Experimental analysis of fluid mechanical energy losses in aortic valve stenosis: Importance of pressure recovery

Current methods for assessing the severity of aortic stenosis depend primarily on measures of maximum systolic pressure drop at the aortic valve orifice and related calculations such as valve area. It is becoming increasingly obvious, however, that the impact of the obstruction on the left ventricle is equally important in assessing its severity and could potentially be influenced by geometric factors of the valve, causing variable degrees of downstream pressure recovery. The goal of this study was to develop a method for measuring fluid mechanical energy losses in aortic stenosis that could then be directly related to the hemodynamic load placed on the left ventricle. A control volume form of conservation of energy was theoretically analyzed and modified for application to aortic valve stenosis measurements. In vitro physiological pulsatile flow experiments were conducted with different types of aortic stenosis models, including a venturi meter, a nozzle, and 21-mm Medtronic-Hall tilting disc and St. Jude bileaflet mechanical valves. The energy loss created by each model was measured for a wide range of experimental conditions, simulating physiological variation. In all cases, there was more energy lost for the nozzle (mean = 0.27 J) than for any other model for a given stroke volume. The two prosthetic valves generated approximately the same energy losses (mean = 0.18 J), which were not statistically different, whereas the venturi meter had the lowest energy loss for all conditions (mean = 0.037 J). Energy loss correlated poorly with orifice pressure drop (r² = 0.34) but correlated well with recovered pressure drop (r² = 0.94). However, when the valves were considered separately, orifice and recovered pressure drop were both strongly correlated with energy loss (r² = 0.99, 0.96). The results show that recovered pressure drop, not orifice pressure drop, is directly related to the energy loss that determines pump work and therefore is a more accurate measure of the hemodynamic significance of aortic stenosis.