Background and aim of the study: In 1988, valve manufacturers petitioned the U. S. Food & Drug Administration (FDA) to replace catheter with Doppler ultrasound measurements of pressure gradient (DP) in clinical studies. Manufacturers agreed to submit bench data validating the Bernoulli equation used to calculate DP: DP = K(Vd2 - Vp2), where K = constant, Vd = distal Doppler velocity, and Vp = proximal Doppler velocity. Previous studies suggest that K may vary from the idealized 4.0, which could lead to incorrect valve assessment and clinical errors.
Methods: Variation in K-values in marketing application data submitted to the FDA was assessed. Pulse duplicator data included four bileaflet valves, two stented bioprostheses, and seven stentless bioprostheses, sized from 19 to 33 mm. Effects of valve type, valve size, blood-mimicking fluid used, and distal pressure tap position (DPTP) were evaluated via an analysis of variance.
Results: K-values varied from 2.50 to 7.40 (n = 90). K was found

to be dependent on valve type (p <0.0001), blood-mimicking fluid (p <0.0001) and DPTP (p <0.0001), but not valve size. At DPTP = 30 mm, K = 3.43 ± 0.56, 5.15 ± 0.81, and 4.81 ± 1.02, for bileaflet, stented and stentless valves, respectively. K averaged 10% less using the 100-mm DPTP, due to pressure recovery. Variations due to blood-mimicking fluid were likely related to the fluid density.
Conclusion: Variations due to DPTP and fluid used are consistent with physical mechanisms of pressure recovery and fluid density. Results from previous studies have suggested that effects of valve type on K are also real. The magnitude of these effects appeared to be ±25%. Extrapolation to patients is difficult, but clinicians should be aware that Doppler measurements may vary by similar amounts. Doppler pressure gradients should be interpreted qualitatively and moderated by other diagnostic measures of valve performance.
The Journal of Heart Valve Disease 2004;13:461-466