Intraoperative validation of mitral inflow determination by transesophageal echocardiography: Comparison of single-plane, biplane and thermodilution techniques

Objectives.: This study investigated the accuracy of mitral inflow quantification using biplane transesophageal echocardiography. Background.: Mitral stroke volume can be reliably quantified by transthoracic Doppler echocardiography, but previous studies involving monoplane transesophageal echocardiography have yielded mixed results. Methods.: Thirty patients without mitral regurgitation were prospectively examined immediately before cardiovascular surgery. Mitral annulus diameter was measured in the transverse (d₁) and longitudinal views (d₂) by biplane transesophageal echocardiography. Assuming an elliptic shape, the annular area was calculated as πd₁d₂/4; area was also calculated from single-plane data assuming a circular annular shape as πd²4. The time-velocity integral of mitral annular Doppler velocity was then multiplied by annular area to yield stroke volume. These data were compared with simultaneous thermodilution measurements by linear regression. Results.: Good correlations were observed between thermodilution (x) and Doppler (y) measurements of stroke volume (SV) (r = 0.86, p < 0.01, ΔSV [y-x]= 2.64 ± 9.86 ml for single four-chamber view; r = 0.77, p < 0.01, ΔSV = 1.82 ± 12.59 ml for two-chamber view; r = 0.94, p < 0.001, ΔSV = 1.78 ± 5.90 ml for biplane measurements) with similar data for cardiac output (r = 0.82, r = 0.74 and r = 0.92, respectively). The biplane measurements were most accurate and had less variability in individual patients (p < 0.05). This finding was supported by a numerical model that demonstrated (for an ellipse of eccentricity 1.5: 1) that even maximal misalignment of biplane diameters yielded only 8% area overestimation, whereas single-plane calculations assuming a circular shape produced a variation in area of 225%. Conclusions.: This study validates the accuracy of measurements of mitral inflow using biplane transesophageal echocardiography with potential application for quantification of valvular regurgitation in the operating room. The results are further generalizable, indicating that orthogonal biplane measurements are both necessary and sufficient to ensure accuracy in area calculation for any elliptic structure.