A device designed for accurate injection of an indicator solution (e.g., cooled saline, dye) into the blood stream that includes a mechanical mechanism used to inject the solution, in a closed loop, into a blood vessel through an indwelling catheter (e.g., Swan-Ganz cardiac catheter) mainly to assess heart blood flow and measure cardiac output. It may be used in conjunction with a densitometer or a thermodilution device to determine cardiac output. This is a reusable device.