Measurement of CO remains a cornerstone in the hemodynamic assessment of the critically ill patient. Its determination can classify patients in high- or low-CO states, thus indicating categories of circulatory failure and subsequently help in providing the appropriate treatment. Thermodilution, utilizing a method based on the Fick principle, is considered the “gold standard” to determine CO at the bedside. This technique requires placement of a PAC, and, although a useful technique, carries considerable risks and a potential for inaccuracies. Unreliable values are particularly common in the ICU, where extremes of hemodynamic and respiratory conditions are often found with either very low or very high CO and frequent TR related to high pulmonary artery pressure (PAP). Several methods for determining CO have been described using both 2D and Doppler echocardiography. With this technique, SV and CO can be determined directly by combining Doppler-derived instantaneous blood flow velocity through a conduit with the cross-sectional area (CSA) of the conduit. Blood flow can be calculated through various cardiac structures, including the pulmonary valve,52the mitral valve,53–54 and the aortic valve.55–58 In the absence of intracardiac shunts, blood flow through these structures should be the same (continuity equation).59 Of these methods, the one using the LV outflow tract (LVOT) and aortic valve as the conduit is probably the most reliable and most commonly used, with an excellent agreement with thermodilution in most situations.55–58 The LV SV is obtained by measuring the CSA of the LVOT (area [centimeters squared] = (LVOT diameter [centimeters]2) × [π/4], assuming that just below the aortic annulus, the LVOT is circular) multiplied by the transaortic flow velocity time integral derived from a spectral Doppler tracing. The SV thus obtained is then multiplied by the heart rate to give the CO: CO = CSA × velocity time integral × heart rate. With the transthoracic approach, the LVOT diameter is usually obtained from the parasternal long-axis view, just below the insertion of the aortic valve leaflets. The Doppler interrogation is then performed through the aortic valve from the apical view. With the transesophageal approach, the LVOT diameter is usually obtained from the five-chamber view of the LV. The transgastric view is usually used to obtained an apical long axis-view of the aortic valve through which Doppler interrogation is then performed.,60Using TTE, McLean et al61 demonstrated an excellent correlation (r = 0.94) between CO determined by the LVOT Doppler method and the thermodilution method in a population of critically ill patient. Other studies,55 have shown similar results. In a study by Feinberg et al,58 CO, as determined by TEE Doppler imaging, was obtainable in 88% of a population of 33 critically ill patients with good correlation (r = 0.91) with the thermodilution method. Descorps-Declère et al,60 also showed that transgastric pulsed Doppler measurement across the LVOT with TEE to be a clinically acceptable method for CO measurement in the critically ill (r = 0.975, when compared to the thermodilution method).