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Five laboratories carried out measurements of cavitation threshold for a common set of six mechanical prosthetic heart valves, two each from three different manufacturers. This study was intended to evaluate to what extent FDA's current guidance for cavitation testing would lead to consistent results in a variety of laboratory settings and to seek areas for improvement in the recommended test protocol. The inter-laboratory study protocol specified: (1) characterization of the test fluid by oxygen content and electrical conductivity, (2) location and frequency response of pressure sensors, (3) determination of ventricular and atrial pressures (P) and loading rates (dP/dt) averaged over the time period of valve closure and over the time periods of 1 ms, 5 ms, and 20 ms prior to video visualization. The protocol did not specify: (1) the fluid pumping equipment to be used to generate cavitation, (2) the pump or fluid parameters adjusted to raise or lower the loading rate, (3) the equipment, technique, or sensitivity used to visualize cavitation, and (4) a specific definition of the threshold for cavitation. Results from the five laboratories are reported. Significant differences in results were observed in dP/dt and in the pressure difference across the valves during closure at cavitation threshold. Specific differences in test systems included a wide range of ventricular compliance and single valved versus double valved test systems. Three single valve systems with compliant ventricles produced results in reasonable agreement with one another. Further similarity in test equipment should be specified to assure adequate interlaboratory reliability for cavitation testing. Areas needing better specification include the design of the valve mount, the design of the cavitation generators, and qualitative criteria for detection of threshold cavitation.

An in vitro study was designed to evaluate the effect of structural compliance on cavitation threshold measurements of mechanical heart valves. Using a dual channel high speed video image analysis method, the experiment was carried out in a pulse duplicator under a single, simulated physiologic condition. Each valve was mounted on a compliance adjustable fixture. One video camera served as a displacement monitor, while the other monitored cavitation bubbles at the inflow side of the valve. On-line adjustment of the mounting compliance allowed for cavitation threshold detection with respect to compliance without influencing the testing condition. Visible cavitation bubbles disappeared when the compliance was increased. Ten 29 mm mitral valves, including two each of St. Jude Medical, Carbomedics, Edwards-Duromedics (ED), Edwards Tekna (ET) and Medtronic-Hall (MH) were tested. All the bileaflet valves tested in this study showed similar cavitation thresholds at a compliance range of 0.5-0.65 mil/lb at a dp/dt of 4,000 mmHg/sec (averaged dp/dt during valve closure). Under the same dp/dt, MH monoleaflet valves showed a higher compliance range of 3.5-3.8 mil/lb to achieve the cavitation threshold. This study demonstrated that the mounting compliance of the valve must be known and well controlled in order to quantify the cavitation threshold with respect to dp/dt. In addition, the more compliant sewing ring of the ET valve may be responsible for higher cavitation thresholds with respect to dp/dt as observed in previous studies. The present in vitro study suggests that the investigation of the sewing structural compliance may be essential for the evaluation of in vivo cavitation potential.