Deduction of biphasic phase response curves from ventricular parasystole.

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.