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Cardiac dysfunction is one of many causes for unsuccessful weaning from mechanical ventilation. Although cardiac dysfunction can be detected via direct measurement of cardiac output during weaning, available methods are not feasible.To investigate the role of noninvasive monitoring of cardiac output during weaning and determine if a relationship exists between serial measurements during the spontaneous breathing trial and weaning outcomes.A prospective, observational study was conducted in the intensive care unit at a university-affiliated teaching hospital. The sample consisted of patients intubated for more than 24 hours who were being weaned off of mechanical ventilation according to a validated weaning protocol. Before the first spontaneous breathing trial, a noninvasive cardiac output monitor was connected to the ventilator circuit. Measurements were made before, at the beginning of, and at the end of the trial.Among the 85 patients tested, baseline cardiac output was similar (P = .93) for those in whom the first trial was successful (mean [SD], 5.7 [2.1] L/min) and those in whom the trial was unsuccessful (5.6 [1.8] L/min). Unlike patients with unsuccessful trials, patients with successful trials were able to augment their cardiac output from baseline. Mean cardiac output increased to 7.1 (SD, 3.1) L/min for patients in whom weaning was successful and to 6.2 (SD, 2.3) L/min for those in whom weaning was unsuccessful (P = .001).A noninvasive method of monitoring cardiac output can be easily applied while patients are being weaned off of mechanical ventilation.