Mechanisms of intermittent ventricular parasystole due to type II second degree entrance block

Three patients with intermittent ventricular parasystole are reported in whom the presence of second degree entrance block of type II or a type similar to that was shown. In all the patients, when a sinus beat occurred within a certain (the first) critical period after the preceding ectopic beat, the parasystolic focus was protected from this sinus impulse. When, on the other hand, a sinus beat occurred beyond another (the second) critical period after the ectopic beat, this sinus impulse reached and discharged the focus without an appreciable conduction delay. In one patient the second critical period was equal to the first one, while, in the other two, the second one was longer than the first one. In these two patients, when a sinus beat occurred between the two critical periods after the ectopic beat, this sinus impulse reached and discharged the focus after marked delay, and thereafter became a manifest or concealed re-entrant ventricular extrasystole. On the basis of these observations, an attempt was made to clarify the difference in mechanism between type I and type II second degree entrance block.     

Intermittent ventricular parasystolic bigeminy accompanied by reentrant ventricular extrasystoles.

An extremely rare case of intermittent ventricular parasystolic bigeminy is presented, in which coexisting reentrant ventricular extrasystoles of sinus origin, showing mainly trigeminy, and those of ventricular parasystolic origin, showing a close coupling to the ventricular parasystolic beat, are demonstrated. Such a unique case has never been reported. Timely parasystolic exit block occurring just prior to the anterograde ventricular activation of the sinus impulse was considered to be of critical importance in developing the reentrant extrasystolic trigeminy.     

Mechanism of escape, extrasystolic, and parasystolic arrhythmias. Study on an electrical analogue.

A simple analogue of the heart consisting of a system of neon relaxation oscillators is presented. The analogue may display rhythm patterns similar to sinus rhythm, escape rhythm, isorrhythmic dissociation with synchronization, atrial extrasystoles, ventricular extrasystoles, and parasystole. The strict rules followed by these arrhythmias, as well as the deviations from the rules commonly followed by the equivalent heart arrhythmias, may be easily reproduced on the analogue. Such features are the Treppe phenomenon and captured beats in escape rhythm, fixed coupling intervals in extrasystoles, partial or complete atrioventricular block in very premature atrial extrasystoles, prolongation of the period following an atrial extrasystole, interpolated premature beats, complete compensatory pause and the rule of bigeminy in ventricular extrasystoles, slight instability of the parasystolic period, multiple length parasystolic periods slightly different from the exact multiples of the parasystolic idioperiod, preference of the parasystoles for certain phase in the sinus cycle, synchronization at a phase difference and fluctuation repeatedly and without interruption from a parasystolic to an extrasystolic rhythm and synchronization in escape rhythm with isorrhythmic dissociation. The mechanisms involved in these phenomena are discussed in detail. The striking similarity between the properties of the cardiac pacemakers and those of the relaxation oscillators on the one hand and betwen the rhythm patterns of the heart and those of the analogue on the other may permit the hypothesis that the mechanisms operating in the analogue may be used in analyzing and understanding heart arrhythmias.     

Intermittent parasystole with concealed extrasystolic bigeminy during myocardial infarction.

A 72-year-old man demonstrated persistent intermittent ventricular parasystole for 6 days during an acute inferior wall myocardial infarction. Entrance block failure occurred at a critical interval of 1.24 sec. after parasystolic beats with resetting of the parasystolic cycle length. The first ectopic beat in each parasystolic series showed coupling to its second preceding conducted beat, and successive cycle lengths in each series usually showed gradual shortening. Concealed extrasystolic bigeminy was also demonstrated between parasystolic series with the number of intervening conducted beats conforming to the formula 2n + 2. After interpolated parasystolic beats, this formula became 2n + 3.     

Mechanisms of ventricular parasystole.

Eight cases of ventricular parasystole are reported. In all these cases, regardless of whether parasystole seems intermittent or "continuous," the presence of second degree entrance block of the Mobitz type I was suggested. Parasystole alternated with concealed extrasystolic bigeminy showing occasional reentrant extrasystoles. Such intermittent parasystole appears to originate in the reentrant path of extrasystoles. Reentrant extrasystolic bigeminy was seen in a comparatively rapid sinus rhythm, whereas parasystolic bigeminy was seen in a comparatively slow sinus rhythm. The difference between the interectopic intervals during parasystolic bigeminy and during (manifest or concealed) extrasystolic bigeminy was comparatively small so that occasionally the difference was not distinct; on such an occasion the case showed a seemingly "continuous" parasystole. These observations strongly suggest the possibility that most cases of parasystole, whether intermittent or "continuous," may be governed by incomplete entrance block of the second degree.     

