How much does aortic and pulmonary artery area vary during the cardiac cycle?

Cardiac output (CO), as measured by Doppler, is computed by multiplying vessel area by mean velocity. One unresolved problem is the magnitude of vessel area change during the cardiac cycle. Prior estimates have ranged from 2% to 30% area change during the cardiac cycle. The purpose of this study was to measure cyclic dimensional changes in the aorta and pulmonary artery of dogs by a very sensitive technique, mercury strain gauge plethysmography. Gauges were fixed around the pulmonary artery and the aorta and changes in vessel circumference were recorded under the following conditions: increased preload, increased afterload, decreased preload, and during administration of dobutamine, 5 and 10 micrograms/kg/min. Percent change in circumference of the aorta at baseline was small (mean circumference change = 4%, SD = 1%, SEE = 0.5). Percent change in the pulmonary artery at baseline was slightly larger (mean circumference change = 6%, SD = 5%, SEE = 2%). Mean aortic area change was 9% (SD = 2%) and for the pulmonary artery it was 12% (SD = 10%). No experimental manipulation significantly altered the percent cyclic change in vessel size from baseline except for clamping the inferior vena cava, in which mean percent change in aortic circumference decreased to 1% (SD = 1%, SEE = 5%) (p less than 0.05). Percent change in the aorta correlated only weakly with heart rate (r = 0.48), blood pressure (r = 0.4), and CO (r = 0.46). Similar values were obtained for the pulmonary artery. These data demonstrate a small but definite cyclic change in pulmonary artery area, and to a lesser extent in aortic area.