Anatomic versus Effective Orifice Area in a Bicuspid Aortic Valve

In their article, House et al. provide an excellent discussion of the differences between effective orifice area (EOA) and anatomic orifice area (AOA), and their respective ability to predict bicuspid aortic valve (BAV) function and stenosis. Their case study illustrates the weak correlation between those two metrics in BAV patients with severe stenosis. The authors conclude that AOA may overestimate EOA in BAV patients and that EOA may be a better predictor for the functional assessment of stenosis in those patients. The authors have raised a critical point and we would like to highlight the fluid mechanical complexity of the BAV and its impact on the clinical assessment of stenosis, as reported in our recent study published in the Journal of Biomechanical Engineering. Specifically, we conducted in vitro steady-flow measurements to quantify valvular performance and energy loss in porcine tricuspid aortic valve (TAV) and BAV models under normal and simulated calcified states. First, our measurements demonstrated that the difference between AOA and EOA is increasingly significant in calcified valves. At near peaksystolic flow rate (20 L/min), the differences between those two metrics were 18% and 13% in the noncalcified TAV and BAV, respectively, but attained 21% and 33% in the moderately calcified TAV and BAV, respectively. Those results support the increasingly weak correlation between EOA and AOA described by House et al. in calcified BAVs. Second, we would like to point out the importance of the pressure recovery in the assessment of BAV function. While this phenomenon may be insignificant in the case described by House et al. due to the strong jet eccentricity and enlarged aorta, it should not be systematically discarded for the evaluation of all BAV patients. In fact, our flow measurements at near peak-systolic flow rate indicated a 34% reduction in jet skewness in the calcified BAV relative to the noncalcified BAV, presumably due to the structural alterations caused by the presence of calcific nodules on the leaflets. In addition, all our measurements were performed using a normal aortic diameter. In this particular context (i.e. mild orifice jet skewness and normal/mildly dilated aorta), we demonstrated the low sensitivity of the EOA estimated by the Gorlin equation to changes in workload through the valve, as quantified by the transvalvular energy loss: EL 1⁄4 DPþ qQ 2 2 Aa Am ðAaAmÞ ; where Am and Aa are the ventricle and aorta cross-sectional areas, respectively, q is the blood density and Q is the flow rate. Those results suggest the inability of the EOA, which is solely a function of the difference in static ventricular and aortic pressures, to account for the pressure recovery phenomenon, which might be significant in calcified BAVs. Therefore, the study conducted by House et al. and our measurements highlight the fluid mechanical complexity of calcified BAVs, in which pressure drop cannot be reliably predicted from the sole knowledge of the flow rate and AOA, and justify the use of alternate energy loss metrics for the estimation of BAV function.