Effects of drugs on the negative (backflow) component of velocity patterns in the dog aorta.

Using an accurately calibrated flowmeter in the descending thoracic aorta of the dog, a study was made of alterations in aortic flow patterns following intravenous and intra-arterial injections of vasoactive drugs. When injected distal to the flowmeter, vasopressors caused marked backflow while vasodilators raised the level of the lowest portion of the pulsatile flow curve. When injected into the brachiocephalic artery, vasopressors prevented the development of a significant negative component whereas vasodilators caused the appearance of backflow, or augmented that which was already present. All acute elevations or decreases in peripheral resistance due to drugs were accompanied by increases in the negative or "backward" phase of flow in diastole. A significant negative flow component was noted only during these acute adjustments. A temporary differential in resistances of the upper and lower portions of the arterial circulation protected cerebral and coronary blood flow during acute haemodynamic adjustments.