Mechanism of Synchronization in Isorhythmic Dissociation

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Abstract
Third degree heart block was produced in anesthetized dogs by injecting 95% ethanol into the region of the A-V node. When the ventricles were paced artificially at a constant rate near the spontaneous rate of the S-A node, then the atria and ventricles became synchronized. During synchronization, the P wave oscillated rhythmically around the QRS. When the P preceded the QRS, the arterial blood pressure increased, whereas when the P wave followed the QRS, the blood pressure fell. Synchronization depended on such rhythmical fluctuations in blood pressure because (1) when the blood pressure changes were severely attenuated, synchronization ceased, and (2) simulation of the blood pressure changes by means of a servo-controlled pump also produced synchronization. A biological control system operates in such a way that (1) the P-R interval affects blood pressure by virtue of changes in the atrial contribution to ventricular filling; (2) the blood pressure has an inverse effect on atrial frequency through the baroreceptor reflex, and perhaps other mechanisms; and (3) changes in atrial frequency alter the P-R interval, to complete the control loop.