Supernormal phase of conduction in human heart demonstrated by subthreshold pacemakers.

Subthreshold stimuli may produce ventricular excitation when they fall within a limited period of the cardiac cycle. This phenomenon, known as the supernormal phase of conduction (Adrian, 1921), was shown in the anaesthetized animal to be from the beginning of the descending limb of the T wave, and the beginning of the U wave (Hoff and Nahum, 1938). Its presence in the human heart was first reported by Lewis and Master (1924). Wenckebach and Winterberg (1927) later doubted its existence, but several authors have since confirmed its presence (Scherf and Schott, 1939; Kline, Conn, and Rosenbaum, 1939; Pick, Langendorf, and Katz, 1962; Soloff and Fewell, 1960; Linenthal and Zoll, 1962; Burchell, Connolly, and Ellis, 1964; Linenthal and Zoll, 1965). This period of hyperexcitability of the heart has been invoked to explain the genesis of a number of ventricular dysrhythmias, including those extrasystoles occurring early in diastole immediately after the T wave (Mack, Langendorf, and Katz, 1947; Soloff and Fewell, 1960; Burchell, 1963). In this paper, we show the supernormal phase of conduction of the human heart by observation on the effects of subthreshold artificial stimuli from failed implanted pacemakers. The duration of this phase is determined and its relation to ventricular extrasystoles established.