Pressure Recovery Explains Doppler Overestimation of Invasive Pressure Gradient across Segmental Vascular Stenosis

Aims: This investigation sought to discern the accuracy of Doppler predictions of pressure drops in stenotic jets mimicking aortic coarctation (AC) across a broad spectrum of conditions, using an in vitro nonpulsatile flow model. Methods and results: The model allowed for different AC diameter and length, different cardiac output, and different diameter of the aorta distal to the AC. For each study condition, pressure drops across a model of AC were measured both noninvasively using the simplified Bernoulli equation (SBE) and invasively using the instantaneous actual pressure gradient. We observed a good correlation of SBE-predicted and actual pressure gradients (slope = 0.929; r = 0.907, P < 0.0001) when AC diameter was small and the diameter of the aorta distal to the AC was large. However, we observed various degrees of overestimation by Doppler when mild AC was simulated (P = 0.0003), or when the diameter of the aorta distal to the coarctation was reduced (P < 0.0001). Increased AC length was associated with underestimation of actual pressure gradient by Doppler when AC diameter was small. Accounting for pressure recovery and viscous losses reduced the discrepancies between Doppler-predicted and actual pressure gradients in all study conditions (slope = 0.964; r = 0.989, P < 0.0001). Conclusions: Caution should be used in applying the SBE to the noninvasive assessment of AC severity. Significant segmental stenosis is associated with significant underestimation of the actual pressure gradient because of viscous losses. A substantial overestimation of the actual pressure drop due to a pressure recovery effect may occur in mild AC, especially when the distal aorta is not dilated. (Echocardiography 2010;27:21-31).