Comparison of Valvular Resistance, Stroke Work Loss, and Gorlin Valve Area for Quantification of Aortic Stenosis

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Abstract
Background Valvular resistance and stroke work loss have been proposed as alternative measures of stenotic valvular lesions that may be less flow dependent and, thus, superior over valve area calculations for the quantification of aortic stenosis. The present in vitro study was designed to compare the impacts of valvular resistance, stroke work loss, and Gorlin valve area as hemodynamic indexes of aortic stenosis. Methods and Results In a pulsatile aortic flow model, rigid stenotic orifices in varying sizes (0.5, 1.0, 1.5, and 2.0 cm 2 ) and geometry were studied under different hemodynamic conditions. Ventricular and aortic pressures were measured to determine the mean systolic ventricular pressure (LVSP m ) and the transstenotic pressure gradient (ΔP m ). Transvalvular flow (F m ) was assessed with an electromagnetic flowmeter. Valvular resistance [VR=1333 · (ΔP m /F m )] and stroke work loss [SWL=100 · (ΔP m /LVSP m )] were calculated and compared with aortic valve area [AVA=F m /(50√ΔP m )]. The measurements were performed for a large range of transvalvular flows. At low-flow states, flow augmentation (100→200 mL/s) increased calculated valvular resistance between 21% (2.0-cm 2 orifice) and 66% (0.5-cm 2 orifice). Stroke work loss demonstrated an increase from 43% (2.0 cm 2 ) to 100% (1.0 cm 2 ). In contrast, Gorlin valve area revealed only a moderate change from 29% (2.0 cm 2 ) to 5% (0.5 cm 2 ). At physiological flow rates, increase in transvalvular flow (200→300 mL/s) did not alter calculated Gorlin valve area, whereas valvular resistance and stroke work loss demonstrated a continuing increase. Our experimental results were adopted to interpret the results of three clinical studies in aortic stenosis. The flow-dependent increase of Gorlin valve area, which was found in the cited clinical studies, can be elucidated as true further opening of the stenotic valve but not as a calculation error due to the Gorlin formula. Conclusions Within the physiological range of flow, calculated aortic valve area was less dependent on hemodynamic conditions than were valvular resistance and stroke work loss, which varied as a function of flow. Thus, for the assessment of the severity of aortic stenosis, the Gorlin valve area is superior over valvular resistance and stroke work loss, which must be indexed for flow to adequately quantify the hemodynamic severity of the obstruction.