Where is the injectate typically put into on a Pulmonary Artery Catheter (PAC) to measure Cardiac Output (CO) in an adult patient?

Injectate Location for Cardiac Output Measurement via Pulmonary Artery Catheter

The injectate for thermodilution cardiac output measurement is injected into the right atrium (or proximal port of the PAC located in the right atrium/superior vena cava), and the temperature change is detected downstream in the pulmonary artery. 1

Technical Mechanism of Thermodilution

• The pulmonary artery catheter measures cardiac output through thermodilution by injecting a known volume and temperature of fluid into the right atrium, with the temperature change measured downstream in the pulmonary artery. 1

• The PAC is a balloon flotation catheter that measures pressures in the superior vena cava (SVC), right atrium, right ventricle, and pulmonary artery, with the thermistor located distally in the pulmonary artery to detect the temperature change after proximal injection. 2

• For accurate central venous oxygen saturation (ScvO2) measurements, the catheter tip must be positioned at or close to the SVC-right atrial junction or inferior vena cava-right atrial junction, which is also the optimal location for the proximal injection port. 2

Injection Port Specifications

• The proximal injection port (typically the blue port on standard PACs) is located approximately 30 cm from the catheter tip, positioning it in the right atrium when the distal thermistor is properly positioned in the pulmonary artery. 1

• Modern catheters can measure cardiac output semi-continuously without requiring repeated manual injections, providing real-time data on cardiac function. 2

Critical Technical Considerations

• Severe tricuspid regurgitation can overestimate or underestimate cardiac output measured by thermodilution, as the regurgitant flow disrupts the indicator dilution curve between the injection site (right atrium) and detection site (pulmonary artery). 2

• The accuracy of thermodilution requires proper catheter positioning with the distal thermistor in the pulmonary artery and the proximal injection port in the right atrium, ensuring adequate mixing of the injectate before reaching the thermistor. 1

• Measurements should be obtained at end-expiration during spontaneous breathing when intrathoracic pressure approaches atmospheric, or at standardized points in the respiratory cycle during mechanical ventilation. 3

Alternative Measurement Sites

• Transpulmonary thermodilution systems inject into a central venous catheter and detect temperature changes in the femoral or axillary artery, providing cardiac output along with extravascular lung water measurements, but this represents a different technique than standard PAC thermodilution. 1