The two Staircase Phenomena

AbstractThe well known changes of amplitude of the ventricular contractions, which have been attributed to a staircase phenomenon, were confirmed in perfused dog hearts (figs. 1 and 2). All these changes are obtained by varying the frequency or the interval between the stimuli applied to the heart.Gradual changes of amplitude can also be obtained by modifying the amount of work performed by the ventricle. With a constant rate of stimulation, if the work increases because of an augmentation of either the input or the output pressures, the increased contractions cause a growth of those that follow. This growth persists for some time after the input or output pressures are returned to their initial values (figs. 3, 4 and 5).There are thus two staircase phenomena, one due to changes of the intervals between the stimuli and the other to changes in the amount of work performed by the ventricle. The two are independent (fig. 6).