An array of electrical bioimpedance electrodes are provided on a hollow, flexible tube that is insertable into the esophagus of a patient. Four electrodes are positioned on the outer wall of the tube. Four wires are run through the hollow center portion of the tube and are connected through the wall of the tube to a respective one of the four electrodes. The four wires are connectable to an electrical bioimpedance measurement device. Two of the electrodes are connected to a high frequency, low magnitude constant current source so that a current can be injected into the thorax of the patient between the two electrodes. The other two electrodes are connected to the sensing input of the electrical bioimpedance measurement device and detect a voltage induced in the thorax of the patient that varies in accordance with the electrical bioimpedance changes in the patient. The tube is positioned in the esophagus proximate to the descending thoracic aorta so that the primary source of electrical bioimpedance changes is the flow of blood in the descending thoracic aorta. The present invention is used to determine the cardiac output of the patient that results in the flow of blood in the descending thoracic aorta.