Fick Cardiac Output Calculator
Estimate cardiac output from oxygen consumption and arterial-venous oxygen content difference using the Fick principle.
Formula used: CO (L/min) = VO₂ / ((CaO₂ − CvO₂) × 10). All values should be measured clinically when possible.
What is the Fick cardiac output method?
The Fick principle is a classic hemodynamic method for calculating cardiac output (CO), the volume of blood pumped by the heart each minute. It is based on conservation of mass: oxygen uptake by the body must equal blood flow times the arterial-venous oxygen content difference. In clinical cardiology and critical care, the Fick approach remains important, especially when interpreting right heart catheterization data.
Fick cardiac output formula and units
CO = VO₂ / (CaO₂ − CvO₂)
If CaO₂ and CvO₂ are entered in mL O₂/dL, convert to liters with:
CO (L/min) = VO₂ / ((CaO₂ − CvO₂) × 10)
- VO₂ = oxygen consumption (mL O₂/min)
- CaO₂ = arterial oxygen content (mL O₂/dL blood)
- CvO₂ = mixed venous oxygen content (mL O₂/dL blood)
- CaO₂ − CvO₂ = arteriovenous oxygen content difference
How oxygen content is commonly estimated
When direct oxygen content is not reported, it is often estimated from hemoglobin and saturation values:
- CaO₂ ≈ (1.34 × Hb × SaO₂) + (0.0031 × PaO₂)
- CvO₂ ≈ (1.34 × Hb × SvO₂) + (0.0031 × PvO₂)
Here, Hb is in g/dL, saturation is a fraction (e.g., 0.97), and partial pressure is in mmHg. The dissolved oxygen term (0.0031 × PO₂) is usually small compared with hemoglobin-bound oxygen.
How to use this calculator
- Enter measured VO₂ in mL/min (or validated estimated VO₂ if direct measurement is unavailable).
- Enter CaO₂ and CvO₂ in mL O₂/dL.
- Click Calculate to get cardiac output in L/min.
- Optionally add BSA to get cardiac index and heart rate to estimate stroke volume.
Worked example
Suppose VO₂ is 250 mL/min, CaO₂ is 20 mL/dL, and CvO₂ is 15 mL/dL. The arteriovenous difference is 5 mL/dL.
CO = 250 / (5 × 10) = 5.0 L/min
If BSA is 1.9 m², cardiac index = 5.0 / 1.9 = 2.63 L/min/m².
Interpreting your result
| Metric | Typical Adult Resting Range | Clinical Note |
|---|---|---|
| Cardiac Output (CO) | ~4 to 8 L/min | Depends on body size, metabolic demand, and physiology |
| Cardiac Index (CI) | ~2.5 to 4.0 L/min/m² | CO adjusted for body surface area |
| Stroke Volume (SV) | ~60 to 100 mL/beat | Derived from CO and heart rate |
Ranges vary by context. Exercise, fever, sepsis, anemia, sedation, and advanced heart disease can shift expected values. A single number should always be interpreted with blood pressure, filling pressures, perfusion markers, and trend data.
Direct Fick vs estimated Fick
Direct Fick
Uses directly measured VO₂ (often by metabolic cart) and invasive blood sampling. This is typically more accurate and preferred when precision is critical.
Estimated Fick
Uses predicted VO₂ equations rather than measured oxygen consumption. It is faster and often used in routine cath lab workflows, but may introduce error in patients with atypical metabolism.
Common pitfalls
- Using central venous oxygen instead of true mixed venous oxygen when the clinical protocol requires SvO₂.
- Unit mismatch (mL/dL vs mL/L), which can cause a 10-fold error.
- Inaccurate hemoglobin or saturation values from delayed/poor samples.
- Relying on estimated VO₂ in unstable patients where direct measurement would be better.
- Interpreting results without considering shunts, severe valvular disease, or changing ventilatory status.
Clinical reminder
This calculator is for educational and informational use. It does not replace clinician judgment, institutional protocols, or full hemodynamic assessment. If you are managing a patient, confirm assumptions, verify units, and correlate with the full clinical picture.