Functional Refractory Period of Cardiac Tissues

¹ From the Department of Physiology, Instituto Nacional de Cardiologia, Mexico, D. F., Mexico

In dogs anesthetized with pentobarbital, with the heart denervated, the adrenals ligated, and the S.A. node crushed, pairs of stimuli (S₁–S₂) were applied to the auricle or ventricle, varying the delay between S₁ and S₂. When the auricle was stimulated, the following relations were measured: S₁–S₂: A₁–A₂; A₁–A₂; A₂–V₂; A₁–A₂; V₁–V₂. The corresponding curves for ventricular stimulation were also plotted. Calling R₁ and R₂ the responses at a given site, the S₁–S₁: R₁–R₂ curves usually showed a horizontal branch. This branch indicates that impulses stimulated at different moments in the refractory cycle may reach the recording site at the same moment of the cycle. A theorem is demonstrated which proves that the constancy of the R₁–R₂ interval cannot be explained merely on the basis of slowed conduction rate of R₂, but implies that the impulse stops at some point in the conducting path. For A-A or V-V propagation this stop is due to delay in the initiation. For A-V or V-A propagation, the stop occurs at some site in the intermediate conducting tissues. The stop in these cases is due to the existence of a prolonged functional refractory period (F.R.P.).

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