Effect of aortic compliance on Doppler diastolic flow pattern in coarctation of the aorta.

The spectral Doppler pattern at the site of an aortic coarctation (CoA) generally displays increased maximal velocity (Vmax) during systole with a slow velocity decay, resulting in the characteristic "sawtooth" pattern. If there is rapid velocity decay, the obstruction is often judged to be mild. The purpose of this study was to investigate if velocity decay is affected by proximal aortic compliance (C(p)). The relation between the velocity decay measured from the Doppler pattern and C(p) was studied with the use of an in vitro pulsatile flow model. The time (tau) between Vmax and 33% Vmax was the measure of velocity decay. The C(p) was varied from 0.7 to 2.6 mL/mm Hg for each of 4 levels of CoA severity. The various obstructions produced a Vmax range of 2.7 to 5.5 m/s. There was a positive linear relation between tau and C(p) (r(2) = 0.76). For a low C(p) (compliance = 0.7 mL/mm Hg), velocity decay was rapid (tau = 0.2 to 0.3 seconds) with no diastolic gradient. For equivalent obstructions, a high C(p) (2.6 mL/mm Hg) produced a persistent diastolic gradient and slow velocity decay (tau = 0.5 to 0.6 seconds). The Doppler pattern across a CoA is affected by C(p). Therefore, the absence of a sawtooth pattern should not exclude the diagnosis of significant CoA obstruction.