A COMPARISON OF VALVE RESISTANCE, THE CONTINUITY EQUATION, AND THE GORLIN FORMULA AGAINST DIRECTLY OBSERVED ORIFICE AREA IN BIOPROSTHETIC VALVES IN THE MITRAL POSITION - AN IN-VITRO STUDY

Background and aims of the study: There is no consensus over how to de scribe forward flow through valves in the mitral position. There are t hree main candidate hydraulic formulae; resistance, the Gorlin formula and the continuity equation, However, virtually no work has been perf ormed to validate resistance and the continuity equation for valves in the mitral position. The aim of this study, therefore, was to compare the three formulae against an independent standard provided by direct ly observed orifice areas. Materials and methods: Five bioprosthetic v alves with orifice areas between 0.14 cm(2) and 2.33 cm(2) were studie d in a pulse simulator at up to 20 different stroke volume/rate combin ations using quasi-physiologic flow curves. Orifice areas were measure d using a video camera, pressure difference using strain gauge transdu cers and Doppler signals using a 1.9 MHz Pedoff probe with a Vingmed S D50 system. Results: The Gorlin ratio (flow/root mean Delta P) had a d irect curvilinear relationship with the orifice area (log(y) = 0.31 0.36x; r = 0.94, SEE 0.08 cm(2), p < 0.0001). Resistance (mean Delta P /flow) had an indirect curvilinear relationship (log(y) = 0.19 - 0.55x , r = -0.93, SEE 0.13 cm(2), p < 0.0001). The continuity equation was directly related to observed orifice area although with high scatter ( y = 1.13 + 0.79x; r = 0.90, SEE 0.23 cm(2), p < 0.0001). Although both the Gorlin ratio and resistance changed with flow, there was also a t endency for observed orifice areas to increase with flow. Empirical ef fective orifice areas calculated using the regression equations closel y resembled observed orifice areas and agreement was reasonable, with 95% limits of -0.33 cm(2) to +0.33 cm(2) (Gorlin), -0.41 cm(2) to +0.4 2 cm(2) (resistance) and -0.40 cm(2) to +0.48 cm(2) (continuity). Conc lusion: In conclusion, no single formula adequately predicted all obse rved orifice areas although a resistance and the Gorlin formula gave u seful predictions after empirical correction.