An orifice formula independent of mitral pressure gradient for the evaluation of prosthetic mitral valve obstruction.

We have studied an hydraulic orifice equation capable of quantifying the obstructive properties of mitral prostheses from data of a potentially less invasive nature than is required by conventional methods. The results indicate that effective prosthetic orifice area may be computed by the pressure independent equation A = 21SV/DFP2, where A is the effective prosthesis orifice area in cm2, SV is the stroke volume in ml and DFP is the diastolic filling period in s min-1. Areas computed with the pressure independent equation correlate with the results of the modified Gorlin formula (given in the appendix) at a level of r = 0.91 for a series of 17 cases within a range of effective valve areas of 0.50 cm2 to 1.60 cm2. These results suggest that simplified nontraumatic procedures may be developed to screen and follow patients with prosthetic valve implants.