Bileaflet mechanical heart valves at low cardiac output.

Several earlier studies have indicated that bileaflet mechanical heart valves behave irregularly at low cardiac output and low pulse rate conditions, and that their hydrodynamic performances are generally inadequate. The authors conducted in vitro experiments in a pulsatile mock circulatory loop to compare the performance of the St. Jude Medical (SJM) valve and a long body bileaflet prosthesis recently introduced by Medical Carbon Research Institute (MCRI) (Austin, TX). The new MCRI mechanical heart valve model was designed with emphasis on improved hydrodynamic efficacy by introducing a long body with parallel leaflets and by leaflets increasing the flow area. Experimental studies were conducted on five test valves (MCRI 19 mm, MCRI 25 mm, SJM 19 mm, SJM 23 mm, and SJM 29 mm) with cardiac outputs of 2.0, 2.5, 3.0, and 3.5 L/min at a pulse rate of 40 beats/min, and 3.5, 4.0, 4.5, and 5.0 L/min at a pulse rate of 70 beats/min. Transvalvular pressure drop and closure volume were assessed by measuring the instantaneous ventricular and aortic pressures and aortic flow. The leaflet motions of the tested valves were observed by direct video recording using a charge coupled device camera while the flow measurements were being conducted. Testing under simulated physiologic ventricular and aortic pressure waveforms, the results of this study show that the MCRI bileaflets remained fully open during the entire ejection phase, even at very low cardiac output conditions (2.0 L/min). The closure volume (defined as percentage of forward flow volume) increased with decreasing cardiac output, as reported earlier by others. Comparative results also indicate that the MCRI design has nearly a two size pressure drop advantage over the SJM, with significantly smaller closure volume.