Modulated ventricular parasystole as a mechanism for concealed bigeminy

Apparent disappearance of ventricular parasystole due to a marked difference between the long form and the short form of the ectopic cycles

Electrocardiograms were taken from a 44-year-old man with irregular ventricular parasystole in whom pure parasystolic cycles without any intervening nonectopic QRS complexes were found. When a sinus impulse fell late in the parasystolic cycle, it hastened occurrence of the next parasystolic discharge. This suggested that type I second degree entrance block occurred in the re-entrant pathway containing the parasystolic focus. When a sinus impulse fell early in the parasystolic cycle, it delayed occurrence of the next parasystolic discharge. This suggested that electrotonic modulation occurred in the parasystolic focus. As a result, the difference in length between the short form and the long form of the parasystolic cycle became markedly great. When the length of two adjacent sinus cycles ranged between the short and the long parasystolic cycle, manifest parasystolic QRS complexes disappeared for a long time. In true ventricular parasystole with pure ectopic cycles, such long disappearance has never been reported before.

Effect of standing on ventricular parasystole: shortening of the parasystolic cycle length

OBJECTIVE

To investigate the effect of standing on the parasystolic cycle length in cases of "true" ventricular parasystole.

METHODS

Parasystolic cycle length and sinus cycle length were measured during lying and standing in eight men with true ventricular parasystole. These cycle lengths were also measured after exercise in the lying position.

RESULTS

In all cases, parasystolic cycle length and sinus cycle length both shortened on standing, by a mean of 6.4% and 17.8%, respectively, compared to lying. In all cases, the rate of shortening of the parasystolic cycle length was less than that of the sinus cycle length. Parasystolic cycle length was prolonged after exercise, in contrast to a shortening of the sinus cycle length.

CONCLUSIONS

Influences on the parasystolic cycle length are not always in the same direction as on the sinus cycle length. This suggests that the effect of autonomic changes on parasystolic rhythm is not always parallel to that on sinus rhythm.

Reverse effects of exercise on the sinus and parasystolic cycle lengths

Ventricular arrhythmias are the primary concern in the exercise laboratory. It has recently been suggested that in not a few cases, ventricular premature complexes are governed by modulated parasystole and not by ordinary extrasystolic rhythm. In many cases, however, it is difficult to differentiate between parasystole and ordinary extrasystoles. Few reports are available on the effect of exercise in cases of "true" parasystole. This study investigated the effect of exercise on the parasystolic cycle length 11 cases of true ventricular parasystole, in which one or more "pure" parasystolic cycles containing no intervening nonectopic QRS complexes were found. In all cases, in contrast to an acceleration of the sinus rate, a definite decrease in the parasystolic rate was found. In no case did complete suppression occur. These findings suggest that the effects of exercise on ordinary ventricular extrasystolic rhythm and on ventricular parasystole may be considerably different from each other.

Differentiation between parasystole and extrasystoles. Influence of vagal stimulation on parasystolic impulse formation

Recently, it has been shown that when a sinus impulse falls late in the parasystolic cycle, it usually hastens the next ectopic discharge. Thus, in many cases, the classic criteria for the diagnosis of parasystole (ie, varying coupling intervals and constant shortest interectopic intervals) cannot be used. To differentiate between parasystole and extrasystoles in such cases, the influence of vagal stimulation on parasystolic impulse formation was investigated in seven cases of "true" parasystole in which one or more "pure" ectopic cycles without any intervening nonectopic QRS complexes were found spontaneously. In all cases pure ectopic cycles were found during sinus arrest caused by vagal stimulation; namely, none of the cases showed extreme prolongation of the parasystolic cycle. These results strongly suggest that instead of the classic criteria, vagal stimulation causing temporary sinus arrest is the optimal method for differentiation between parasystole and extrasystoles in cases without spontaneous pure ectopic cycles.

