Validation of the proximal flow convergence method. Calculation of orifice area in patients with mitral stenosis

It has been proposed recently that measuring the flow convergence region proximal to an orifice by Doppler flow mapping can provide a means of calculating regurgitant flow rate. Although verified in experimental models, this approach is difficult to validate clinically because there is no ideal gold standard for regurgitant flows in patients. However, this method also can be used to derive cardiac output or flow rate proximal to stenotic orifices and therefore to calculate their areas by the continuity equation (area = flow rate/velocity). Applying this method in mitral stenosis would provide a unique way of validating the underlying concept because the predicted areas could be compared with those measured directly by planimetry.

We studied 40 patients with mitral stenosis using imaging and Doppler echocardiography. Doppler color flow recordings of mitral inflow were obtained from the apex, and the radius of the proximal flow convergence region was measured at its peak diastolic value from the orifice to the first color alias along the axis of flow. Flow rate was calculated assuming uniform radial flow convergence toward the orifice, modified by a factor that accounted for the inflow funnel angle formed by the mitral leaflets. Mitral valve area was then calculated as peak flow rate divided by peak velocity by continuous-wave Doppler. The calculated areas agreed well with those from three comparative techniques over a range of 0.5 to 2.2 cm2: 1) cross-sectional area by planimetry (y = 1.08x-0.13, r = .91, SEE = 0.21 cm2); 2) area derived from the Doppler pressure half-time (y = 1.02x-0.14, r = .89, SEE = 0.24 cm2); and 3) area calculated by the Gorlin equation in the 26 patients who underwent catheterization (y = 0.89x + 0.08, r = .86, SEE = 0.24 cm2). Agreement with planimetry was similar for 22 patients with mitral regurgitation and 18 without it (P > .6), as well as for 6 in atrial fibrillation (P > .2).

These results validate the proximal flow convergence concept in the clinical setting and also demonstrate that it can be extended to orifice area calculation using the continuity equation.