The catheter is flow-directed through the heart of the patient by a balloon on its distal end to pass through the right atrium, right ventricle and into the pulmonary artery. A through lumen in the catheter tube is open at the distal end to monitor blood pressures in the pulmonary artery or a branch, or take blood samples. A thermistor proximal to the balloon permits monitoring of blood temperature in this region and thus allows the determination of cardiac output by the thermodilution technique. A second lumen has an orifice, which is characteristically located in the region of the junction of the superior vena cava and the right atrium, for pressure monitoring, infusion of liquid media, or blood sampling. A stylet in the second lumen assists in advancing the catheter and, by means of a sudden variation in stiffness, facilitates forming a sharp bend within the right ventricle to press distal electrodes against the myocardial wall which separates the left and right ventricles. Proximal electrodes on the catheter tube are positioned in the right atrium. In addition to the two lumens mentioned, a third lumen serves as a balloon inflation lumen and a fourth lumen contains the wires for the thermistor, distal electrodes and proximal electrodes.