A comparison of the classical and modified forms of the continuity equation in the On-X (R) prosthetic heart valve in the aortic position

Background and aim of the study: The use of echocardiography to determine p rosthetic valve hemodynamics has become generally accepted; however, there are still many differing methodologies in use. The continuity equation, whi ch uses the ratio of the subaortic and transaortic velocity-time integrals for determining aortic effective orifice area (EOA), has been established a s an accurate method. Another method using the more easily measured peak ve locities in ratio has also been employed. These methods were compared to de termine if the simpler method gave equivalent results. Methods: Early postoperative echocardiographic data on prosthetic valves fr om the MCRI Multicenter Trial were used to compare the two methods of calcu lating EOA (A(2)). Results using the two methods were compared by paired t- tests, the Wilcoxon signed rank test, regression and Bland-Altman analysis. Results: Despite a good correlation between the two methods (r = 0.91), res ults were different when compared by a paired t-test. On average, results b y the modified method were 0.2 cm(2) lower, but in 28% of cases they were i n fact higher than the classical method. Conclusion: The modified continuity equation based on the peak velocity rat io does not give the same result as the classical formula based on the velo city-time ratio. The modified method cannot reliably be substituted for the classical method in normally functioning On-X valves.