In vitro studies of gas bubble formation by mechanical heart valves

Background and aim of the study: Recent clinical research using transcrania l Doppler ultrasonography has shown the presence of emboli in the cranial c irculation of some mechanical heart valve patients. Due to the high-intensi ty signals produced by these emboli, it has been suggested that they are sm all gas bubbles. Meanwhile, transesophageal echocardiography of mechanical heart valve patients has shown images of bright, mobile particles (also con sidered to be gas bubbles) near the valve. Motivated by these reports, a se ries of in vitro studies was performed to investigate the relationship betw een 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 wi th various levels of dissolved CO2. A Doppler ultrasound probe served as a bubble detector on the outflow side of the EVAD. Measurements of vaporous c avitation intensity with a high-fidelity pressure transducer were also made . Similar experiments were then performed in porcine blood, using an imagin g ultrasound transducer to detect bubbles. Results: Bubbles were seen movin g off the valve in the retrograde direction just after closure. The ultraso und probe detected these bubbles downstream, indicating a bubble lifetime o n the order of seconds. It was observed with high-speed video that bubble f ormation and cavitation are separate events and occur at different times du ring valve closure. Bubbles were more likely to be observed when CO2 levels were higher. Experiments in blood provided images of bubbles near the valv e, predominantly at higher CO2 levels and high valve loading conditions. Conclusions: These results show that stable gas bubbles can form during mec hanical heart valve operation. The bubbles likely form from the combined ef fects of gaseous nuclei formed by cavitation, low-pressure regions associat ed with regurgitant flow, and the presence of CO2, a highly soluble gas.