Interaction of oscillators: effect of sinusoidal stretching of the sinoatrial node on nodal rhythm

In this study of factors affecting and integrating activity of cardiac tissues, isolated strips of rabbit atria containing the sinoatrial node were subject to sinusoidal stretch. Effects on electrical discharge rates and rhythm of this pacemaker were observed. Repetitive stretch of a few millimeters (20-40% increase in length) caused acceleration and produced oscillations in rate which were roughly proportional to frequency of stretching. At fast frequencies of stretching, the sinus rates oscillated faster but not synchronously. When rate of stretching approached the "resting" rate of the sinus, a "lock'in" occurred, in that sinus rate accelerated but oscillations disappeared. During stretching rates approximately twice resting rate, the oscillations again disappeared. At rates of stretching during which oscillations were synchronous, the time of pacemaker firing changed to coincide with the peak phase of the applied sinusoidal stretch, although several seconds were required for this adjustment. Interval histograms also showed that application of sinusoidal stretch imposed a greater regularity on pacemaker action. It was thus demonstrated that oscillatory processes of the cardiac pacemaker and an interaction of oscillators conceivably can occur in the heart. (Preliminary Publication, The Physiologist, 16 "3, 1973.)