Hemodynamic effects of acute experimental aortic coarctation in the dog

Pressure and flow pulses were recorded immediately proximal and distal to a temporary stenosis of the thoracic aorta of the dog. The severity of the stenosis was varied, and the magnitude of the pulse changes was correlated with the degree of stenosis expressed as the area ratio, As/All, where As is the cross-sectional area of the stenotic section and A-s that of the normal vessel. The harmonic amplitudes of the pressure and flow waves during the stenotic period were obtained by Fourier analysis and normalized to their values when the stenosis was released. It was found that as A-s/All yields 0 the proximal pressure amplitude increased and the distal pressure and the proximal and distal flow amplitudes decreased. These changes were such that the proximal fluid impedance calculated from the corresponding pressure and flow amplitude ratios increased as A-s/A-ll yields 0; the distal impedance remained relatively unaffected. These findings can be interpreted in terms of reflection of the pulse waves at the stenosis. The normalized proximal pressure amplitudes approximate to 1 + R and the distal pressure and both the proximal and the distal flow vary according to 1 minus R, where R is the reflection coefficient. The variation of R with A-s/A-ll can be determined from the proximal fluid impedance changes; we found that R increased relatively slowly for moderate stenoses, varying from 0 at A-s/A-ll 1.0 (no stenosis) to similar to 0.2 at A-s/A-ll equals 0.2. At more severe stenoses, it increased more rapidly approaching 1.0 at A-s/A-ll equals 0. The generation of turbulence was most marked at A-s/A-ll yields 0.2.