LIMITATIONS IN THE APPLICABILITY OF THE SIMPLIFIED BERNOULLI RELATIONTO AORTIC-STENOSIS

Transvalvular pressure gradients (TPG) may be directly measured by inv asive catheterization of the heart or indirectly estimated by echocard iography through Doppler measurement of fluid velocity. A significant body of work has recently shown that pressure recovery distal to the p oint of maximum velocity must be taken into account in the evaluation of aortic flows. Supported by this new work, conceptual arguments are presented here that challenge the common computation of pressure gradi ent across a restricting orifice from measurements of maximum fluid ve locity, V. We argue that ideal, quasi-steady flow proximal to and high ly dissipative flow distal to the orifice are inappropriate simplifica tions. Rather, the relationship, TPG congruent-to K x V2, which freque ntly provides an acceptable estimate of TPG for values of K near 4, sh ould be viewed as an empirical estimate of pressure drop due to loss m echanisms. Calculations of energy losses using one particular flow mod el demonstrate this. Such a reorientation in the interpretation of Dop pler data provides critical insights into valve and patient variations , which heretofore have been assigned to measurement uncertainties. Ba sed upon this analysis, the Food and Drug Administration's Office of D evice Evaluation now asks that the coefficient K be determined in vitr o for new prosthetic heart valves prior to human implantation.