Parasystolic Ventricular Tachycardia With Exit Block

Nikiforos G. Koulizakis, MD, Kostas G. Kappos, MD, and Pavlos K. Toutouzas, MD

Abstract:

A rare case of parasystolic ventricular tachycardia with variable exit block concurring with runs of reentrant ventricular tachycardia is described. The reentrant ventricular tachycardia or isolated ventricular extrabeats modified the exit block patterns of the parasystolic tachycardia. Key words: ventricular tachycardia, parasystole, exit block.

Exit block in ventricular parasystole has been recognized in both clinical electrocardiography and experimental electrophysiology.‘-4 Parasystolic ventricular tachycardia may be associated with some type of exit block. Second-degree 2: 1 or Mobitz type 2 exit block between the parasystolic focus and ventricular myocardium is not unusual, but Wenckebath exit block is rare,5 making it difficult to detect the arrythrnia. The occurrence of another coexisting ventricular arrythrnia may make the above-mentioned pattern even more obscure. We present a rare case of parasystolic ventricular tachycardia with Wenckebach type 2:1 or highgrade exit block occurring with reentrant ventricular extrabeats or runs of reentrant ventricular tachycardia.

Case Report A 40-year-old man with no evidence of organic heart disease complained of palpitations and bouts of dizziness. An ECG (Fig. 1) showed, in some leads From the Department of Cardiology, University of Athens Medical School, Hippokration Hospital. Athens, Greece. Reprint requests: Nikiforos G. Koulizakis, MD, 114 Vasilissis Sofias Avenue, Athens 115 27 Greece.

(1, 11, III, VI, V5, V,) sinus beats alternating with ventricular extrabeats or an irregular ventricular rhythm; in other leads (aVR, aVL, aVF, VZ, V3, V,) there were only irregular beats with wide QRS complexes. A long tracing at the bottom of Figure 1 shows a long period of irregular wide QRS complexes and no evidence of atria1 activity. Long electrocardiographic recordings revealed a wide variation of coupling intervals between sinus beats and extrabeats. In Figure 2 all time intervals are expressed in hundredths of second. The coupling intervals between sinus beats and extrabeats in tracings B, C, and E ranged from 42 to 64, and in tracing E a fusion beat is noted. The interbeat intervals during brief periods of pure ectopic rhythm were remarkably irregular (Fig. 2A). A possible explanation for this irregularity is some type of exit block between the ectopic focus and ventricular myocardium. Since we found no common denominator of the successive interbeat intervals, we postulated a Wenckebach type of exit block. In such a case the shortest R-R interval of the arrythmia should be equal to or somewhat longer than the cycle length of the ectopic focus. The longest R-R interval should include the blocked beat of a Wenckebach period or the blocked beat of 2 : 1 block if Wenckebath periodicity converted periodically to 2 : 1 block, as it frequently does during type 1 block. The shortest

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Fig.

1. Electrocardi-

ogram.

R-R interval noted in tracing A of Figure 2, between the second and third beat, is 46. The longest R-R interval, in the same tracing between the first and second beat, is 90, approximately double the shortest one; this suggests 2 : 1 exit block of an ectopic ventricular tachycardia with a cycle length of 45. The diagram under the tracing illustrates the conduction route of the ectopic beats. The interval between the second and fifth beat is 180, which is an exact multiple of 45, denoting a 4: 3 Wenckebach period. Three consecutive Wenckebach periods follow, with a cycle length fluctuating between 43 and 44. The ventricular extrabeats (marked with a dot) with a different QRS configuration, are connected to the ectopic beats with an almost fixed coupling interval ranging from 34 to 36. These extrabeats interfering with the ectopic rhythm at times prevented the emergence of the ectopic beats. The broken lines mark two ectopic beats not conducted to the ventricles due to depolarization of the latter by the extrabeats. Such ventricular extrabeats were frequently observed in long electrocardiographic recordings either as isolated extrabeats or in runs of three or four beats, and they were coupled only to the ectopic beats and not to the sinus beats. The fact that the coupling interval between these extrabeats and the ectopic beats was fixed, combined with the finding that they were coupled only to the ectopic beats, favors reentry as the mechanism of their genesis.

Tracings B-E in Figure 2 were taken 24 hours later than tracing A and represent ECG lead VI. Tracing B shows a period of irregular ectopic ventricular rhythm. The interval between the third and fourth beat of 84 was the longest found in long ECG recordings. This interval probably represents 2:l exit block of the ectopic impulses, for reasons explained in tracing A, suggesting an ectopic cycle length of 42. The next interval corresponding to a multiple of 42 is between the fourth and sixth beat, reflecting a 3 : 2 Wenckebach period. The interval between the sixth and ninth beat is also a multiple of 42, probably reflecting another Wenckebach period terminating in 4: 1 block. The intervening sinus beat does not seem to modify the ectopic cycle length. This finding, which is also observed in tracing C, combined with the wide variation of coupling intervals between sinus and ectopic beats and the presence of fusion beat in tracing E, suggest a parasystolic nature of the ectopic focus. In tracing C the interval between the second and fourth beat of 84 corresponds also to two parasystolic cycles of 42 and probably reflects 2: 1 exit block. The intervening beat, with a different QRS contour (marked with a dot) is a reentrant ventricular extrabeat similar to the ones in tracing A, as was evident in simultaneous multiple lead recordings. The reentrant extrabeat does not affect the ectopic cycle length, giving additional evidence of the parasystolic nature of the ectopic focus. Two consecutive Wenck-

