[Modulated parasystole. An arrhythmia waiting for diagnostic criteria. Deductive analysis of a case]

We report a case of ventricular parasystole in which the long interectopic intervals are always less than a multiple of the short interectopic intervals. Assuming a regular ectopic discharge, the above said phenomenon could theoretically be due to relative prolongation of the short interectopic intervals caused by delayed ectopic-ventricular conduction of the second consecutive conducted impulse (a Wenckebach-like phenomenon). Nevertheless, this hypothesis may be ruled out in this case by the analysis of a long tracing. The unusual relationship between the interectopic intervals may be explained assuming the modulation of the parasystolic activity by sinus beats. Experimental work has shown that despite the protection (or entrance) block, the sinus or dominant rhythm may affect the automaticity of a parasystolic focus by means of electrotonic influence. It has been pointed out that sinus impulses falling in the first part of the ectopic cycle delay the next ectopic discharge, while relatively late sinus beats (falling in the last 30-40% of the ectopic cycle) are able to anticipate the delivery of the parasystolic impulse. The phenomenon of modulation is marked by a characteristic biphasic phase-response curve. In the reported case a phase-response curve has been obtained, which explains the parasystolic cycle variations according to the phase of the cycle in which the sinus beats fall. The obtained curve is quite similar to those observed experimentally, therefore the diagnosis of modulated parasystole appears to be very likely.(ABSTRACT TRUNCATED AT 250 WORDS)