Mechanism of supraventricular parasystole

Two patients with supraventricular parasystole (one atrioventricular and one auricular) are reported. In both patients, reentrant extrasystoles appeared to occur as the result of Mobitz type I second-degree entrance block. We believe that when a sinus impulse fell soon after the absolute refractory period of the pathway containing the parasytolic focus, it reached and discharged the focus after marked delay, and thereafter became a reentrant extrasystole. In interectopic intervals containing more than one sinus beat, the number of intervening sinus beats was always even, suggesting the presence of concealed reentrant extrasystolic bigeminy. The observations in the present report and in our previous patients with ventricular parasystole strongly suggest that most cases of parasystole, whether ventricular or supraventricular, or whether intermittent or "continuous," may be governed by second-degree entrance block.     

Simultaneous but separate intermittent and continuous ventricular parasystole

A 19 year old female with end-stage biventricular congestive cardiomyopathy and digitalis toxicity demonstrated double ventricular parasystole with separate but simultaneous intermittent and continuous parasystolic rhythms. The intermittent parasystolic focus, left ventricular in origin, showed failure of entrance block when conducted beats followed its ectopic beats at a critical compensatory interval. Each new parasystolic series was coupled to this interrupting conducted beat at an interval equaling the parasystolic cycle length. Even numbers of conducted beats intervened between ectopic beats during intermittency and odd numbers during uninterrupted parasystole. The mechanism of intermittency was explained by a concealed ventricular tachycardia with 3 3n1 exit block with the demonstration of occasional 3 3n2 Wenckebach exit block within a parasystolic interectopic interval.A second continuous parasystolic rhythm, right ventricular in origin, occurred at a later time without altering the first intermittent parasystolic rhythm.A 4% parallel and proportionate lengthening of both the sinus node and intermittent parasystolic cycle lengths was shown at an even later time.     

Role of the compensatory pause in the production of concealed bigeminy

In experiments conducted on anesthetized dogs, a single prolonged cardiac cycle caused an increase in the duration of the refractory period of the ventricular myocardium. The refractory period remained prolonged for one or two additional cardiac cycles after the basic cycle length was resumed. In concealed bigeminy, a 2:1 block in a reentry path is postulated. The compensatory pause that follows a manifest extrasystole results in a prolongation of the refractory period. If the refractory period exceeds the propagation time of the next ectopic impulse conducted through the reentry path, that ectopic impulse will be concealed. However, if the refractory period does not exceed the propagation time, another extrasystole will occur, and a bigeminal sequence will ensue. Consecutive compensatory pauses appear to exert a cumulative effect on refractory period duration, so that for critical propagation times, the bigeminal pattern will be terminated briefly when an ectopic impulse becomes concealed. A quadrigeminal pattern occurs when the propagation time in the reentry loop lies between the refractory period durations of the beat terminating a compensatory pause and the beat that occurs two sinus cycles later. In concealed bigeminy where an odd number of conducted beats greater than five lies between extrasystoles, the propagation time in the reentry path is just less than the refractory period of the ventricular cells during normal sinus rhythm at the prevailing heart rate. Random increases in propagation time or decreases in refractory period result in manifest extrasystoles.     

Intermittent ventricular parasystole with observations on its relationship to extrasystolic bigeminy

1. 1. Two cases of intermittent parasystole are described in which there is alternate extrasystolic and parasystolic impulse formation from the same focus; transitions between the two rhythms are repeatedly observed.

2. 2. It seems that intermittence results when the parasystolic focus is subjected to the enhancing effect of the sinus discharge whereby the automatic beat is prematurely precipitated, thus becoming a forced beat.

3. 3. This phenomenon can be explained on the basis of a property of automatic centers; namely, that their resting potentials exhibit a gradual upward “slope” of depolarization. With critical timing, the terminal part of the “slope” (i.e. its near threshold level) may encounter the enhancing effect of the preceding sinus beat and thus be precipitated prematurely.

4. 4. It is suggested that the form of extrasystolic bigeminal rhythm described may constitute a link between parasystolic and extrasystolic rhythm.

     

Ventricular parasystolic couplets originating in the pathway between the ventricle and the parasystolic pacemaker: mechanism of "irregular" parasystole.

