The authors hypothesized that bioimpedance cardiography measured by the Endotracheal Cardiac Output Monitor (ECOM; ConMed, Utica, NY) is a convenient and reliable method for both cardiac index (CI) assessment and prediction in fluid responsiveness.A prospective observational study.A teaching university hospital.Twenty-five adult patients.Admission to the intensive care unit after conventional cardiac surgery and investigation before and after a fluid challenge.Simultaneous comparative CI data points were collected from transpulmonary thermodilution (TD) and ECOM. Correlations were determined by linear regression. Bland-Altman analysis was used to compare the bias, precision, and limits of agreement. The percentage error was calculated. Pulse-pressure variations (PPVs) and stroke-volume variations (SVVs) before fluid challenge were collected to assess their discrimination in predicting fluid responsiveness. A weak but statistically significant relationship was found between CI(TD) and CI(ECOM) (r = 0.31, p = 0.03). Bias, precision, and limits of agreement between CI(TD) and CI(ECOM) were 0.08 L/min/m(2) (95% confidence interval, -0.11 to 0.27), 0.68 L/min/m(2), and -1.26 to 1.42 L/min/m(2), respectively. The percentage error was 51%. A nonsignificant positive relationship was found between percent changes in CI(TD) and CI(ECOM) after fluid challenge (r = 0.37, p = 0.06). Areas under the ROC curves for both PPV and SVV to predict fluid responsiveness were 0.86 (95% confidence interval, 0.67-1.06) and 0.89 (95% confidence interval, 0.74-1.04, respectively; p = 0.623).Continuous measurements of CI under dynamic conditions are consistent and easy to obtain with ECOM although not interchangeable with transpulmonary thermodilution. SVV given by ECOM is a dynamic parameter that predicts fluid responsiveness with good accuracy and discrimination.