Inputs
O₂ Consumption (VO2)(mL/min)
Typical adult ≈ 250 mL/min or 125 × BSA
Hemoglobin(g/dL)
Normal range: 12-16 g/dL
SaO₂(fraction 0–1)
Arterial oxygen saturation (e.g., 0.98 for 98%)
SvO₂(fraction 0–1)
Mixed venous oxygen saturation (e.g., 0.70 for 70%)
Result
Enter values to calculate
Formula
CO = VO₂ / ((CaO₂ - CvO₂) × 10) where CaO₂ = 1.34 × Hb × SaO₂ and CvO₂ = 1.34 × Hb × SvO₂VO₂ Calculator
Calculate oxygen consumption from patient height and weight using body surface area.
Theory and Practice
Theory
Fick's principle states that uptake (or release) of a substance equals blood flow multiplied by the arteriovenous concentration difference of that substance. Applied to oxygen:
CO = VO₂ / (CaO₂ – CvO₂)• VO₂: oxygen consumption, determined by measuring inspired vs. expired O₂.
• CaO₂ and CvO₂: arterial and mixed venous O₂ content, derived from blood gases using the hemoglobin-oxygen binding equation.
• Physiologic basis: oxygen taken up from alveoli must equal oxygen extracted by tissues; thus cardiac output links systemic oxygen use to the arteriovenous O₂ gradient.
• The same principle can be applied with CO₂ or other substrates, or to regional organ blood flow.
• CaO₂ and CvO₂: arterial and mixed venous O₂ content, derived from blood gases using the hemoglobin-oxygen binding equation.
• Physiologic basis: oxygen taken up from alveoli must equal oxygen extracted by tissues; thus cardiac output links systemic oxygen use to the arteriovenous O₂ gradient.
• The same principle can be applied with CO₂ or other substrates, or to regional organ blood flow.
Practice
• Direct Fick method: Collect exhaled gas to measure VO₂, obtain arterial and pulmonary artery blood samples, calculate O₂ contents, and compute CO. Considered the reference standard but rarely feasible in critically ill patients.
• Indirect methods: Replace one or more measured values with estimates (e.g., VO₂ from nomograms). More practical, but error-prone.
• Indirect methods: Replace one or more measured values with estimates (e.g., VO₂ from nomograms). More practical, but error-prone.
Sources of inaccuracy
• Pulmonary oxygen consumption inflates VO₂ estimates.
• High-output states narrow Ca–Cv difference, magnifying error.
• Intracardiac shunts distort measured oxygen contents.
• High-output states narrow Ca–Cv difference, magnifying error.
• Intracardiac shunts distort measured oxygen contents.
Applications today
• Serves as physiologic benchmark for other cardiac output methods.
• Applied in cath lab or research environments, with some limitations to applying in the ICU.
• Applied in cath lab or research environments, with some limitations to applying in the ICU.
Contributors
Dr. Ross Prager
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