This article explains the mechanism of "irregular" parasystole. Two theories have been suggested: "electrotonic modulation" and "type I second degree entrance block." This study attempts to clarify the mechanism of irregular parasystole in cases of true ventricular parasystole associated with ventricular parasystolic couplets. Cases associated with ventricular parasystolic couplets were selected from 37 clinical cases of true ventricular parasystole in which one or more pure parasystolic cycles with no intervening nonectopic QRS complexes were found. Of the 37 cases of true ventricular parasystole, ventricular parasystolic couplets were found in 4 cases. In none of the other 33 cases, ventricular parasystolic couplets were found. In all the cases coexisting with ventricular parasystolic couplets, the latter ectopic QRS complex of the couplet failed to reset the parasystolic rhythm. The above findings suggest that the latter ectopic QRS complex of the parasystolic couplet originated not in the parasystolic pacemaker but in the pathway between the ventricle and the parasystolic pacemaker. It seems that when a sinus impulse fell late in the parasystolic cycle, it passed through the site of second degree entrance block and that the parasystolic couplets originated from the reentrant pathway between the ventricle and the pacemaker. This strengthens our previous suggestion that the mechanism of irregular parasystole is governed by "type I second degree entrance block" and not by "electrotonic modulation."     

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.     

Atrio-ventricular junctional parasystole: modulation by sinus impulses

A rare case of atrio-ventricular (A-V) junctional parasystole with a quinquageminy and a trigeminy in the presence of a right bundle branch block (RBBB) is reported, in which the ectopic beats showed the same QRS complex as that of the conducted sinus beats. After taking a bath and after intravenous administration of 1.0 mg of atropine, viz. the maneuver to block the vagal effects, the short interectopic interval shortened from control of 2.65-2.70 sec to 2.08 sec, when the shortest interval of 0.98 sec was observed. Thus, the short interectopic intervals resulted from a 2:1 exit block. When the intrinsic ectopic cycle length was about 1.52 sec, the second of four intervening sinus beats in quinquageminy reset the parasystolic pacemaker and a 2:1 exit block ensued, which was attributed to entrance block failure due to a supernormal phase of excitability and conductivity in the His bundle.     

Ventricular Extrasystoles with Interpolation or Postponed Compensatory Pause during Atrial Fibrillation: Fact or Fiction?

A Holter recording obtained from a patient with atrial fibrillation showed ventricular extrasystoles often in bigeminal rhythm. Most extrasystoles were followed by a long return cycle, and only in a few instances the postextrasystolic interval was short. The latter phenomenon was interpreted as a manifestation of poor retrograde concealed penetration of the ventricular impulse into the atrioventricular (A‐V) junction: accordingly, an ensuing relatively early fibrillation impulse reached the ventricular chamber, since it did not find the A‐V node refractory. These events are similar to what happens in interpolated ventricular extrasystoles occurring during sinus rhythm, the absent or minimal concealed retrograde penetration of the ectopic impulse into the A‐V node being necessary to permit anterograde conduction of the ensuing sinus impulse. Analysis of the recording also revealed that a very long (>2 second) interval between two consecutive narrow beats only occurred after an “interpolated” extrasystole. This was interpreted with the same mechanism underlying the “postponed compensatory pause” observed at times after interpolated ventricular extrasystoles during sinus rhythm: the minimal or nil penetration of the ventricular ectopic impulse into the A‐V junction, followed by conduction of an ensuing early atrial impulse, “shifts to the right” the A‐V nodal refractory period, preventing conduction of several further supraventricular impulses and generating a pause. Both interpolated ventricular extrasystoles and the phenomenon of “postponed compensatory pause” are, thus, conceivable during atrial fibrillation, although no definite demonstration is possible.     

Protection of the ventricular parasystolic focus due to interference of sinus and parasystolic impulses in the ventricular-ectopic junction

A 50-year-old man with intermittent ventricular parasystolic bigeminy is reported in whom the parasystolic focus was protected from late intervening sinus impulses. This is the first report to suggest the presence of protection due to interference in parasystolic bigeminy. The findings in this case suggest that when a sinus impulse falls in a late period of the parasystolic cycle, it travells so slowly along the ventricular-ectopic junction that it is unable to reach the parasystolic focus before the spontaneous occurrence of the next parasystolic impulse; as a result, the sinus impulse interferes with the next parasystolic impulse in the ventricular-ectopic junction. Thus it is suggested that the parasystolic focus is protected from the sinus impulse because of the interference and not because of an entrance block. This reinforces the concept of a second-degree entrance block as a mechanism of parasystole.     

Ventricular tachycardia in bursts. Electrophysiological and therapeutical study

22 patients with salvoes of ventricular tachycardia were investigated electrophysiologically and pharmacologically to determine the mechanism of the arrhythmia. The appearance of the ventricular complexes was suggestive of a septal origin. The arrhythmia appeared to occur and regress within precise limits of sinus rhythm. 24 hour ECG recording and endocavitary stimulation techniques showed its essentially variable character, but occasionally a fixed coupling interval with the first extrasystole was observed, suggesting the presence of an unprotected parasystolic focus. The repetition of the extrasystolic activity, which could be triggered and terminated at will, is explained by phenomena of delayed after potentials. The results of the systematic investigation of the effects of quinidine-like drugs, beta-blockers and verapamil do not invalidate these hypotheses and show a difference of degree but not of nature between the two populations A and B of patients classified according to the quantitative character of the extrasystolic activity over 24 hours (more or less than 1000/hour). The observation of intermediate forms with simple extrasystoles and other forms with sustained tachycardia extends the significance of these results to many forms of ventricular arrhythmia.     

