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Cardiac Output by the Fick Principle

Fick cardiac output = O₂ consumption ÷ arteriovenous O₂ content difference, where O₂ content ≈ 1.34 × Hb × saturation (× 10 per litre). With estimated VO₂ of 125 mL/min/m², normal CO is 4–8 L/min and cardiac index 2.5–4.0 L/min/m²; CI <2.2 suggests low output.

Haemoglobin (g/dL or g/L)
Arterial O₂ saturation (%)
Mixed venous O₂ saturation (PA) (%)
Weight (kg or lb)
Height (cm or m or in)
Measured VO₂ (blank = 125 mL/min/m²) (mL/min)

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Interpretation

Result	Meaning
CI < 2.2	Low output (cardiogenic shock if with hypotension and congestion)
CI 2.2–4.0	Normal
CI > 4.0	High output

Formula

CO (L/min) = VO₂ (mL/min) ÷ [1.34 × Hb (g/dL) × (SaO₂ − SvO₂) × 10]. Estimated VO₂ = 125 mL/min/m² × BSA. CI = CO ÷ BSA.

Clinical pearls

Assumed VO₂ (125 mL/min/m²) is the largest error source: true VO₂ varies by age, sex, sepsis and fever, with errors >25% common.
Dissolved O₂ (0.003 × PaO₂) is ignored; it matters only at high FiO₂.
SvO₂ must come from the pulmonary artery (true mixed venous), not a central line or right atrium, especially with shunts.

Examples

fick cardiac output hb 15 sao2 98 svo2 75 70kg 175cm cardiac index by fick: haemoglobin 10 g/dL, art sat 95%, PA sat 50%, weight 80 kg height 180 cm

FAQ

How do you calculate cardiac output with the Fick method?

Divide oxygen consumption by the arteriovenous oxygen content difference: CO = VO₂ ÷ [13.4 × Hb × (SaO₂ − SvO₂)] when saturations are fractions.

Fick or thermodilution?

Thermodilution is less accurate with severe tricuspid regurgitation or low output; assumed-Fick is less accurate when VO₂ is abnormal. Disagreement should prompt review of both.

Evidence

LaFarge CG, Miettinen OS. The estimation of oxygen consumption. Cardiovasc Res 1970;4:23–30.

Cardiology · Reviewed 2026-10-14 · Methodology