Anatomic versus Effective Orifice Area in a Bicuspid Aortic Valve

Bicuspid aortic valve (BAV) is the most common congenital cardiac anomaly, with an estimated 1–2% of the population being affected. Pathology associated with BAV includes increased rates of aortic stenosis, aortic insufficiency, endocarditis, aortic dissection, and aortic coarctation. Aortic stenosis is the most frequently observed clinically, as well as in the echocardiography laboratory. Aortic valve area (AVA) is an integral component for assessing aortic stenosis severity. The primary reason for this is that AVA is theoretically less affected by flow variation in comparison with velocities and pressure gradients. AVA can be calculated both by the continuity equation, which provides functional or effective orifice area (EOA), and planimetry, which provides the geometric or anatomic orifice area (AOA). An important limitation of planimetry is that the measurement must be taken at the leaflet tips, as displacement toward the ventricle can result in an overestimation of AOA. This is due to the funnel-shaped outflow associated with aortic stenosis. The evaluation of AVA in BAV can be challenging when compared to trileaflet aortic valves. A strong correlation exists between AOA and EOA in trileaflet, stenotic valves. This same relationship is much weaker in BAV, with AOA often overestimating EOA. Potential causes or contributors to this in BAV include eccentric systolic flow patterns, valve shape, and valve profile. Eccentricity results in forward flow that collides with the aortic wall, which may potentially increase resistance. Eccentric flow also results in inefficient use of the AOA, resulting in a vena contracta (the narrowest point of flow, corresponding with the EOA and peak velocity) occurring distal to the AOA. This is referred to as the contraction of flow. The ratio of the AOA and EOA is called the coefficient of contraction. A smaller coefficient of contraction, and therefore a larger difference between AOA and EOA results in higher peak and mean pressure gradients than would be expected by the AOA alone. Valve shape and valve profile also impact the coefficient of contraction. Circular orifices are associated with higher coefficients of contraction, while eccentric orifices are associated with lower coefficients of contraction. This is likely due to the greater frictional losses associated with eccentric orifices, with the end result being a greater functional stenosis severity. Funnelshaped valves, or those with a progressive decrease in cross-sectional area, are associated with larger coefficients of contraction. Valves that have a flatter surface and abruptly narrow are associated with smaller coefficients of contraction and greater functional stenosis severity. Any of these variables alone or in combination can result in a worse functional stenosis severity than would be predicted by the AOA. Pressure recovery is an important consideration in aortic stenosis and especially in those cases when the Doppler data are discordant with two-dimensional or clinical data. Pressure recovery is considered “the increase of pressure downstream from a stenosis due to reconversion of kinetic energy into potential energy. ” This phenomenon is most relevant when the proximal aorta is small, and should be considered clinically when the proximal aorta is less than 3 cm in diameter. The impact of pressure recovery is reduced when the proximal aorta is large, the left ventricular outflow tract is large, and when there is eccentric systolic flow. Therefore, BAV is Address for correspondence and reprint requests: Chad House, B.S., R.D.C.S. , Mail Stop 11102M, 640 Jackson Street, St. Paul, MN 55101, USA. Fax: 651-254-3526; E-mail: chad.m. house@healthpartners.com