Intermittent Parasystole—Mechanism of Protection

Analysis of ten cases of intermittent ventricular parasystole suggested temporary loss of protection of the parasystolic focus resulting in discharge and "resetting" of the parasystole by sinus or other supraventricular beats. Parasystolic centers appear to be protected from supraventricular and other ventricular ectopic impulses early in their cycle by their refractory periods, and late in the cycle by diastolic depolarization. Between these two areas of protection is a period of "susceptibility" during which supraventricular beats can discharge the parasystolic focus. This phenomenon accounted for fixed coupling of the first parasystolic beat of a series to a preceding sinus beat. Under certain conditions all parasystolic beats may have such fixed coupling, a new and previously unconsidered mechanism for parasystole with fixed coupling. In one case of intermittent ventricular parasystole, the parasystole was shown to originate in the posterior fascicle of the left bundle branch on the basis of the shape of the standard electrocardiogram and by His bundle recordings. These parasystolic impulses produced a compensatory pause regardless of whether their discharge was manifest or concealed, and thus imitated a second degree (Type II) atrioventricular (A-V) block. This represents the first description of completely blocked atrial impulses resulting from concealed parasystolic beats arising within the ventricular conduction system.