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 con-