Influence of Cycle Length Upon Refractory Period of Auricles, Ventricles, and A-V Node in the Dog

The functional refractory periods (FRP) of auricular and ventricular muscle, and of the A-V transmission system were estimated in the denervated dog heart. In all three tissues the FRP was found to be a curvilinear function of the immediately preceding cycle length, with values for all but very early premature beats falling close to the curve describing the basic cycles; that is, there was no evidence for a cumulative effect of frequency. In the auricle, the FRP of even the earliest possible premature beats appeared to fit the curve relating cycle length to refractory period. In both the A-V node and ventricles very early responses deviated from an otherwise smooth relationship. It is postulated that the deviations may represent dissociation of the properties of two different elements in each of these tissues.