Cavitation

  Reservoir at the CVE pulse duplicator Copyright: CVEAME Figure 1: open reservoir at the CVE pulse duplicator

Background

Cavitation is one reason for material damage on mechanical heart valve prostheses. Cavitation occurs at or directly after valve closure. Because of the abrupt stopping of the prostheses leaflets at prosthesis valve closure, the fluid column upstream the prosthesis stalls, what results in a huge local pressure drop in the fluid. The physical fluid-gas is dissolved and gas bubbles develop at the contact area leaflet-housing. The cavitation bubbles have a very short lifetime of approximately some micro- to milliseconds. The bubble collaps generates extremely high pressures and material pitting at the valve leaflets and housing occurs. The pitting – a kind of material erosion- firstly occurs in a microscopic range, but because of permanent impact the erosion areas increase and surface fractures and notches develop. This effect leads finally to prosthesis failure.

The determination of the cavitation threshold is an important aspect for the examination of mechanical heart valve prostheses according the valid standards.

Method

The assessment of the cavitation threshold is performed in the CVE pulse duplicator (figure 1). For cavitation testing the test prosthesis is mounted in mitral position and the flexible atrium is exchanged by an open reservoir, which permits the observation of the closing motion of the test prosthesis by means of a high speed video system. With a frame rate of 18.000 frames/s and more cavitation bubbles can be detected (figure 2). The simultaneous acquisition of the ventricular pressure is performed to determine the cavitation threshold.

According to FDA guidance, the cavitation threshold is defined as the left ventricular pressure increase velocity before valve closure, when cavitation occurs barely.

  Cavitation bubbles on a heart valve prostheses Copyright: CVEAME Figure 2: Cavitation bubbles on the upstream side of a mitral valve prostheses; recorded in the CVE pulse duplicator