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.