A 56-year-old male with acute on chronic systolic heart failure and septic shock from pneumonia is admitted to the ICU. A recent transthoracic echo reveals moderate tricuspid regurgitation and an ASD with significant left to right shunting. A pulmonary artery catheter is placed to guide hemodynamic management.
Which of the following would most consistently underestimate CO if measured by thermodilution?A. Left to right intra cardiac shunt
Correct Answer: D
CO measurement using thermodilution is the gold standard in current practice owing to its ease of use, safety, and reproducibility over time. To run thermodilution CO, 10 mL of saline cooler than blood is rapidly injected into the RA, and the change in temperature after injection is measured by a thermistor in the PA and is integrated over time. The area under the curve of this injectate is inversely proportional to CO. To provide a more reliable measurement, the test is run three times and values within 10% of each other are averaged. An initial steep positive deflection with high amplitude represents rapid delivery of the cold injectate to the thermistor causing a rapid maximal change in temperature. A steep negative deflection with prompt return to baseline represents forward flow and resolution of temperature change.
Low CO states demonstrate an attenuated rise and fall and overall larger area under the curve. Left to right shunting may overestimate CO by diluting out the injectate, whereas right to left shunting overestimates CO by allowing the injectate to quickly bypass the pulmonary circulation. Right-sided valvular lesions can also overestimate or underestimate CO making thermodilution unreliable. A larger than programmed injectate would cause a larger area under the curve than expected and thus underestimates actual CO. Warmer than programmed injectate would cause a smaller area under the curve than expected and thus overestimates actual CO. Injectate that is warmer than programmed into the computer would be the only answer that would consistently overrestimate CO. Other sources of error include extremely low flow states causing injectate heat loss from slow transit, rapid fluid administration causing temperature fluctuation, improper injection technique, improper placement, and thermistor clot.