Fick Principle
Fick Cardiac Output Calculator
CO = VO₂ / (CaO₂ − CvO₂) × 10. Enter VO₂, hemoglobin, and arterial/venous oxygenation.
Inputs
Typical resting VO₂ ≈ 200–300 mL/min (adults).
Dissolved O₂ term is small (0.0031×PaO₂), but included for completeness.
Optional: Cardiac Index (add height & weight for BSA)
Results
Typical Normal Ranges (Adults)
Oxygen Content
CaO₂ (arterial): — mL O₂/dL
CvO₂ (venous): — mL O₂/dL
Cardiac Output
CO: —
CI: —
Formulas: CaO₂ = 1.34×Hb×SaO₂ + 0.0031×PaO₂; CvO₂ =
The Fick principle is used to calculate cardiac output by relating oxygen consumption to the arteriovenous oxygen difference. It provides a measure of how effectively the heart is pumping blood to meet the body’s metabolic demands.
In practice, the Fick equation is often applied in cardiac critical care, such as when assessing cardiac output trends while titrating or weaning inotropes like dobutamine.
Clinical use & interpretation
- High CO/CI can be seen in sepsis, anemia, hyperthyroidism, pregnancy, or after vasodilators.
- Low CO/CI suggests pump failure or inadequate preload/afterload conditions (e.g., cardiogenic shock, tamponade, severe hypovolemia).
- Cardiac index (CI) normal is roughly 2.5–4.0 L/min/m²; CO depends on body size (≈ 4–8 L/min in adults).
Assumptions & inputs
- VO₂ should be measured when possible (metabolic hood/catheter). Estimating VO₂ (e.g., 125 mL/min/m² × BSA) is common but introduces error.
- Use mixed venous samples for SvO₂/PvO₂ (pulmonary artery). Central venous values (ScvO₂) are not interchangeable.
- Hemoglobin and saturations must be temporally aligned with VO₂; changing oxygen delivery/consumption can skew results.
Pitfalls
- Small errors in SaO₂/SvO₂ or Hb cause large swings in ΔC and therefore CO.
- Shivering, fever, or agitation increase VO₂ and can artifactually raise the calculated CO.
- Sampling errors (e.g., wedged instead of mixed venous, air exposure) alter measured contents.
- Very low ΔC (CaO₂ − CvO₂) magnifies noise; confirm inputs and consider repeat sampling.