PRESSURE WAVE-PROPAGATION AND INPUT IMPEDANCE IN THORACIC AORTA OF CONSCIOUS NEWBORN SHEEP

Aortic hemodynamics were examined in eight conscious newborn sheep. Fl ow and pressure in the thoracic aorta and pressure in the distal abdom inal aorta were measured under control conditions and during pressure changes caused by bottle feeding or during intravenous infusions of ni troprusside, norepinephrine, or angiotensin II. Vasoconstriction affec ted aortic impedance, pressure wave amplification, and wave velocity s imilarly whether induced by feeding or by drugs. Central hemodynamics in the lamb were surprisingly similar to hemodynamics in the sheep fet us despite major changes in cardiovascular function at birth, largely because pressure-related increases in pulse wave velocity postpartum c ompensated for increased arterial lengths and increased heart rate. Wa ve reflection effects on pressure-flow relations were more prominent d uring vasoconstriction and less prominent during nitroprusside. Wave r eflections in both lambs and fetal sheep return to the heart in early diastole; therefore, they do not add to ventricular afterload. Early d iastolic return of reflected waves characterizes adults of many specie s, and demonstration of the phenomenon throughout the perinatal period reinforces arguments for its adaptive value.