The Stewart-Hamilton equation is used to calculate cardiac output by thermodilution. Which technical error will falsely INCREASE the calculated cardiac output?

• A Injecting too slowly (over 6 seconds)
• B Tricuspid regurgitation causing recycling of thermal signal
• C Using warm injectate rather than cold ✓
• D Atrial fibrillation with variable stroke volumes

Explanation

Thermodilution CO is calculated from the area under the temperature-time curve after injecting cold injectate into the right atrium. A smaller temperature change (smaller area under the curve) produces a mathematically higher calculated CO. Warm injectate produces a smaller temperature difference from blood, reducing the detectable thermal signal and falsely calculating a higher CO. Cold injectate (<5°C) maximises the signal-to-noise ratio and accuracy. Tricuspid regurgitation causes recycling of cooled blood, underestimating CO (falsely low).

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.