In Vivo Pulsatile Hemodynamics of Homografts

valves have centered on the measurement of life expectancy of patients and failure rates of the valves. Difficulties in obtaining reproducible hemodynamic data in the postoperative, closed chest patient have resulted in a reliance on relatively crude measures of function such as the assessment of valve gradients with correlation to subsequent regression of left ventricular hypertrophy. The hemodynamic characteristics of individual valves as they function in vivo are critical since failure rates of the valve are likely related to these properties. Our own in vivo data regarding the energy dissipation of homograft valves in the sheep model demonstrates not only a comprehensive laboratory method for quantifying differences in valves, but also shows critical differences between fresh and cryopreserved valves that may effect long-term results. The ideal valve would be biological and require no anti-coagulation, with a durability that equals or exceeds mechanical valves. Ricou et al. compared first and second generation (zero-pressure fixation) porcine prosthetic valves to St. Jude mechanical valves. The negligible differences in pressure gradients across the mechanical valves were mostly accounted for by annulus size. Zero-pressure fixation (Medtronic Intact and Freestyle) may prove to be a more durable process since this retains the natural collagen crimp of the valve cusp and maintains the tissue’s shock absorbing capacity which aids in resisting cyclic fatigue. The zero pressure fixation method is also thought to increase leaflet compliance and extensibility, improvements that may extend valve life. This study also pointed out discrepancies that may arise when studies are not standardized to cardiac output. Cardiac output across a given valve size may be of critical importance in smaller mechanical valves.The favorable hemodynamics of the smaller valve were demonstrated by Rashtian who demonstrated a resting mean gradient of 5mmHg and a peak gradient of less than 15mmHg in the 19mm St. Jude bileaflet valves. Some studies have suggested that the gradients across the 19mm St. Jude valve are small and acceptable only in patients with a body surface area (BSA) less than 1.7m. Others found that the valve functioned well regardless of BSA. O’Brien notes that the switch from formaldehyde to glutaraldehyde preservation occurred at the same time as the advent of stents. It is possible that the adverse effects of the stents were initially obscured by improvements in preservation methods. David’s work with the unstented porcine valve has shown a number of beneficial aspects in short-term follow-up. These valves showed regression of left ventricular hypertrophy and a reduction in residual gradient. It was shown in a separate study that the stentless porcine valve in the aortic position is associated with a greater decrease in left ventricular systolic wall stress, a decrease in transvalvular pressure gradient, and a decrease in valvular energy loss, when compared to the stented biological valve. Presence of a rigid stent contributes to a poor 11 In Vivo Pulsatile Hemodynamics of Homografts