Dependence of Gorlin formula and continuity equation valve areas on transvalvular volume flow rate in valvular aortic stenosis.

Abstract

BACKGROUND Valve areas derived by the Gorlin formula have been observed to vary with transvalvular volume flow rate. Continuity equation valve areas calculated from Doppler-echo data have become a widely used alternate index of stenosis severity, but it is unclear whether continuity equation valve areas also vary with volume flow rate. This study was designed to investigate the effects of changing transvalvular volume flow rate on aortic valve areas calculated using both the Gorlin formula and the continuity equation in a model of chronic valvular aortic stenosis. METHODS AND RESULTS Using a canine model of chronic valvular aortic stenosis in which anatomy and hemodynamics are similar to those of degenerative aortic stenosis, each subject (n = 8) underwent three studies at 2-week intervals. In each study, transvalvular volume flow rates were altered with saline or dobutamine infusion (mean, 10.3 +/- 5.1 flow rates per study). Simultaneous measurements were made of hemodynamics using micromanometer-tipped catheters, of ascending aortic instantaneous volume flow rate using a transit-time flowmeter, and of left ventricular outflow and aortic jet velocity curves using Doppler echocardiography. Valve areas were calculated from the invasive data by the Gorlin equation and from the Doppler-echo data by the continuity equation. In the 24 studies, mean transit-time transvalvular volume flow rate ranged from 80 +/- 33 to 153 +/- 49 mL/min (P < .0001). Comparing minimum to maximum mean volume flow rates, the Gorlin valve area changed from 0.54 +/- 0.22 cm2 to 0.68 +/- 0.21 cm2 (P < .0001), and the continuity equation valve area changed from 0.57 +/- 0.18 cm2 to 0.70 +/- 0.20 cm2 (P < .0001). A strong linear relation was observed between Gorlin valve area and mean transit-time volume flow rate for each study (median, r = .88), but the slope of this relation varied between studies. The Doppler-echo continuity equation valve area had a weaker linear relation with transit-time volume flow rate for each study (median, r = .51). CONCLUSIONS In this model of chronic valvular aortic stenosis, both Gorlin and continuity equation valve areas were flow-dependent indices of stenosis severity and demonstrated linear relations with transvalvular volume flow rate. The changes in calculated valve area that occur with changes in transvalvular volume flow should be considered when measures of valve area are used to assess the hemodynamic severity of valvular aortic stenosis.