Mechanism of irregular parasystole: Differentiation of second-degree entrance block from electrotonic modulation

Experts had believed for a long time that the parasystolic rhythm is regular and independent of sinus rhythm. In 1974, Kinoshital reported a case of ventricular parasystole in which when a sinus impulse fell late in the parasystolic cycle, it hastened the next ectopic discharge. Since then, many cases of such “irregular” parasystole have been reported,2-14 and it has been shown that in most clinical cases of parasystole, such hastening of the ectopic discharge can be observed.15, l6 There are two theories to explain hastening of the ectopic discharge: one, which was postulated by Kinoshita et a1.,l-12, 15* I6 is type I second-degree entrance block; the other, which was postulated by Jalife and Moe,17 is electrotonic modulation. According to both theories, the parasystolic cycle can be divided into the early period and the late period, and only when nonparasystolic impulses fall in the late period do they hasten the next ectopic discharge. Thus the coupling intervals of ectopic QRS complexes to the preceding sinus QRS complexes have a discontinuous distribution; that is, there is a gap in distribution bet,ween the coupling intervals to early intervening sinus QRS complexes and those to late intervening sinus QRS complexes. The upper limit of the gap is seen when a sinus QRS complex occurs at the terminal portion of the early period; the lower limit of the gap is seen when it occurs at the initial portion of the late period. At the site of entrance block, the length of the early period is invariable according to both theories. Although these characteristics resemble each other, their mechanisms are quite different.