In vitro evaluation of the long-body On-X bileaflet heart valve

In order to optimize the length-to-diameter ratio, a series of circular aluminum rings with flared inlet and varying ring lengths, with internal diameters corresponding to that of 19 mm replacement prosthetic heart valve orifices, were tested in a steady-flow hydraulic system. The study aim was to determine the ring length-to-internal diameter ratio that produces the best hydraulic efficiency (i.e. lowest pressure gradient) within the physiologic flow rate range.

Each ring was tested at flow rates of 10, 15, 20, 25 and 30 l/min and length-to-diameter ratio effect on hydraulic efficiency determined experimentally. The hydraulic effect was most significant for a ratio of about 0.6, with an increase to 1.2 providing little additional benefit. Thus, a ratio of about 0.6 was considered optimum in terms of hydraulic efficiency and incorporated into the design of the On-X bileaflet mechanical heart valve (BHV) series. An in vitro hydrodynamic study of the smallest (19 mm) and largest (25 mm) clinical On-X aortic valves was performed at two independent laboratories. Standard St. Jude Medical BHVs were used as the study controls.

Steady-flow experiments showed that the pressure gradient in the On-X valve was about 50% less than that of the comparable size control. The pulsatile flow study demonstrated a similar pressure gradient advantage. Laser Doppler anemometer velocity profiles taken downstream of the On-X valve at the aortic root showed typical characteristics of bileaflet valves, with three velocity peaks. The peak velocity reached 1.6 m/s for the On-X and 1.75 m/s for the control valve. A recirculating vortex was seen in the sinus cavity during the ejection period. This vortex, found in most aortic valves (including bioprostheses), is believed to provide a beneficial wash-out of the valve region and assist in valve closure.

These two independent studies clearly demonstrated that the elongated valve body and comparably larger flow area helped to improve the valve hydrodynamic performance, which is especially beneficial in the smallest (19 mm) size valve.