Noninvasive Estimation of the Severity of Aortic Stenosis: Going Beyond Bernoulli.

Ever since the original reports by Hatle et al, echocardiography has been the mainstay for diagnosing the severity of aortic stenosis noninvasively. They reported that the modified Bernoulli equation could be applied to estimate the pressure drop across a valve and that the continuity equation could be used to estimate its area. The behavior of fluids is described by the Navier–Stokes differential equations, derived from the conservation of momentum (Newton second law) and applying the conservation of energy. Using these equations and assuming negligible gravity, viscous forces, and flow acceleration, a relation can be derived between the pressure gradient between 2 points and the velocities of flow at those 2 points. This is the Bernoulli equation, the cornerstone of Doppler hemodynamics, and its application largely supplanted invasive hemodynamic measurements in clinical practice. Now, with renewed interest in the precise quantification of aortic stenosis driven by the possibility of percutaneous valve implantation as well as open heart surgery, tomographic imaging is used increasingly, and new 3-dimensional (3D) imaging techniques are being exploited to generate 4D flow fields. In light of these developments, the accuracy of the existing methods is being re-examined.