SUMMARY An analysis of electrocardiograms from a patient with spontaneous double irregular ventricular parasystole is presented. Irregularity in one of the two parasystoles was produced by intermittence based on rate-dependent (phase 3) entrance block, and in the other parasystole it was attributed to "supernormal" exit conduction. Critical analysis of electrocardiograms revealed that first degree block, ratedependent block, and "supernormal" conduction in the exit pathway may account for the alterations in the arrangement and manifestation of the parasystolic beats. An electrocardiographic approach to these properties of the parasystolic structure and demonstration of double ventricular parasystole with irregularity in both parasystoles has not previously been found in the literature.AMONG THE FEW REPORTS on spontaneous double and multifocal ventricular parasystole,1-7 only three cases with intermittence in one of the parasystoles were demonstrated.5-7 Temporary loss of protection of the parasystolic focus was postulated for the interpretation of intermittence in these cases. Resetting of the parasystolic pacemaker was ascribed to supernormal conduction5 and to rate-dependent conduction disturbance in the surrounding tissues.6'7 The purpose of our report is: 1) to demonstrate a patient with double irregular ventricular parasystole, 2) to approach some electrophysiological properties of the parasystolic structure, and 3) to review the diagnostic criteria of intermittence based on phase 3 conduction defect.Demonstration and Interpretation of Electrocardiograms The ECG tracings analyzed were selected from several long rhythm strips (standard lead II) obtained over a seven-day period from a 64-year-old male patient with coronary heart disease. ECG on admission ( fig. IA, continuous tracings) shows QRS complexes of sinus origin (labelled R) and two types of ectopic QRS complexes (labelled A, ventricular ectopy and labelled B). The wide variations in coupling intervals and spacing of both A and B ectopic beats fulfill the criteria for regular double ventricular parasystole with short runs of the double parasystolic rhythm. Beats labelled F are fusions of B and R in the first strip, and of A and R in the second one. Occasionally, the ventricles and the atria were controlled by pure bifocal parasystolic rhythm over longer periods, as seen in figure 1 time intervals in this and in subsequent figures and in the text are expressed in hundredths of a second. The consecutive records in figure 2A were obtained the next day. Parasystole B appears for only one instance in the form of a fusion beat (the second beat in the third strip). The smallest manifest interectopic A-A intervals range from 184-188. The longer interectopic intervals are not simple multiples of the smallest ones. The differences between calculated and manifest cycle lengths exceed that accepted for simple parasystole,8 and the characteristic structure of Wenckebach type exit block cannot be recognized.9 Analysis of the ECG reveals that if the coupling time ...
