Graeser K, et al. - Researchers systematically compared cardiac output (CO) with Doppler-derived 3-dimensional (3D) transesophageal echocardiography (TEE) and CO by thermodilution in a broad population of patients undergoing cardiac surgery, to test their hypothesis that Doppler-derived CO by 3D TEE would agree well with CO measured with pulmonary artery catheter thermodilution as a reference method based on accurate measurements of the cross-sectional area of the left ventricular outflow tract. Although low bias was observed, wide limits of agreement were found between Doppler-derived CO by 3D TEE and CO by thermodilution. This will limit clinical application and hence, Doppler-derived CO by 3D TEE cannot be considered interchangeable with CO obtained by thermodilution. The lack of agreement of the 3D technique did not explain the lack of agreement.