A system for measuring performance of a beating heart, comprising: an intracardiac blood pump (400; 600) having a tubular cannula with proximal and distal openings (636, 638), a cylindrical surface disposed between the proximal and distal openings and being configured to be positioned in the aorta; an electrically driven motor, and a rotor disposed within the cannula, and an electrical line configured to supply current to the motor; a catheter having proximal and distal end regions, the distal end region being connected to the cannula; a repositioning sheath (452; 652) disposed about the catheter; a thermistor (424; 624) disposed in the distal end region of the catheter, configured to detect blood temperature flowing in the heart's aorta; a bolus of fluid at an initial temperature different than physiologic blood temperature; a first sensor that detects changes in the motor current during operation; a second sensor (423; 623) configured to detect the pressure within the ascending aorta; and a processor configured to: receive a first signal from the first sensor, the first signal being indicative of a change in the motor current, receive a second signal from the second sensor (423; 623), the second signal being indicative of the pressure within the ascending aorta, and receive a third signal from the thermistor (424; 624) indicative of a temperature of blood flowing in the heart's ascending aorta, calculate a pump flow output based on the first signal and the second signal; calculate a total cardiac output based on the third signal; calculate a native cardiac output of the beating heart based on the pump flow output and the total cardiac output, and output the calculated native cardiac output as an output variable (CO NAT). an intracardiac blood pump (400; 600) having a tubular cannula with proximal and distal openings (636, 638), a cylindrical surface disposed between the proximal and distal openings and being configured to be positioned in the aorta; an electrically driven motor, and a rotor disposed within the cannula, and an electrical line configured to supply current to the motor; a catheter having proximal and distal end regions, the distal end region being connected to the cannula; a repositioning sheath (452; 652) disposed about the catheter; a thermistor (424; 624) disposed in the distal end region of the catheter, configured to detect blood temperature flowing in the heart's aorta; a bolus of fluid at an initial temperature different than physiologic blood temperature; a first sensor that detects changes in the motor current during operation; a second sensor (423; 623) configured to detect the pressure within the ascending aorta; and a processor configured to: receive a first signal from the first sensor, the first signal being indicative of a change in the motor current, receive a second signal from the second sensor (423; 623), the second signal being indicative of the pressure within the ascending aorta, and receive a third signal from the thermistor (424; 624) indicative of a temperature of blood flowing in the heart's ascending aorta, calculate a pump flow output based on the first signal and the second signal; calculate a total cardiac output based on the third signal; calculate a native cardiac output of the beating heart based on the pump flow output and the total cardiac output, and output the calculated native cardiac output as an output variable (CO NAT). receive a first signal from the first sensor, the first signal being indicative of a change in the motor current, receive a second signal from the second sensor (423; 623), the second signal being indicative of the pressure within the ascending aorta, and receive a third signal from the thermistor (424; 624) indicative of a temperature of blood flowing in the heart's ascending aorta, calculate a pump flow output based on the first signal and the second signal; calculate a total cardiac output based on the third signal; calculate a native cardiac output of the beating heart based on the pump flow output and the total cardiac output, and output the calculated native cardiac output as an output variable (CO NAT).