What is the Fick principle for cardiac output?

The Fick principle states that cardiac output equals whole-body oxygen consumption divided by the arteriovenous oxygen content difference: CO = VO₂ / (CaO₂ − CvO₂). It assumes steady-state conditions and that all oxygen uptake occurs in the systemic circulation.

What units should I use in the Fick equation?

Clinically, VO₂ is often expressed in mL O₂/min and CaO₂ and CvO₂ in mL O₂/dL blood. In that case, cardiac output in L/min is CO = VO₂ / (CaO₂ − CvO₂) ÷ 10. If you use SI units (VO₂ in mL/min, contents in mL/L), then CO in L/min is simply CO = VO₂ / (CaO₂ − CvO₂).

What is a normal cardiac output and cardiac index?

In healthy adults at rest, normal cardiac output is about 4–8 L/min. Cardiac index (CI), which is cardiac output normalized to body surface area, is typically 2.5–4.0 L/min/m².

When is the Fick method used clinically?

The Fick method is used during right heart catheterization to measure cardiac output, especially when thermodilution is unreliable (e.g., severe tricuspid regurgitation, intracardiac shunts, very low output states). It can use directly measured VO₂ or an estimated VO₂ based on predictive equations.

What are the limitations of the Fick principle?

Limitations include the need for accurate VO₂ and blood gas measurements, assumption of steady state, potential errors with intracardiac or intrapulmonary shunts, and the fact that estimated VO₂ formulas may be inaccurate in critically ill or obese patients.

Fick Principle Cardiac Output Calculator (CO, VO₂, A

Use this Fick principle calculator to compute cardiac output (CO), oxygen consumption (VO₂), or arteriovenous oxygen content difference (A–V O₂) from the classic Fick equation. Supports both SI (mL/L) and clinical (mL/dL) units and automatically calculates cardiac index (CI).

Fick principle: definition and clinical use

The Fick principle is a fundamental concept in cardiovascular physiology. It states that the uptake or release of a substance by an organ is equal to the blood flow through that organ multiplied by the arteriovenous concentration difference of the substance.

Applied to the whole body and oxygen, it gives a way to calculate cardiac output (CO) from oxygen consumption and the difference between arterial and mixed venous oxygen content.

General Fick principle

\( \text{Flow} = \dfrac{\text{Uptake or release of substance}}{\text{Arterial concentration} - \text{Venous concentration}} \)

For cardiac output using oxygen:

\( \text{CO} = \dfrac{\dot V_{O_2}}{C_{aO_2} - C_{vO_2}} \)

\( \text{CO} \): cardiac output
\( \dot V_{O_2} \): whole-body oxygen consumption (VO₂)
\( C_{aO_2} \): arterial oxygen content
\( C_{vO_2} \): mixed venous oxygen content

Fick equation: practical forms and units

1. SI units (recommended)

If you express oxygen contents in mL O₂/L blood and VO₂ in mL O₂/min:

\( \text{CO (L/min)} = \dfrac{\dot V_{O_2} \; (\text{mL/min})}{C_{aO_2} - C_{vO_2} \; (\text{mL/L})} \)

2. Clinical units (mL O₂/dL)

In many clinical references, oxygen content is given in mL O₂/dL blood. In that case:

\( \text{CO (L/min)} = \dfrac{\dot V_{O_2} \; (\text{mL/min})}{C_{aO_2} - C_{vO_2} \; (\text{mL/dL})} \div 10 \)

The extra division by 10 converts dL to L (1 L = 10 dL).

Oxygen content from blood gases

Oxygen content is usually derived from hemoglobin concentration and oxygen saturation:

\( C_{O_2} = 1.34 \times \text{Hb} \times S_{O_2} + 0.0031 \times P_{O_2} \)

Hb: hemoglobin (g/dL)
\( S_{O_2} \): oxygen saturation (fraction, e.g. 0.97)
\( P_{O_2} \): partial pressure of O₂ (mmHg)
Result is in mL O₂/dL blood

Worked example: Fick cardiac output

Consider a 70 kg adult at rest with:

VO₂ = 250 mL/min
CaO₂ = 200 mL O₂/L blood
CvO₂ = 150 mL O₂/L blood
BSA = 1.8 m²

Step 1 – A–V O₂ difference:

\( C_{aO_2} - C_{vO_2} = 200 - 150 = 50 \; \text{mL O₂/L} \)

Step 2 – Cardiac output:

\( \text{CO} = \dfrac{250}{50} = 5.0 \; \text{L/min} \)

Step 3 – Cardiac index (CI):

\( \text{CI} = \dfrac{\text{CO}}{\text{BSA}} = \dfrac{5.0}{1.8} \approx 2.8 \; \text{L/min/m²} \)

This is within the normal resting range for both CO and CI.

Interpreting results

Normal CO: ~4–8 L/min in adults at rest.
Normal CI: ~2.5–4.0 L/min/m².
High A–V O₂ difference (large CaO₂ − CvO₂) usually indicates low cardiac output or increased tissue extraction.
Low A–V O₂ difference can be seen with high cardiac output states (e.g., sepsis, hyperthyroidism) or impaired extraction.

Assumptions and limitations of the Fick method

Steady-state conditions (VO₂ and blood flow are stable during measurement).
All oxygen uptake occurs in the systemic circulation (no significant shunts).
Accurate measurement of VO₂ and blood oxygen contents.
Mixed venous sample truly represents the average venous blood (pulmonary artery catheter, not central venous line).

This calculator is for educational and research support only and does not replace clinical judgment, institutional protocols, or direct measurement methods.

Frequently asked questions

What is the difference between direct and indirect Fick?

In the direct Fick method, VO₂ is measured directly using a metabolic cart. In the indirect Fick method, VO₂ is estimated from predictive equations based on age, sex, and body size. Direct measurement is more accurate but less practical in routine clinical settings.

Can I use central venous oxygen saturation (ScvO₂) instead of SvO₂?

Some simplified approaches substitute ScvO₂ (from a central venous catheter) for SvO₂, but ScvO₂ is not always equivalent to true mixed venous saturation, especially in shock or sepsis. This introduces additional uncertainty into the Fick calculation.

How does anemia affect Fick cardiac output?

Anemia reduces CaO₂ and CvO₂ because oxygen content depends on hemoglobin. For the same VO₂ and CO, the A–V O₂ difference will be larger in anemic patients. If you ignore hemoglobin and only look at saturations, you may misinterpret the hemodynamic state.

Fick Principle Cardiac Output Calculator

Fick Equation Calculator

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