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The use of transesophageal echocardiography for the determination of cardiac output (CO) has been limited to date. We assessed the capability of aortic continuous-wave Doppler transesophageal echocardiography to determine CO (DCO) in a transgastric long-axis imaging plane of the heart by comparing DCO to thermodilution CO (TCO).DCO was determined in 63 consecutive patients undergoing cardiac surgery. Aortic valve area was obtained from the transverse short-axis view of the valve assuming a triangular shape for the valve orifice. Stroke volume was calculated as the product of velocity–time integral and aortic valve area: stroke volume = velocity–time integral × aortic valve area. DCO was calculated off-line, by multiplying stroke volume with heart rate: DCO = stroke volume × heart rate.The aortic valve orifice was easily imaged in all patients. Excellent-quality continuous-wave Doppler flow profiles were obtained in nearly all (62 of 63). A total of 109 DCO determinations were performed. Mean DCO was 4.35 ± 1.18 1·min-1 (range 2.02-7.42 1·min-1), and mean TCO was 4.41 ± 1.17 1·min-1 (range 2.24-8.94 1·min-1). Very high correlation and agreement were found between the two methods: DCO = 0.94 × TCO + 0.19, r=0.94, SEE (standard error of the estimate) = 0.41 1·min-1; 95% confidence interval=0.06 ± 0.83 1·min-1. Relative changes from pre- to postbypass CO (Δ) also showed a strong correlation (ΔDCO=0.93 × ΔTCO + 5.4%, r=0.82, SEE=17.8%). For CO changes greater than 10%, Doppler was in accordance with thermodilution in 43 of 45 measurements. DCO repeatability coefficient was 0.51 1·min-1.Compared to thermodilution, continuous-wave Doppler measurements of blood flow velocity across the aortic valve in the transesophageal echocardiographic transgastric view allow accurate CO determination.