The continuity equation, derived from the study of fluid mechanics, may serve as the basis for calculation of orifice area of stenosed cardiac valves. As applied to aortic stenosis, the continuity equation states that the flow across the narrowed valve is equal to the flow in the left ventricular (LV) outflow tract such that A1 X v1 = A2 X v2, where A1 = LV outflow tract area, v1 = prestenotic velocity, A2 = stenotic orifice area and v2 = poststenotic velocity. Accordingly, at each point in time during pulsatile flow, the respective valve orifice area can be calculated. Hence, from the sum of all areas throughout the ejection time, the mean valve orifice area can be constructed as integral of A2/ET = A1 X integral of (v1/v2)/ET, assuming A1 to be constant, where integral of denotes the integral over the ejection time ET. To assess the usefulness of this method with respect to its clinical relevance, in 36 patients with aortic stenosis, the Doppler echocardiographically-determined orifice areas were compared with those calculated by the Gorlin formula based on invasively-obtained data. LV outflow tract area A1 was measured by echocardiography from a parasternal long-axis view. Prestenotic velocity v1 was recorded in the LV outflow tract by pulsed Doppler from an apical transducer position, whereby care was taken in positioning the sample volume not too close to the stenotic valve to avoid the prestenotic area of increased velocity. Continuous-wave Doppler was used, usually from an apical or right parasternal transducer position, to record the stenotic jet velocity v2.(ABSTRACT TRUNCATED AT 250 WORDS)