of rhythm.* How- ever, primarily for technical reasons, few studies have dealt with retrograde activation of the His bundle. With 5 or 10 mm bipolar HBE leads the retrograde H deflections can be missed easily if buried in the QRS complexes, whereas the 0.5 mm and 1 mm leads are superior in this respect (Fig. 8 and 9). This is because the magnitude of the local potential depends on the interelectrode distance, and the shorter the distance the more “local” the potential recorded. Yet with the 1 mm electrode, the extrinsic potentials were not completely abol- ished, at least with the frequency of the filters used, possibly due to valve motion.

Moreover, special care had to be taken to deter- mine whether an H deflection or a P wave was retrograde in origin, since in the HBE leads they did not appear as the mirror image of the waves recorded during antegrade conduction. Moreover, the polarity of the P waves could not be determined with certainty when they fell in the beginning of the T waves of the surface leads. The high bipolar atria1 lead (BAE) was most helpful in differentiat- ing between sinoventricular and ventriculoatrial conduction. This distinction was more difficult in the case of the H deflections. Yet, when an H-H interval was shorter than the control interval (Fig. 3, 5, 8 and 9), the absence of a P wave in the pre-

A I I I A-H / / P. /5>

H 7 7 I H-V //N/r

V I

Figure 10. Case 7. His bundle electrogram during intermittent paired ventricular pacing at short intervals. The first 2 beats were produced by the driving impulse (rate lOO/min, cycle length 600 msec). The third stimulus artefact fell in the vulnerable period of the antecedent T wave and produced repetitive firing (2 extra QRS complexes). Retrograde activation of the His bundle with considerable infra-His bundle delay occurred after the third and fourth ventricular beats.

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ceding R-R cycle (Fig. 2, 3, 5, 8 and 9) and the presence of a retrograde P wave after the QRS complex (Fig. 2 to 5 and 7 to 9) favored a retrograde origin of the H deflection under consideration. Even with these limitations, Figures 2 to 7 and 10 show that retrograde activation of the His bundle can be identified in some patients with the 5 or 10 mm HBE leads. Knowledge thus obtained has aided understanding of the mechanisms of retrograde conduction in the human heart.

Retrograde block below the His bundle: Re- cordings from the specialized conducting tissues of the heart showed that retrograde conduction was delayed below as well as above the His bundle. Infra-Hfs bundle block in early ventricular ectopic beats can occur within the ventricular myocardium itself, that is, between the stimulated area and the point where the impulse enters (in a retrograde fashion) the Purkinje system. However, delay could also be present in the Purkinje- bundle branch system ; in fact, some of the patients studied had evidence of forward block in this region.

Retrograde block below the His bundle is not unexpected. In 1926, Scherf and Shookhoff13 first postulated that ventricular extrasystoles not conducted to the A-V node could delay A-V transmission of the subsequent beat. However, as stated by Moore,21 most studies of concealment of premature ventricular responses have concentrated mainly on antegrade conduction.

The presence of retrograde block between the Purkinje-bundle branch system and the His bundle seems paradoxical since the duration of the action potential and of the functional refractory period of the latter is shorter than that of the former. Moore’s observations21 are interesting in this respect. He observed that progressive reduction of cycle lengths produced by a series of consecutive ventricular premature beats could make the functional refractory period of the right bundl,e branch shorter than that of the His bundle. He also suggested that His bundle fibers have a higher threshold for excitation at the functional refractory period so that retrograde block could also result when the impulse arrives at the less excitable His bundle fibers.21

Retrograde intranodal conduction: His bundle recordings in man also demonstrated the existence of retrograde block above the His bundle. This finding is in keeping with the work of Moe et al.18JQ In fact, the greatest delay appeared in this region. The most interesting studies of retrograde conduction in the human heart were made by Schuillen- burg and Durrerl4 in 1969. They paced the ventricles at a constant driving rate ( VI-V1 interval). Testing stimuli (V,) were delivered after every. eighth driving stimulus, at progressively shorter V,-V1 intervals. A similar technique was used in the patient presented in Figure 10. Schuillenburg

and Durrer observed, as in the present study, both supra- and infra-His bundle retrograde block.14 In addition, their work corroborated the concepts of Moe et al.,lQ who had postulated the presence of 2 (high) intranodal separate pathways converging into a final common pathway feeding into the sub- nodal specialized conduction system.

The mechanisms of retrograde intranodal conduction are closely related to the mechanism of ventricular echoes. Classically, there are 2 intrano- da1 pathways (although there can be more) : a slower one (a) and a faster one (p) . A ventricular echo will occur if a retrograde impulse, after traversing the His bundle and the final (nodal) common pathway, is adequately timed to be conducted

‘through the (Y pathway while being blocked in the p pathway. If the latter condition is met, the impulse will reach the atria after considerable delay. In fact, as mentioned, the greatest area of delay in the human heart can be at the A-H region. Ac- cording to Moe and Mendez,16 if this impulse (after reaching the atria) finds that the faster (p) pathway has recovered, it will propagate through the latter in a forward direction, traversing the final common pathway, and finally arriving at the ventricles through the His bundle. However, the impulse descending through the p pathway will be blocked on its return journey to the ventricles if it finds the final common pathway completely refractory. In this case, an echo will not occur.

His bundle recordings in our cases convincingly show that ventricular echoes engage the His bundle in both a retrograde and antegrade direction. Yet, evidence of intranodal events is still indirect because recordings were not made from this struc- ture itself. Nevertheless, our data are in keeping with previously mentioned studies. Therefore, it appears that information obtained from His bundle studies has corroborated previous concepts based on animal experiments.18JQ Taking these concepts into consideration, the circuit of a ventricular echo in the human heart can be repre- sentedasfollows:V+H+FCP+a-+A+/3+ FCP + H + V (FCP = final common pathway). In Figures 7, 8, and 10, the atria appeared to be a necessary link for the production of echoes. More studies are required to reach a definite conclusion in this respect.16s20

Analysis of His bundle recordings in man has also led to the introduction of new theories. For instance, Damato et al.3 have suggested that QRS complexes that are not preceded by P waves, but in which the H-R distance and QRS configuration are identical to those of sinus beats, originate in the His bundle. The last 2 complexes in Figure 3 ful- fill the criteria of these investigators for His bundle rhythms. This concept is worthy of consideration although the possibility that these rhythms may arise in the N-H cells (and even in the A-N area) cannot be dismissed.22,23



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