Apparatus and methods for the measurement, control, or both, of thermodilution injectate flow for calculation of transpulmonary thermodilution parameters. An injectate delivery system includes a syringe for holding an injectate and a conduit configured at one end to be connected to a catheter. A flow measurement device is interposed in the conduit to generate a signal for determining the flow rate of the fluid from the syringe to the catheter. A processor receives the signal from the flow measurement device and calculates the injectate volume to be used as input for calculating a transpulmonary thermodilution parameter such as cardiac output. A GUI is provided to direct a user on whether the injection rate is too fast or too slow. Rather than measuring or calculating flow rate, a system may include a constant flow valve to provide a constant flow rate from the syringe.
