Fluid dynamics of aortic stenosis: Mechanisms for the presence of subvalvular pressure gradients

Intraventricular flow velocity waveforms and pressure gradients measured by high-fidelity multisensor catheters in isolated valvular aortic stenosis (AS) were analyzed. In 12 patients, valve area 1.0 ± 0.3 cm², intraventricular (subvalvular) pressure drops were 42 ± 16 mmHg and transvalvular ones 59 ± 22 mmHg. A fluid dynamic model for ejection through the tapering subvalvular field was developed to assess dissipative and nondissipative mechanisms. The striking augmentation of pressure gradients in the immediate vicinity of the stenosed orifice is underlaid mainly by the intensification of the convective acceleration effect. Whereas the convective component requires a confluent flow, the local accelaration component is always operative with a pulsed flow. At peak flow, when the local acceleration δV/δt is zero, the convective effect accounts fully for the measured gradients. For negative δV/δt, following peak ejection, the contribution of the local acceleration to the total pressure drop opposes the simultaneous effect of the convective component, whereas they both act in the same sense to peak ejection when δV/δt is positive. The influence of the taper in AS is much stronger on convective than on local acceleration gradients, since the former depend on the square, whereas the latter depend on the square root of the ratio of downstream to upstream flow-section areas of the subvalvular region. Thus, in AS, pressure drops and ejection velocities are more in phase as opposed to normal ejection dynamics. The measured gradient values also depend strongly on the distance between the exact placement of the pressure sensors along the tapering region. Viscous dissipation makes only a small contribution to the subvalvular gradients.