[The many faces of parasystolic rhythm]

Parasystole is the expression of a pacemaker that is protected from, and thus independent of, the dominant rhythm. The arrhythmia is not always associated with the 3 classic signs: variable coupling, mathematically related interectopic intervals, and fusion beats. A large amount of experimental and clinical data have pointed out several atypical phenomena that make the recognition of parasystole difficult. This especially occurs in the presence of influence exerted from sinus impulses upon the parasystolic rhythm.

A pattern of ventricular parasystole was evident throughout a 24-hour Holter recording obtained from a 55-year-old female. The following data were analyzed: a) distribution of ectopic complexes; b) parasystolic cycle duration; c) regularity of parasystole, as assessed by means of the variation index. In some sections of the tracing in which an electrotonic influence (modulation) exerted by the sinus rhythm on the parasystolic rhythm appeared as very likely, phase-response curves were constructed in order to express the time-dependent modulation effected by sinus impulses.

Parasystole occurred in two separate periods, lasting 4 hours and 90 minutes, respectively. On both occasions, the arrhythmia was apparently precipitated by a brief episode of atrial tachycardia. Phases of regular parasystole, as well as periods of irregular modulated parasystole, were observed. The distribution of ectopic complexes was at times typical for concealed bigeminy (intervening beats always in odd numbers), but on other occasions reflected the even variant of concealed bigeminy where the intervening beats conform to the formula 1 + n (n being zero or an odd number). Some couplets of identical ectopic complexes were also observed.

The study shows that several different expressions of parasystolic rhythm may be present within the same tracing. A minimal or absent modulation results in the classical picture of parasystole; when a mild modulating influence is present, the typical pattern of modulated parasystole ensues, whereas a strong modulation leads to disappearance of the typical features of parasystole and manifestation of concealed bigeminy. Finally, supernormal modulation is responsible for the occurrence of couplets.