Atrial parasystole and tachycardia. Modulation and automodulation of a parasystolic focus

This report deals with a patient reflecting atrial parasystole and episodes of atrial tachycardia. The P' waves during tachycardia were identical to the parasystolic P' waves. Atrial parasystole was at times regular, as revealed by a precise mathematical relationship between the interectopic intervals, and on other occasions irregular. Irregularity was due to modulation, namely electrotonic influence exerted by the sinus impulses upon the parasystolic focus. Atrial tachycardia occurred only during the periods when atrial parasystole was modulated. Atrial tachycardia has been interpreted as due to automodulation, a situation where the propagated parasystolic impulse exerts an electrotonic influence on the ectopic focus itself, leading to a marked unexpected acceleration of the ensuing parasystolic discharge.

Concealed rhythms by double ventricular parasystole

Electrocardiograms taken from 11 patients in sinus rhythm with ventricular ectopic rhythms from two different foci were analyzed to find the number of sinus beats, S, between the ectopic rhythms (S values). Three out of 11 patients had the S values typical for concealed ectopic rhythms. One of them had concealed bigeminy of 2n-1 form that occasionally shifted to 2n form. Following the shift, S values of 2n-1 form were always achieved by the occurrence of double ventricular ectopic rhythms in succession. Concealed trigeminy of 3n and 3n-2 form was seen in the other two patients. Double ventricular ectopic rhythms had bizarre abnormal QRS complexes of two different morphologies and were inscribed in opposite directions. Ectopic rhythms in each case had parasystolic characteristics. These observations suggest bifocal automaticity as a mechanism for bidirectional ventricular tachycardia.

Intermittent ventricular bigeminy as an expression of two-level Wenckebach periodicity in the reentrant pathway of extrasystoles

A patient with intermittent ventricular bigeminy is reported in whom the presence of two-level Wenckebach periodicity in the reentrant pathway of extra-systoles is suggested. When sinus arrest was caused by vagal stimulation, no ectopic QRS complex occurred. This indicated that ventricular bigeminy was not parasystolic bigeminy but ordinary extrasystolic bigeminy. Observations of the electrocardiogram suggested that Wenckebach block occurred at two different levels in the reentrant pathway of ventricular extrasystoles. When extrasystoles were noninterpolated, Wenckebach block occurred at the distal level of the pathway and caused termination of ventricular bigeminy. On the other hand, when extrasystoles were interpolated, Wenckebach block occurred at the proximal level of the pathway. This is the first report to suggest the presence of two-level Wenckebach periodicity in a reentrant pathway of extrasystoles.

Modulated parasystole complicating permanent ventricular pacing: the different influence of spontaneous and paced impulses upon the parasystolic rhythm

A case of modulated ventricular parasystole observed in a patient with a VVIM pacemaker is reported. Analysis reveals that the electronic influence (modulation) effected upon the parasystolic focus by the sinus impulses is different from that exerted by the paced impulses. Furthermore, fusion beats reflect an intermediate modulating effect according to the prevalence of the sinus or the paced wavefront.

A computer-based model of parasystolic activity modulated by ventricular activation in the open-chest dog

The modulated parasystole hypothesis proposed by Moe has been suggested as a common mechanism of a wide variety of arrhythmias. To represent the arrhythmic patterns defined by the model as an actual ECG trace, a "hybrid" experimental system was devised. The system consisted of the intact canine heart connected to a microcomputer that operated as the modulated parasystolic pacemaker. An intrinsic cycle length of the "ectopic" pacemaker was modulated by the phase-dependent effect of the activity of the intact heart in situ. Discharges in the "ectopic" pacemaker were transmitted to the right ventricle. The result was that the theoretically defined patterns of the ectopic activity were represented as real ECG traces. Some published examples of intermittent parasystole that had been attributed to other complex mechanisms were accurately reproduced when the estimated phase response curves were set in the "ectopic" pacemaker. A wide variety of clinical eCG tracings of ventricular arrhythmias may be reproduced by this hybrid experimental model.

