Unusual electrocardiographic patterns of modulated parasystole.

Modulation of a parasystolic rhythm implies that the latter is affected by nonparasystolic beats in predictable ways. When modulation occurs the diagnosis of ventricular parasystole cannot be made by applying the well-known 'classical' criteria. This report deals with clinical tracings from three cases having modulated parasystole with unusual characteristics. Case 1 showed a 24-hour diurnal variability of parasystolic modulation characterized by its occurrence during only part of the period of sleep (from 1 to 5 am). In case 2, modulated ventricular parasystole produced episodes of intermittent ventricular bigeminy with fixed coupling resembling those attributed to a reentry mechanism. The proper diagnosis was made when the sinus cycle length changed abruptly. Finally, in case 3, the idionodal rhythm from a patient with complete AV block was shown to be not only parasystolic, but also modulated. In addition, the idionodal rhythm was entrained (captured) in a concealed fashion by paced beats so that the post-pacing events did not conform with those occurring during overdrive pacing of parasystolic nonmodulated, or nonparasystolic, idionodal rhythms. These findings constitute the clinical counterpart of experimental studies performed with microelectrode techniques.