Fluid mechanics of aortic stenosis

In vitro qualitative and quantitative flow-mapping studies were conducted in an adult size aortic flow chamber, using bioprosthetic valves (0.5-5.0 cm2) to mimic varying degrees of aortic stenosis. The studies were performed under physiologic conditions in a left heart stimulator using: flow visualization, laser Doppler anemometry (LDA), continuous-wave (CW) Doppler and colour Doppler flow mapping (CDFM) techniques. Pressure gradients in the range 15-150 mmHg were accurately predicted by CW Doppler using the Bernoulli equation (r = 0.99). The flow visualization and CDFM studies revealed that all degrees of aortic stenosis led to jet-type flow fields, in which jet orientation was not necessarily symmetric and was skewed to varying degrees. Therefore, in aortic stenosis, Doppler measurements should be conducted in multiple views in order to visualize the flow field properly. Measurements with cross-sectional LDA revealed that as aortic stenosis increased: jet size narrowed; the peak velocity and turbulent intensities of the jet increased; jet instability increased; and acceleration of the jet proximal to the valve increased. Peak velocities as high as 4-7 ms-1 with turbulence levels (i.e. root mean square axial velocities) of 1.0-2.3 m s-1 were measured, with the moderately and severely stenotic valves. These elevated levels of turbulence could cause damage to the formed elements of blood and the walls of the ascending aorta. The high velocities and turbulence levels created by the moderately and severely stenotic valves, made quantitative interpretation of CDFM recordings very difficult, if not impossible.