A method and apparatus for use in determining the cardiac output. The method includes measuring a first parameter indicative of a patient's oxygen uptake and a second parameter indicative of the patient's fractional arterial oxygen concentration. The method also includes inducing a change in the patient's arterial oxygen concentration while taking these measurements to monitor the effects of the change in the patient's arterial oxygen concentration. The cardiac output is determined from the data collected regarding the effects of the change in the patient's arterial oxygen concentration.