Pulmonary artery catheter (Swan-Ganz) thermodilution measures cardiac output. The Fick principle can also calculate cardiac output if which three values are known?

• A Heart rate, stroke volume, and systemic vascular resistance
• B PCWP, mean arterial pressure, and central venous pressure
• C Oxygen consumption (VO2), arterial O2 content (CaO2), and mixed venous O2 content (CvO2) ✓
• D Haemoglobin concentration, SaO2, and SvO2

Explanation

The Fick principle states that oxygen consumption equals cardiac output multiplied by the arteriovenous oxygen content difference: VO2 = CO × (CaO2 − CvO2). Therefore, CO = VO2 ÷ (CaO2 − CvO2). CaO2 is measured from arterial blood and CvO2 from mixed venous blood (pulmonary artery sample). VO2 is either measured by metabolic analysis or assumed (assumed values of 125 mL O2/min/m² are commonly used). While haemoglobin and saturation contribute to oxygen content, Hb and saturations alone are insufficient — the full content calculation [Hb × 1.34 × SaO2 + 0.003 × PaO2] for both arterial and mixed venous blood is required.

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

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