title: A comparison of the classical and modified forms of the continuity equation in the On-X prosthetic heart valve in the aortic position.
abstract: OBJECTIVE
  The use of echocardiography to determine prosthetic valve hemodynamics has become generally accepted; however, there are still many differing methodologies in use. The continuity equation, which uses the ratio of the subaortic and transaortic velocity-time integrals for determining aortic effective orifice area (EOA), has been established as an accurate method. Another method using the more easily measured peak velocities in ratio has also been employed. These methods were compared to determine if the simpler method gave equivalent results.
  
  
  METHODS
  Early postoperative echocardiographic data on prosthetic valves from the MCRI Multicenter Trial were used to compare the two methods of calculating EOA (A2). 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), results were different when compared by a paired t-test. On average, results by the modified method were 0.2 cm2 lower, but in 28% of cases they were in fact higher than the classical method.
  
  
  CONCLUSIONS
  The modified continuity equation based on the peak velocity ratio does not give the same result as the classical formula based on the velocity-time ratio. The modified method cannot reliably be substituted for the classical method in normally functioning On-X valves.
authors.affiliation: Adult Echocardiography, Guy's and St. Thomas' Hospitals, London, UK.
authors.forename: J
authors.initials: J
authors.lastname: Chambers
authors.forename: J
authors.initials: J
authors.lastname: Ely
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001022:Aortic Valve Insufficiency / Q000503:physiopathology / Q000601:surgery*; D001024:Aortic Valve Stenosis / Q000503:physiopathology / Q000601:surgery*; D001783:Blood Flow Velocity / Q000502:physiology; D004452:Echocardiography; D019544:Equipment Failure Analysis; D005260:Female; D006350:Heart Valve Prosthesis; D006801:Humans; D008297:Male; D008875:Middle Aged; D015233:Models, Statistical; D011183:Postoperative Complications / Q000175:diagnosis / Q000503:physiopathology; D011474:Prosthesis Design