Bubble formation on St. Jude Medical mechanical heart valves: an in-vitro study

Bubble formation in blood, which has been observed during valve closure in patients with mechanical heart valves (MHVs), raises concern because of cognitive changes and potential activation of blood elements associated with thromboembolic phenomena. Bubble formation in the vicinity of MHVs has been described previously; the present in-vitro study was undertaken to quantitate bubbles leaving the heart and entering the systemic circulation, and to gain a closer perspective of this phenomenon and its potential clinical implications.

Experiments were performed in a left heart pulsed flow simulator with 29 mm mitral and 23 mm aortic St. Jude Medical MHVs; mitral and aortic bioprosthetic valves were used as controls. De-aerated water was pumped through the left heart pulsed flow simulator at a fixed rate of 70 beats/min and at different cardiac outputs. Bubble numbers and sizes were monitored photographically at the simulator exit.

Numbers of bubbles per frame generated in MHVs in the mitral position ranged from 8 to 14 at cardiac outputs of 3 to 6.5 1/min; this corresponded to gas volumetric flow rates of 330 to 830 mm3/min. The number of bubbles per frame for the aortic MHV was 3 to 7, reflecting the less severe flow conditions and milder valve closure impact. The diameter of bubbles generated by the mitral valve was almost uniform (0.45 mm), while bubble diameters in the aortic valve ranged from 0.36 to 0.69 mm.

Bubble formation was found to be an inherent flaw of the present generation of MHVs, and this problem should be addressed in future valve designs.