Fick Principle Calculator
Calculate cardiac output using the Fick principle: CO = VO2 / (CaO2 - CvO2). Used in catheterization labs to measure cardiac output from oxygen consumption and arteriovenous O2 difference.
❤️ Physiology📐 CO = VO2 / (CaO2 - CvO2)🧬 Biology
O2 consumption VO2 (mL/min)
Arterial O2 content CaO2 (mL/dL)
Mixed venous O2 content CvO2 (mL/dL)
Please enter valid values.
Formula & Reference
| Variable | Symbol | Formula | Units |
|---|---|---|---|
| Fick Principle Calculator | — | CO = VO2 / (CaO2 - CvO2) | L/min |
Step-by-Step Examples
Example 1
Normal Catheterization
VO2=250 mL/min, CaO2=19 mL/dL, CvO2=14 mL/dL.
- A-V diff = 19-14 = 5 mL/dL
- CO = 250/(5x10) = 250/50 = 5 L/min
- Normal cardiac output
✓ CO = 5 L/min (normal)
Example 2
Low Output State
VO2=200, CaO2=18, CvO2=11 mL/dL.
- A-V diff = 7 mL/dL (high — more extraction)
- CO = 200/70 = 2.86 L/min
- Severely reduced CO — heart failure or shock
- High extraction compensates for low flow
✓ CO = 2.9 L/min — low output state
Example 3
High Output State
Septic shock: VO2=350, CaO2=18, CvO2=16 mL/dL.
- A-V diff = 2 mL/dL (low — poor extraction)
- CO = 350/20 = 17.5 L/min
- Hyperdynamic circulation — sepsis pattern
- Despite high CO, poor tissue O2 extraction
✓ CO = 17.5 L/min — septic hyperdynamic
Real-World Applications
Cardiac Catheterization
Gold standard for CO measurement in cardiac catheterization lab.
Heart Failure Management
Guides inotrope, diuretic, and vasodilator therapy.
Hemodynamic Research
Fick principle underlies all oxygen-based hemodynamic calculations.
Anesthesia
Modified Fick equation used in non-invasive CO monitoring.
Common Mistakes to Avoid
⚠️
Using assumed VO2 (Lafarge equation)
VO2 is often assumed rather than measured directly. Assumed values (Lafarge, Bergstra) introduce significant error. Measure VO2 directly when possible.
⚠️
Sampling error in mixed venous
Mixed venous sample must be from pulmonary artery (truly mixed). CvO2 from superior vena cava is not equivalent.
⚠️
Not accounting for shunts
In patients with intracardiac shunts (ASD, VSD), Fick calculation requires modified equations accounting for shunt flow.
Frequently Asked Questions
What is the Fick principle? ▾
The amount of a substance consumed (VO2) equals the blood flow (CO) times the difference in concentration of that substance in arterial vs venous blood.
Why is Fick CO considered a gold standard? ▾
Directly measures the relationship between blood flow and O2 transport. Less affected by tricuspid regurgitation than thermodilution. Preferred in low CO states.
What is thermodilution CO? ▾
Cold saline injected into right heart; temperature change detected downstream. Fast, easy but inaccurate in low flow states or tricuspid regurgitation.
What is ScvO2 vs SvO2? ▾
ScvO2 (central venous O2 saturation) from superior vena cava ≈ SvO2 (mixed venous from pulmonary artery) ± 5%. ScvO2 >70% is a hemodynamic resuscitation goal in sepsis.
What does high A-V O2 difference mean? ▾
High extraction — peripheral tissues taking more O2 per unit blood. Indicates low CO (heart failure) or increased metabolic demand.