THE HEMODYNAMIC-EFFECTS OF MECHANICAL PROSTHETIC VALVE TYPE AND ORIENTATION ON FLUID MECHANICAL ENERGY-LOSS AND PRESSURE-DROP IN IN-VITRO MODELS OF VENTRICULAR HYPERTROPHY

Background and aims of the study: When choosing a prosthetic replaceme nt for a natural heart valve, one objective should be to minimize the workload placed on the heart. This workload can be raised by fluid mec hanical energy losses imposed by the valve. For a patient with left ve ntricular hypertrophy, certain aortic valve types and orientations cou ld be hemodynamically superior to others. Methods: This study used a c ontrol volume analysis to investigate the effects of prosthetic mechan ical aortic valve type and orientation on fluid mechanical energy loss es in four in vitro models of the left ventricular outflow/aortic infl ow tract in various degrees of hypertrophy. Flow visualization studies were performed to qualitatively validate this analysis. The two most commonly used mechanical valve designs were studied: the St. Jude Medi cal (SJM) bileaflet valve and the Medtronic Hall (MH) tilting disk val ve. Experiments were performed in pulsatile flow at a constant heart r ate of 60 beats per min for five valve type/orientation combinations, The stroke volume was varied between 40 and 120 ml in five increments for each model and valve/orientation studied. Results: Valve type and orientation was found to have a significant effect on energy losses in these models (p < 0.05). Valve/orientation combinations with leaflets or disks approximately parallel to the proximal flow direction create d lower energy losses than others. The MH valve in the 180 degrees ori entation caused significantly less energy losses and pressure drops (o rifice and recovered) than any of the STM valve/orientations studied ( p < 0.05). The STM and MH valves in the 0 degrees orientation were res ponsible for significantly more energy loss than other valve/orientati ons studied (p < 0.05). An aortic inflow tract model with severe (45 d egrees) curvature created significantly more energy loss (p < 0.05) th an those with less curvature (15 and 30 degrees). However, the inserti on of an obstruction simulating a hypertrophic tissue outgrowth caused much more energy loss than increasing the severity of outflow tract c urvature from 15 to 45 degrees. Both orifice pressure drop and recover ed pressure drop had excellent linear correlations with energy losses found in these models. Conclusions: These results imply that: (i) pros thetic valve type and orientation should be considered when replacing the aortic valve of a hypertrophic patient; (ii) removal of obstructio ns within the aortic inflow tract will decrease ventricular workload; and (iii) the Doppler-estimated pressure gradients commonly used by ca rdiologists to assess the performance of a prosthetic valve, correlate very well with left ventricular energy loss and work load.