Characteristics of Aortic Diastolic Pressure Decay with Application to the Continuous Monitoring of Changes in Peripheral Vascular Resistance

Large changes in stroke volume and peripheral vascular resistance were induced by varying the heart rate and by intra-aortically infusing acetylcholine or angiotensin II in six dogs with heart block and electromagnetic flowmeters chronically implanted around their ascending aortas. Changes in stroke volume, aortic and atrial pressures, and systemic resistance were monitored continuously for 3–6 hours under morphine-pentobarbital anesthesia. The characteristics of diastolic pressure decay at heart rates ranging from 60 beats/min to 200 beats/min and during transient periods of asystole were studied, especially with reference to the distortions caused by reflected pressure waves. The diastolic phase of pressure pulses recorded over a segment of the thoracic aorta several centimeters long centered about 4 cm cephalad to the dorsal insertion of the diaphragm could be closely approximated by a straight line on a semilogarithmic scale. Under the conditions of these experiments, changes in the slope of that line and of its reciprocal, the time constant, correlated well with concomitant variations in peripheral vascular resistance. This relationship appears to be of practical value for continuous monitoring of systemic resistance directly from the diastolic segments of pressure pulses recorded from the lower thoracic aorta.