A method for determining cardiac output by use of thermodilution principles and by utilizing a catheter assembly with a passageway therethrough. The catheter assembly is introduced at least into close proximity to the right heart. A small balloon tipped catheter is inserted through the passageway of the catheter assembly and is guided thereby with the balloon uninflated and with a thermistor attached thereto to the desired location. The balloon tipped end of the small catheter is passed out of the passageway through the catheter assembly into the bloodstream and the balloon is inflated so that it is flow directed away from the catheter assembly into the desired position in the heart and spaced a substantial distance from the catheter assembly. The small balloon tip catheter has a central lumen therethrough to effect the inflation and deflation of the balloon. A fluid different in temperature from the bloodstream is injected into the annular space between the passageway of the catheter assembly and the outer surface of the smaller balloon tipped catheter with the passageway through the catheter assembly forming a guideway for the fluid to direct it to the desired location and the spacing between the inflated small balloon catheter and the catheter assembly preventing interference of the flow of the fluid from the catheter assembly so that the thermistor on the balloon catheter can detect the temperature changes in the bloodstream. The thermistor output is then detected to determine the cardiac output.