Assessment of the Björk-Shiley Prosthetic Valve Orifice Area in the Aortic Position

The actual orifice area of a valve is still considered to be a valuable index for assessing prosthetic valve function. Valve orifice area as calculated by Gorlin's formula is, however, not constant but changes in proportion to the transvalvular flow rate. The purpose of the present study was to examine the relationship between orifice area and flow rate across the Björk-Shiley prosthetic valve as calculated by Gorlin's formula, and to modify the formula in a series of patients with the Björk-Shiley prosthetic valve in the aortic position. Fifty-six patients who had received aortic valve replacement with a Björk-Shiley prosthetic valve underwent cardiac catheterization. Prosthetic valve orifice area was calculated by Gorlin's formula and then plotted against flow rate across the valve with respect to valve size. The relationship between orifice area and flow was linear. The discharge coefficient of Gorlin's formula was plotted against flow rate, and a linear correlation was obtained. By substituting Gorlin's formula for an empiric coefficient into the function for transvalvular flow rate, a modified formula that can predict the actual orifice area of the prosthetic valve is obtained.