A cause of paired ventricular extrasystoles.

Eight patients with ventricular extrasystoles are reported in whom coupling intervals of the extrasystoles to the proceding sinus beats were variable and in whom paired ventricular extrasystoles were occasionally seen. In all patients, paired ventricular extrasystoles were initiated only by comparatively late coupled ventricular extrasystoles. However, the interval between the first and the second of these paired extrasystoles was always much shorter than the coupling interval of this first extrasystole to the preceding sinus beat, so that the latter extrasystole often interrupted the T wave of the first one, indicating the R-on-T phenomenon. In two patients there was a gap between the ranges of coupling intervals for single extrasystoles and for the first ones of paired extrasystoles. These observations suggest the presence of longitudinal dissociation in the reentrant pathway as one of the causes of paired ventricular extrasystoles.     

Repetitive supernormal conduction in the right bundle branch in high degree bilateral bundle branch block

Electrocardiograms were taken from a 67-year-old woman with high-degree atrioventricular block in which ventricular escape beats of right bundle branch block pattern occurred, accompanying occasional ventricular capture beats. Only when a sinus P wave occurred 0.60 s after the preceding escape beat, it was followed by a capture beat of left bundle branch block pattern with the RP interval of 0.60 s and the PR interval of 0.19 s. Similar left bundle branch block with left axis deviation pattern had been shown in the electrocardiogram taken 2 years before. Such RP and PR intervals in capture beats were invariable. These suggest that capture beats occurred as a result of supernormal conduction in the right bundle branch, which denies the possibility of ventricular extrasystoles. Such capture beats with the above RP and PR intervals were observed repeatedly.     

Accessory-pathway block on alternate beats in the Wolff-Parkinson-White syndrome: supernormal conduction as the mechanism

The Holter monitor electrocardiograms were taken from 2 patients with intermittent Wolff-Parkinson-White syndrome. In these patients, when the heart rate was increased, accessory-pathway block on alternate beats was found and was maintained for a considerably long period. In one patient, when accessory-pathway block on alternate beats was found, a ventricular extrasystole occurred. After the long compensatory pause after that extrasystole, a sinus impulse was blocked in the accessory pathway, showing that the effective refractory period of the accessory pathway is markedly long. These findings strongly suggest that alternate sinus impulses fell in the supernormal period of the accessory pathway. An attempt was made to explain the mechanism of accessory-pathway block on alternate beats by using the concept of supernormal conduction in the accessory pathway, in the same way as in bundle-branch block on alternate beats.     

Mechanism of bradycardia-dependent appearance of manifest extrasystoles in concealed bigeminy. A theoretical model derived from the concepts of longitudinal dissociation and multilevel block in the reentrant pathway of extrasystoles.

A case of bradycardia-dependent appearance of manifest extrasystoles in concealed bigeminy is presented. To explain the mechanism of such bradycardia-dependent appearance, a theoretical model is derived from the concepts of "longitudinal dissociation" and "multilevel block" in the reentrant pathway of extrasystoles. In the theoretical model, functional longitudinal dissociation divides the reentrant pathway into dual pathways F and S. When manifest extrasystoles are not found for a long time, alternate sinus impulses pass through both pathways F and S, but become concealed extrasystoles because of insufficient conduction delay in the pathways. The other alternate sinus impulses are blocked in the pathways; in pathway F, the impulses are blocked at the entrance, while in pathway S, the impulses are blocked at a more distal level. When sinus cycles gradually lengthen, one of such alternate sinus impulses passes through the entrance of pathway F and, traveling very slowly, is blocked at a more distal level. The next sinus impulse is blocked at the entrance of pathway F; namely, 3:2 Wenckebach block occurs at the entrance of pathway F. Thus this sinus impulse enters only pathway S and passes through pathway S with enough conduction delay to become a manifest reentrant extrasystole.     

Wenckebach-type exit block from an ectopic focus as a cause of variable coupling

Intermittent bigeminal and trigeminal ventricular premature beats were recorded in an otherwise healthy 14 year old male. Coupling intervals progressively lengthened until an ectopic beat was dropped. Odd numbers of sinus beats occurred between bigeminal runs. This rhythm is interpreted as being due to Wenckebach-type block in an exit pathway from the ectopic focus, resulting in concealment of the persistently active extrasystolic mechanism.