Effect of Changing Heart Rate on Cardiovascular Function in the Conscious Dog

The effect of heart rate on the cardiovascular system of the conscious dog was studied by pacing the heart with an implanted right atrial electrode. Left ventricular outflow was measured by an electromagnetic flowmeter with a chronically implanted transducer on the ascending aorta. As heart rate increased, stroke volume fell, and this was linearly related to heart rate over the entire range studied. Peak flow rate and maximum acceleration decreased as heart rate increased, but the change was small compared with that in stroke volume. The decrease in stroke volume resulted principally from a loss of volume ejected in late systole. An increase in heart rate sometimes resulted in a rise of cardiac output, and when this occurred mean arterial pressure rose; when cardiac output did not increase, mean arterial pressure also remained unchanged. The data suggest that there is a pressure for the systemic circulation analogous to the "critical closing pressure" of peripheral vascular beds. It was shown that in these circumstances changes in calculated systemic resistance may not reflect any real alterations in the peripheral vascular beds.