Ventricular Excitability of the Intact Dog Heart *

Since the refractory period was first described by Fontana in 1760,7 a variety of techniques has been employed to study the time-course of recovery of heart excitability and its relation to mechanical and electrical events during the cardiac cycle. Bowditch,7 in 1871, stimulated the perfused apex of the frog heart with rhythmic shocks at intervals of 1, 2, 3, 4, or more seconds. He found that stimuli just strong enough to produce a response failed to do so regularly, and a response to each shock could be obtained only if the strength of the stimulus were considerably augmented. The greater the frequency of stimulation, the more marked was the irregularity of response. At faster rates a much stronger stimulus was required to produce regular responses. The threshold was found to diminish with successive responses. Three years later, Kronecker and Stirling7 performed similar experiments with an improved type of stimulator and were unable to demonstrate this irregularity. They found that if stimuli were of sufficient strength, each produced a response. Shocks of lesser intensity were without effect. They confirmed Bowditch's observation that there was a gradual diminution of threshold with successive responses. Marey,7 in 1876, studied a spontaneously beating frog heart stimulated by single shocks thrown in at random throughout the cardiac cycle. He demonstrated that a refractory period began at the onset of systole for each cycle. The duration of this refractory period depended on the intensity of stimulation and the condition of the preparation. Trendelenburg,6 in 1911, devised an ingenious delayer system which enabled him to apply stimuli at various points along the mechanical contraction curve of the spontaneously beating frog heart. He was the first to carry out a systematic study of the relationship between mechanical events and excitability. He demonstrated the presence of absolute and relative refractory periods, and related them to the components of the mechanical contraction curve. The following year Trendelenburge7 attempted the first correlation between excitability and electrical events, using the monophasic action