Abstract: Background – Mitral regurgitant fraction (MR%), defined as the difference between total stroke volume (TSV) and forward stroke volume (FSV) divided by TSV, is the most direct metric for quantifying mitral regurgitation severity. However, human studies have demonstrated that traditional two-dimensional (2D) estimates of MR% differ from three-dimensional (3D) approaches, including echocardiography and magnetic resonance imaging. Hypothesis/Objectives – To compare stroke volumes and MR% measurements obtained using 2D and real-time 3D echocardiography in dogs. Animals – Fifty-eight client-owned dogs with myxomatous mitral valve disease; mitral annular measurements were available in 41 dogs. Methods – Prospective method-comparison study. Left ventricular TSV was measured using Simpson’s method of discs (TSV_LV-2D) and 3D echocardiography (TSV_LV-3D). Mitral annular TSV and aortic annular FSV were calculated as the product of velocity-time integral and cross-sectional area, derived from linear diameters (TSV_MV-2D, FSV_AV-2D) and 3D planimetry (TSV_MV-3D, FSV_AV-3D). Agreement was assessed using Bland-Altman analysis, and differences among methods were evaluated using the Friedman test. Results – Measurements of TSV_LV-2D overestimated TSV_LV-3D (median difference [95% confidence interval]: 0.18 [–0.07 to 0.35] mL/kg), FSV_AV-2D underestimated FSV_AV-3D (–0.20 [–0.26 to –0.14] mL/kg), and TSV_MV-2D overestimated TSV_MV-3D (1.09 [0.80 to 1.31] mL/kg). Compared with the traditional MR% calculation ([TSV_LV-2D – FSV_AV-2D]/TSV_LV-2D), all alternative formulas except (TSV_LV-3D – FSV_AV-2D)/TSV_LV-3D differed in the 41 dogs (P < 0.05). Conclusions and Clinical Importance – Two-dimensional and 3D echocardiographic estimates of stroke volume and MR% are not interchangeable. Studies incorporating independent reference standards are warranted to determine the accuracy and clinical implications of these differences.
