ASSESSMENT OF SMALL-DIAMETER AORTIC MECHANICAL PROSTHESES - PHYSIOLOGICAL RELEVANCE OF THE DOPPLER GRADIENT, UTILITY OF FLOW AUGMENTATION, AND LIMITATIONS OF ORIFICE AREA ESTIMATION

Background-Noninvasive assessment of functionally stenotic small-diame ter aortic mechanical prostheses is complicated by theoretical constra ints relating to the hemodynamic relevance of Doppler-derived transpro sthetic gradients. To establish the utility of Doppler echocardiograph y for evaluation of these valves, 20-mm Medtronic Hall and 19-mm St Ju de prostheses were studied in vitro and in vivo. Methods aad Results-R elations between the orifice transprosthetic gradient (equivalent to D oppler), the downstream gradient in the zone of recovered pressure (eq uivalent to catheter), and fluid mechanical energy losses were examine d in vitro. Pressure-flow relations across the 2 prostheses were evalu ated by Doppler echocardiography in vivo. For both types of prosthesis in vitro, the orifice was higher than the downstream gradient (P<0.00 1), and fluid mechanical energy losses were as strongly correlated wit h orifice as with downstream pressure gradients (r(2)=0.99 for both). Orifice and downstream gradients were higher and fluid mechanical ener gy losses were larger for the St Jude than the Medtronic Hall valve (a ll P<0.001). Whereas estimated effective orifice areas for the 2 valve s in vivo were not significantly different, model-independent dynamic analysis of pressure-flow relations revealed higher gradients for the St Jude than the Medtronic Hall valve at a given flow rate (P<0.05). C onclusions-Even in the presence of significant pressure recovery, the Doppler-derived gradient across small-diameter aortic mechanical prost heses does have hemodynamic relevance insofar as it reflects myocardia l energy expenditure. Small differences in function between stenotic a ortic mechanical prostheses, undetectable by conventional orifice area estimations, can be identified by dynamic Doppler echocardiographic a nalysis of pressure-flow relations.