Pressure wave propagation and input impedance in thoracic aorta of conscious newborn sheep

Aortic hemodynamics were examined in eight conscious newborn sheep. Flow and pressure in the thoracic aorta and pressure in the distal abdominal aorta were measured under control conditions and during pressure changes caused by bottle feeding or during intravenous infusions of nitroprusside, norepinephrine, or angiotensin II. Vasoconstriction affected aortic impedance, pressure wave amplification, and wave velocity similarly whether induced by feeding or by drugs. Central hemodynamics in the lamb were surprisingly similar to hemodynamics in the sheep fetus despite major changes in cardiovascular function at birth, largely because pressure-related increases in pulse wave velocity postpartum compensated for increased arterial lengths and increased heart rate. Wave reflection effects on pressure-flow relations were more prominent during vasoconstriction and less prominent during nitroprusside. Wave reflections in both lambs and fetal sheep return to the heart in early diastole; therefore, they do not add to ventricular afterload. Early diastolic return of reflected waves characterizes adults of many species, and demonstration of the phenomenon throughout the perinatal period reinforces arguments for its adaptive value.