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The mechanisms behind ventricular premature beats may be divided into two main categories, reentry and modulated parasystole. Although it is relatively easy to distinguish among arrhythmia generating mechanisms on the cellular level, it is far more difficult in the clinic. Modulation of ventricular parasystole by sinus beats has been shown to result in a biphasic phase response curve. Two cases are reported where the deduction of such phase response curves from long-term ECG recordings is discussed and described as a stepwise procedure.

The relation between distribution patterns of ventricular premature complexes (VPCs) and heart rate in Holter recordings abundant in VPCs was analyzed using computer-assisted determination of the number of interectopic sinus beats at different heart rates. Within the complete 24-hour heart rate spectrum, zones were demonstrated that were characterized by manifest or concealed bigeminy or manifest or concealed trigeminy. Bigeminy zones were found in 26 and trigeminy zones in 21 of 42 patients. Bigeminy zones were found at a significantly lower heart rate, on the average, then trigeminy zones. In 10 patients both bigeminy and trigeminy zones were observed. Bigeminy and trigeminy zones probably correspond to the distribution patterns of VPCs predicted from modulation of a pacemaker and reflected reentry, both of which can be induced by electrotonically mediated impulses across a zone of impaired conduction in isolated bundles of Purkinje fibers. The bigeminy and trigeminy zones will correspond at least partly to the entrainment zones found during electrotonic modulation of parasystolic foci. The bigeminy zones will correspond to 2:1 entrainment and the trigeminy zones mainly to 3:1 entrainment.

Modulation of ventricular parasystole by sinus beats has been shown to result in a biphasic phase response curve. Some reports also indicate that the modulation effect could be described by a triphasic response with an early phase of acceleration, which would correspond to the supernormal phase of excitability. Two cases of such triphasic phase response curves are reported. The coexistence of ventricular couplets and parasystole is also discussed. The second beat of the couplet has often been explained by reentry or reflection, but could also be interpreted as due to supernormal modulation, resulting in an accelerated second ectopic response.