Parasystolic VT With Exit Block

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Fig. 2. (A) Lead V,. The diagram below the tracing illustrates the route of conduction of ectopic beats. Numbers represent interbeat intervals. (B-E) Lead V,. The numbers between QRS complexes represent coupling intervals between sinus and ectopic beats. Diagrams as in A. All numbers are expressed in hundredths of a second. V, ventricular myocardium; EV, ectopic ventricular junction; EF, ectopic (parasystolic) focus.

ebach periods follow with a cycle length fluctuating between 42 and 44. The last long interval after the tenth beat is also a multiple of 42; it probably includes a 5 : 4 Wenckebach period and a period of 3 : 1 exit block. The second reentrant beat in tracing C (marked with a dot) probably prevents the emergence of the parasystolic beat (marked with a broken line). The high-grade exit block noted in tracings B and C may’ also reflect a multilevel block between the parasystolic focus and ventricular myocardium.6 Tracing D shows short runs of reentrant tachycardia (marked with dots) as well as isolated extrabeats, obscuring the conduction pattern of the parasystolic tachycardia.

Discussion In the reported case the parasystolic nature of the tachycardia is suggested by demonstration of wide variation of coupling intervals between the sinus and ectopic beats, the presence of fusion beats, and the fact that the sinus beats, as well as the reentrant ventricular extrabeats, did not seem to modify the ectopic cycle length, implying entrance block to the ectopic focus. The nature of protection in clinical parasystole is not quite clear, and several possible mechanisms have been proposed. It certainly is very difficult, if

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not impossible, to prove the mechanism of protection in clinical cases of rapid ventricular parasystole. In cases with a rapid firing ventricular ectopic focus, such as ours, failure of extraneous impulses to alter the rhythmicity of the ectopic focus may be due to interference between the impulses generated by the rapid ventricular ectopic focus and extraneous impulses. Even so, Scherf and associates7” suggested that parasystolic foci have a rapid rate of discharge and explain protection in terms of rate-related refractoriness. In our case, the entrance block may be due to the high rate of ectopic discharge, but there are other possibilities. Unidirectional block in an area of tissue surrounding the parasystolic focus has been proposed as a mechanism of protection.‘,’ Watanabe lo suggested that a greater degree of depression in conductivity around the parasystolic focus may also result in exit block, and therefore the degree of entrance and exit block of a parasystolic focus may be interrelated. Since in our case there is some degree of exit block, sometimes a high-grade one, we may assume that there is also some degree of entrance block providing protection to the parasystolic focus. Concealed reentry in the area surrounding the parasystolic focus may be another cause responsible for early protection of the parasystolic focus.““2 Since in our case there are manifest reentrant beats, concealed reentry cannot be ruled out. Critical analysis of the interbeat intervals suggested that Wenckebach periodicity of exit block converting at times to 2 : 1 or high-grade block could best explain the irregularity of the parasystolic tachycardia. Some of the Wenckebach periods depicted in the figures an atypical pattern, since there was no progressive shortening of R-R intervals. It is well known that in AV nodal type 1 block atypical periods are much more frequent than typical ones.13 Since in AV nodal type I block typical Wenckebach periodicity is the exception, rather than the rule, it seems probable than the same rule may apply to other myocardial tissues. Concealed reentry has also been postulated within or in the vicinity of a parasystolic focus, affecting the timing of the parasystolic beats. ‘* Since in our case manifest reentrant beats have been noted frequently, we suggest concealed reentry as one possible factor to account for the atypicity of type 1 exit block. Self-modulation of the parasystolic focus might also explain the irregular pattern of the parasystolic tachycardia. In recent years, experimental work15p16 has shown that electrotonic effects reflected back to the parasystolic pacemaker cause variation in the

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spontaneous cycle. Nevertheless, considerable exit delay seems to be required to cause a significant variation of the pacemaker cycle. Moreover, in the reported experiments15 large variations of the interbeat intervals, similar to those in our case, have been shown to be due not only to variations of the ectopic cycle length but also to second-degree exit block. Therefore, some form of second-degree exit block, including type 1 block, seems to be required to explain the irregularity of the parasystolic rhythm in our case. Reentrant ventricular extrabeats or runs of ventricular reentrant tachycardia interfered with the parasystolic tachycardia, modifying the type of exit block of the latter. The reentrant nature of those beats was suggested by the fact that they were fixedly and exclusively coupled to the parasystolic beats. Moreover, the presence of exit delay or exit block between the parasystolic focus and ventricular myocardium, as in our case, favors reentry or reflection within or in the vicinity of the parasystolic focus.

References

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15. Jalife J, Moe GK: A biologic model of parasystole. Am J Cardiol 43:761, 1979 16. Jalife J, Michaels DC: Phase-dependent interaction of cardiac pacemakers and mechanisms of control and synchronization in the heart. In Zipes DP, Jalife J (eds): Cardiac electrophysiology and arrhythmias. Grune & Stratton, 1985