DEPENDENCE OF GORLIN FORMULA AND CONTINUITY EQUATION VALVE AREAS ON TRANSVALVULAR VOLUME FLOW-RATE IN VALVULAR AORTIC-STENOSIS

Background Valve areas derived by the Gorlin formula have been observe d to vary with transvalvular volume flow rate. Continuity equation val ve areas calculated from Doppler-echo data have become a widely used a lternate index of stenosis severity, but it is unclear whether continu ity equation valve areas also vary with volume flow rate. This study w as designed to investigate the effects of changing transvalvular volum e flow rate on aortic valve areas calculated using both the Gorlin for mula and the continuity equation in a model of chronic valvular aortic stenosis. Methods and Results Using a canine model of chronic valvula r aortic stenosis in which anatomy and hemodynamics are similar to tho se 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 hemod ynamics using micromanometer-tipped catheters, of ascending aortic ins tantaneous volume flow rate using a transit-time flowmeter, and of lef t ventricular outflow and aortic jet velocity curves using Doppler ech ocardiography. Valve areas were calculated from the invasive data by t he Gorlin equation and from the Doppler-echo data by the continuity eq uation. In the 24 studies, mean transit-time transvalvular volume flow rate ranged from 80+/-33 to 153+/-49 mL/min (P<.0001). Comparing mini mum to maximum mean volume flow rates, the Gorlin valve area changed f rom 0.54+/-0.22 cm(2) to 0.68+/-0.21 cm(2) (P<.0001), and the continui ty equation valve area changed from 0.57+/-0.18 cm(2) to 0.70+/-0.20 c m(2) (P<.0001). A strong linear relation was observed between Gorlin v alve area and mean transit-time volume flow rate for each study (media n, r=.88), but the slope of this relation varied between studies. The Doppler-echo continuity equation valve area had a weaker linear relati on with transit-time volume flow rate for each study (median, r=.51). Conclusions In this model of chronic valvular aortic stenosis, both Go rlin and continuity equation valve areas were flow-dependent indices o f stenosis severity and demonstrated linear relations with transvalvul ar volume flow rate. The changes in calculated valve area that occur w ith changes in transvalvular volume flow should be considered when mea sures of valve area are used to assess the hemodynamic severity of val vular aortic stenosis.