Scientific Publications by FDA Staff

Echocardiography 1994 Sep;11(5):437-43

Limitations in the Applicability of the Simplified Bernoulli Relation to Aortic Stenosis

Carey RF, Herman BA

Transvalvular pressure gradients (TPG) may be directly measured by invasive catheterization of the heart or indirectly estimated by echocardiography through Doppler measurement of fluid velocity. A significant body of work has recently shown that pressure recovery distal to the point 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 gradient across a restricting orifice from measurements of maximum fluid velocity, V. We argue that ideal, quasi-steady flow proximal to and highly dissipative flow distal to the orifice are inappropriate simplifications. Rather, the relationship, TPG ¿K× V2, which frequently provides an acceptable estimate of TPG for values of K near 4, should be viewed as an empirical estimate of pressure drop due to loss mechanisms. Calculations of energy losses using one particular flow model demonstrate this. Such a reorientation in the interpretation of Doppler data provides critical insights into valve and patient variations, which heretofore have been assigned to measurement uncertainties. Based upon this analysis, the Food and Drug Administration's Office of Device Evaluation now asks that the coefficient K be determined in vitro for new prosthetic heart valves prior to human implantation.