Alterations in aortic wave reflection with vasodilation and vasoconstriction in anaesthetized dogs.

Using the reservoir-wave approach, we studied wave propagation, reflection, and re-reflection in the canine aorta with administrations of sodium nitroprusside (NP) and methoxamine (Mtx). In 8 anaesthetized dogs, excess pressures were calculated from pressure and flow measurements at 4 locations along the aorta; wave intensity analysis was employed to identify wavefronts and the type of waves. NP (intravenous; 14 μg/min) decreased mean aortic pressure from 80 ± 3 mm Hg to 48 ± 1 mm Hg; Mtx (intravenous; 10 μg/min) increased mean pressure from 80 ± 3 mm Hg to 104 ± 4 mm Hg. NP increased negative reflection near the kidneys (reflection coefficient: -0.33 vs -0.18; P < 0.01) and produced new negatively reflecting sites just beyond the arch and in the proximal femoral arteries, consistent with a vasodilating effects of nitrates on conducting arteries. Mtx negated negative reflection from near the kidneys (-0.02 vs -0.17; P < 0.01) and increased positive femoral reflection (0.38 vs 0.26; P < 0.01). The large reflected compression wave was re-reflected from the closed aortic valve to produce a prominent increase in middiastolic pressure in the distal aorta. The reservoir-wave approach explains decreasing diastolic pressure without positing waves that travel at near-infinite velocities and reveals the pressure changes that are uniquely due to wave motion.