Double ventricular parasystole. Supernormal phase of conduction as a mechanism of intermittent parasystole. Report of a case

Multiform ventricular ectopic rhythm by combined parasystolic and reentrant activities

Multiform ventricular ectopic rhythm (MVER), i.e., at least two QRS configurations of ventricular ectopic beats (VEBs), was assessed by 24-hour ambulatory ECG recording in four patients with ventricular parasystole (VP). In two of these four patients, VEBs with fixed coupling to the preceding impulses coexisted with VP beats of different configuration. In case no. 1, the VEBs had an identical coupling interval to sinus beats and VP beats, suggesting a mechanism of reentry elicited from both dominant pacemakers. In case no. 2, an intermittent form of VP due to type II second-degree entrance block was present. In this patient, the VEBs were coupled to sinus beats and to sinus-VP fusion beats and appeared to be dependent on the sinus beats reaching the VP focus. A mechanism of reentry determined by the penetration of sinus beats into the VP area, with prematurity-dependent aberrancy of VEBs, was suggested for the coupled VEBs in this patient. These observations suggest that the coexistence of an automatic ventricular ectopic focus and of a reentrant activity determined by, or elicited from, an area of automaticity may constitute the underlying mechanism of MVER in some patients.

Second degree entrance block with supernormal conduction in intermittent ventricular parasystole

A patient with intermittent ventricular parasystole is reported in whom the presence of second degree entrance block with supernormal conduction was suggested for the first time. In this patient, ventricular extrasystoles with variable coupling frequently occurred. The QRS configuration of the extrasystoles was different from that of the parasystolic beats. When extrasystoles did not occur, the parasystolic beat was never seen because the conducted sinus impulse always reset the parasystolic rhythm. When an extrasystole occurred 0.52 sec or more after the preceding sinus beat, this extrasystolic impulse also reset the parasystolic rhythm. On the other hand, when an extrasystole occurred between 0.47 and 0.51 sec after the sinus beat, the parasystolic focus was protected from this extrasystolic impulse. When, however, an extrasystole occurred in a short terminal portion of the T wave of the preceding sinus beat, this extrasystolic impulse reset the parasystolic rhythm again, suggesting entrance block failure during the supernormal phase.

A search for modulation in intermittent ventricular parasystole

As recently described by Moe et al., parasystolic modulation implies that the ectopic cycle length of a parasystolic focus can be increased or decreased by electrical influences transmitted across the zone of protection. A search for this phenomenon was made in nine patients with intermittent ventricular parasystole resulting from protection only during specific portions of the cycle. All nine patients had a period of relatively early protection affecting the initial 55-67% of the cycle preceding a period of resetting during which the parasystolic focus was discharged and fully recycled. In addition, two patients had a period of relatively late parasystolic protection. Pseudo-resetting (apparent resetting in the surface leads without interruption of activity within the parasystolic focus itself), seen throughout the entire period of late protection in these two patients, and during the relatively refractory period in one patient, was identified because the R waves causing apparent resetting were encompassed by parasystolic-to-parasystolic intervals equalling twice the ectopic cycle length. Our findings indicate that parasystolic modulation did not occur in the type of intermittent ventricular parasystole included in this study. That is, the parasystolic focus was either not affected (not modulated) during specific (early or late) portions of the cycle, or normally reset (discharged and fully recycled, but neither depressed, nor enhanced) in other portions of the cycle. This study does not exclude the occurrence of modulation in other types of parasystole, nor in some automatic nonparasystolic rhythms.

Double ventricular parasystole: intermittency and its relation to re-entrant premature beats

Critical analysis of a case of double ventricular parasystole revealed that one parasystolic focus manifested complete protection, whereas the second focus appeared to be intermittent, protected from sinus and other impulse formation both early, as well as late, in the parasystolic cycle. The zone of protection failure, during which extrinsic impulses could penetrate into the parasystolic focus and discharge it, is observed between these early and late phases of protection. When the intermittent parasystolic focus is challenged by the other parasystolic beat, this zone is much wider and is located later in the cycle than the sinus beat. According to this observation, it is supposed that the intermittent parasystolic focus, in this case, is anatomically surrounded by two types of fibers, each showing different degrees of phase 3 and phase 4 block. The fibers located between the major fascicles and the intermittent parasystolic focus may have a shorter phase 3 and a less steep phase 4 depolarization than those located between two parasystolic foci. It is also demonstrated that during the phase 4 depolarization in these fibers around the focus, concealed conduction into this area by the sinus beat evoked exit delay of the parasystolic impulse, resulting in prolongation of the next parasystolic cycle. In this situation, a re-entry movement around the focus was predicted and, as expected, a re-entrant bigeminy coupled with the parasystolic rhythm was observed.