Excitability of the mammalian ventricle throughout the cardiac cycle.

T HE concept that there is an excitability cycle of the heart containing an absolute refractory period, a relative refractory period and a somewhat longer interval of normal excitability had its origin in the work of Fontana, Bowditch, Kronecker, Marey and others (I). A period of supernormality following the relative refractory period was found to occur under certain conditions (2-5) and it has been suggested that there may be a subsequent subnormal period (6). Measurements both of the basal excitability and the phasic changes in excitability have been made on numerous occasions. The generally accepted conclusion is that recovery of excitability throughout the relative refractory period is a smoothly progressive process (7). Recent work (8, g), however, has indicated otherwise. This report is an account of results obtained from a study of the excitability cycle of the mammalian ventricle by means of improved techniques.