The extent of supernormal ventricular excitability in man

The supernormal period is an interval of increased excitability found in Purkinje fibers and thought to discretely neighbor T wave termination. Thereafter, excitability is thought to be constant. We examined the extent of the supernormal period in normokalemic patients at normal heart rates, determining excitability in terms of threshold voltages required for right ventricular bipolar excitation at 10-30 sec msec intervals of diastole. In sinus rhythm (ten patients), thresholds during maximal excitability (Emax) following the relative refractory period were only 5.2% +/- 0.1 less than late diastolic thresholds (P less than 0.05). However, a discrete change from supernormal to normal period was never demarcated. Instead, in each patient all thresholds fell within 95% confidence limits of the linear relation: Threshold = a volts/second + ThresholdEmax. In fact, five of ten patients showed constant or diminished excitability as diastole progressed. In ventricular drive (six patients), threshold was constant throughout diastole. A supernormal period was never seen. Thus, excitability changes were progressive throughout diastole after sinus beats, and diastolic excitability was constant after ventricular beats. T wave termination did not signal a discrete period of increased excitability analogous to the supernormal period of Purkinje fibers.