Apparatus and methods are provided for monitoring cardiac output using bioelectrical impedance techniques in which first and second electrodes are placed in the trachea and/or bronchus in the vicinity of the ascending aorta, while a sense current is injected into the thorax via first and second source electrodes, so that the resulting bioelectrical impedance measurements reflect voltage changes induced primarily by blood flow dynamics, rather than respiratory or noncardiac related physiological effects. Apparatus and methods are also provided so that the measured cardiac output may be used to control administration of intravenous fluids to a patient or to optimize heart rate for those patients having pacemakers.