In vitro studies of gas bubble formation by mechanical heart valves

Background and aim of the study: Recent clinical research using transcranial Doppler ultrasonography has shown the presence of emboli in the cranial circulation of some mechanical heart valve patients. Due to the high-intensity signals produced by these emboli, it has been suggested that they are small gas bubbles. Meanwhile, transesophageal echocardiography of mechanical heart valve patients has shown images of bright, mobile particles (also considered to be gas bubbles) near the valve. Motivated by these reports, a series of in vitro studies was performed to investigate the relationship between dissolved gas concentration and the incidence of bubble formation after valve closure. Methods: A mock circulatory loop was used to study a Medtronic Hall tilting disc valve in the mitral position of the Penn State Electrical Ventricular Assist Device (EVAD). The valve was videotaped as it operated in saline with various levels of dissolved CO2. A Doppler ultrasound probe served as a bubble detector on the outflow side of the EVAD. Measurements of vaporous cavitation intensity with a high-fidelity pressure transducer were also made. Similar experiments were then performed in porcine blood, using an imaging ultrasound transducer to detect bubbles. Results: Bubbles were seen moving off the valve in the retrograde direction just after closure. The ultrasound probe detected these bubbles downstream, indicating a bubble lifetime on the order of seconds. It was observed with high-speed video that bubble formation and cavitation are separate events and occur at different times during valve closure. Bubbles were more likely to be observed when CO2 levels were higher. Experiments in blood provided images of bubbles near the valve, predominantly at higher CO2 levels and high valve loading conditions. Conclusions: These results show that stable gas bubbles can form during mechanical heart valve operation. The bubbles likely form from the combined effects of gaseous nuclei formed by cavitation, low-pressure regions associated with regurgitant flow, and the presence of CO2, a highly soluble gas.