Instantanous dimensional changes of the right atrium of the cat

The sonomicrometer technique was used to study the instantaneous changes of diameter of the right atrium in open chest cats. At heart rates below 150 beats/min, 5 phases of diameter changes could be identified: lengthening (atrial filling), passive shortening (corresponding to rapid ventricular filling), diastasis, active shortening (atrial contraction), and movements of atrioventricular valves. At higher heart rates, passive shortening and diastasis disappeared. The active shortening always followed the systolic rise in atrial pressure after about 30 sec, thus suggesting the existence of an isovolumic phase of atrial contraction. The end diastolic and end systolic diameters and the amount and rate of change of the various phases were studied during acute alterations in venous return, sympathetic and vagal electrical stimulations and heart rate changes.