Electrotonic modulation of parasystole in children

This paper presents four cases of pediatric extrasystole which did not fulfil the diagnostic criteria for parasystole. By calculating various ectopic intervals, we established the phase-response curve demonstrating that sinus electrotonic activities modulated the parasystolic focus in each case. Our results showed these cases to be modulated parasystole.

Deduction of biphasic phase response curves from ventricular parasystole

The mechanisms behind ventricular premature beats may be divided into two main categories, reentry and modulated parasystole. Although it is relatively easy to distinguish among arrhythmia generating mechanisms on the cellular level, it is far more difficult in the clinic. Modulation of ventricular parasystole by sinus beats has been shown to result in a biphasic phase response curve. Two cases are reported where the deduction of such phase response curves from long-term ECG recordings is discussed and described as a stepwise procedure.

Concealed ventricular hexageminy

The authors report a case of concealed ventricular hexageminy in which, with a few exceptions, extrasystoles were separated by sinus beats conforming to the formula 6n - 1. Whenever an exception to this formula occurs, the intervening beats are not all of sinus origin, but include also a ventricular extrasystole that is different from those occurring in hexageminal distribution. The pattern is explained by a parasystolic rhythm modulated by sinus impulses, assuming a 3:1 ratio between the parasystolic cycle and the sinus cycle. Such a ratio would have to be associated with a trigeminal or concealed trigeminal distribution. There is, however, a 2:1 ectopic-ventricular block, leading to a change of the ectopic distributional pattern from the expected concealed trigeminy to that of the concealed hesageminy.

Concealed ventricular bigeminy with exceptions due to time-dependent modulation of an ectopic rhythm

This presentation reflects a case of atypical concealed bigeminy, where some interectopic intervals contain even numbers of sinus beats. Exceptions to the rule of concealed bigeminy only occur during slowing of the sinus node. The pattern is explained on the basis of modulated parasystole, by drawing a phase-response curve which explains all the interectopic intervals on the basis of the modulating effect exerted by the sinus impulses upon a parasystolic focus.

"Irregular" ventricular parasystole: the influence of sinus rhythm on a parasystolic focus

Fifteen cases of ventricular parasystole were analyzed to determine whether the interectopic intervals were regular, as expressed by long intervals being exact multiples of the short ones, or not. The regularity of the interectopic intervals was assessed by means of the variation index: the ratio of the maximal difference between various measurements of the parasystolic cycle length and the mean parasystolic cycle length. Three out of 15 cases had a variation index less than 5, and were classified as "regular parasystole." Twelve cases were associated with a variation index greater than 7.5, and were classified as "irregular parasystole." The cases of irregular parasystole were then analyzed to determine whether the variability of the interectopic intervals was casual or dependent on action of the sinus beats. A parasystolic resetting by critically timed sinus impulses (a form of intermittent parasystole) was evident in three cases. The irregularity in the remaining nine cases was due to modulation (viz., due to electrotonic influence exerted by the sinus beats on the parasystolic focus). In every case of modulated parasystole a phase-response curve was constructed, which enabled an analysis of all the interectopic intervals on the basis of a time-dependent effect exerted by the sinus impulses on an otherwise rhythmic parasystolic focus.

Concealed ventricular quadrigeminy linked to atrial quadrigeminy: a manifestation of modulated parasystole

Analysis of a long electrocardiographic recording including many atrial and ventricular extrasystoles shows that when atrial extrasystoles are in a quadrigeminal distribution, the ventricular extrasystoles also manifest a quadrigeminal distribution or reflect a distributional pattern of concealed quadrigeminy. Conversely, when atrial extrasystoles are other than in a quadrigeminal distribution, the ventricular extrasystoles do not occur in a quadrigeminal or concealed quadrigeminal distribution. This pattern is explained on the basis of modulated parasystole. A biphasic phase-response curve explains the observed phenomena on the basis of variations of the parasystolic cycle length due to the modulating effect of supraventricular beats.