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# Measuring cardiac output: Fick, indicator and thermodilution, Doppler

- Measurement
- Exam priority: medium

Cardiac output is measured from how much oxygen the body takes up, how fast an injected marker is diluted, or how fast blood moves through a valve.

Updated Nov 13, 2026

Also called: Fick principle, Thermodilution, Cardiac index

**See it in 3D:** [Pulmonary arteries](https://durgical.com/en/anatomy/right-pulmonary-artery/) · [Right atrium](https://durgical.com/en/anatomy/right-atrium/) · [Left ventricle](https://durgical.com/en/anatomy/left-ventricle/)

## Key points

- Fick: CO = V̇O₂ / (CaO₂ − CvO₂), using mixed venous blood from the pulmonary artery.
- Indicator and thermodilution: CO = amount of indicator / area under the concentration (or temperature) curve; a smaller area means a higher output.
- Thermodilution errors: tricuspid regurgitation and intracardiac shunts distort the curve; low output states are less accurate.
- Doppler: stroke volume = velocity–time integral × outflow-tract area; CO = SV × HR.
- Cardiac index ≈ 3 L/min/m² in adults; it is highest in childhood (≈ 4 at ≈ 10 years) and falls with age.

## What it is

Cardiac output cannot be seen directly, so it is calculated. The Fick method divides oxygen uptake by the arteriovenous O₂ difference; indicator and thermodilution methods measure how a known amount of dye or cold saline is diluted over time; Doppler echocardiography multiplies blood velocity by the area of the outflow tract. Output is usually scaled to body surface area as the cardiac index.

## Equations

**Fick principle**CO = V̇O₂ / (CaO₂ − CvO₂)

**V̇O₂**
O₂ uptake (mL/min)

**CaO₂**
arterial O₂ content (mL O₂/L)

**CvO₂**
mixed venous O₂ content (mL O₂/L)

Example: 250 / (200 − 150) = 5 L/min.

**Indicator dilution (Stewart–Hamilton)**CO = amount injected / ∫C(t)dt

**∫C(t)dt**
area under the concentration curve, without recirculation

Thermodilution uses cold saline and blood temperature instead of dye concentration.

**Doppler stroke volume**SV = VTI × π × (d/2)²

**VTI**
velocity–time integral (cm)

**d**
outflow-tract diameter (cm)

Errors in diameter are squared, so the diameter is the weakest point.

**Cardiac index**CI = CO / BSA

**BSA**
body surface area (m²)

## Reference values (resting adult)

| Parameter | Value | Note |
| --- | --- | --- |
| Cardiac output | ≈ 5 L/min | ≈ 5.6 L/min in young men, ≈ 4.9 L/min in women |
| Cardiac index | ≈ 2.5–4 L/min/m² | |
| O₂ uptake (V̇O₂) | ≈ 250 mL/min | |
| Arteriovenous O₂ difference | ≈ 40–50 mL O₂/L | |
| Mixed venous O₂ saturation | ≈ 70–75% | |

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

## Methods compared

| Method | Needs | Main pitfall |
| --- | --- | --- |
| Fick (direct) | Measured V̇O₂, arterial and pulmonary-artery blood | Unstable states; leaks in V̇O₂ measurement |
| Dye dilution | Injection into a vein, arterial sampling | Recirculation of dye must be removed from the curve |
| Thermodilution | Pulmonary artery catheter, cold saline into the right atrium | Tricuspid regurgitation, shunts, very low output |
| Doppler echocardiography | Outflow-tract diameter and VTI | Diameter error; angle of the beam |

## Clinical notes

- In shock, low output (cardiogenic, hypovolemic) and high output with low resistance (early sepsis) are told apart by measured output and resistance.
- A low mixed venous O₂ saturation means tissues extract more O₂ because output is too low for demand (Fick in reverse).
- A left-to-right shunt makes pulmonary flow larger than systemic flow; measuring both (Qp/Qs) sizes the shunt.

## Memory aid

Fick follows the oxygen, dilution follows the dye, Doppler follows the speed.

## Test yourself

Think of the answer, then tap the question to see it.

## V̇O₂ is 300 mL/min, arterial O₂ content 200 mL/L and mixed venous content 140 mL/L. What is the cardiac output?

CO = 300 / (200 − 140) = 5 L/min.

## In thermodilution, what does a small area under the temperature curve mean?

High cardiac output: the cold bolus is carried away and diluted quickly, so the temperature change is brief and small.

## Why must the Fick method use pulmonary-artery blood rather than a peripheral vein?

Each organ extracts different amounts of O₂; only the pulmonary artery has fully mixed venous blood from the whole body.

## Why does tricuspid regurgitation make thermodilution unreliable?

The cold indicator moves back and forth across the leaking valve, so it reaches the sensor slowly and the curve is distorted, usually underestimating output.

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## Connections

## See it in 3D 3

The structures of Durgical's 3D anatomy where it happens.

- [Pulmonary arteries](https://durgical.com/en/anatomy/right-pulmonary-artery/)
- [Right atrium](https://durgical.com/en/anatomy/right-atrium/)
- [Left ventricle](https://durgical.com/en/anatomy/left-ventricle/)

## Related topics 4

- [Cardiac output and what sets it](https://durgical.com/en/physiology/cardiac-output-and-what-sets-it/)
- [Oxygen content, delivery and extraction](https://durgical.com/en/physiology/oxygen-content-delivery-and-extraction/)
- [Physiology of congenital shunts](https://durgical.com/en/physiology/physiology-of-congenital-shunts/)
- [Central venous pressure and the jugular pulse](https://durgical.com/en/physiology/central-venous-pressure-and-the-jugular-pulse/)

## Related conditions 3

- [Cardiogenic shock](https://durgical.com/en/conditions/cardiogenic-shock/)
- [Heart failure](https://durgical.com/en/conditions/heart-failure/)
- [Sepsis and septic shock](https://durgical.com/en/conditions/sepsis-and-septic-shock/)

[← All of physiology](https://durgical.com/en/physiology/)

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: [Swan-Ganz - right heart catheterization: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003870.htm) (MedlinePlus, US National Library of Medicine, 2026); [Echocardiogram: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003869.htm) (MedlinePlus, US National Library of Medicine, 2026); [Heart Failure (Congestive Heart Failure)](https://www.ncbi.nlm.nih.gov/books/NBK430873/) (StatPearls (NCBI Bookshelf), 2026).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/measuring-cardiac-output-fick-indicator-and-thermodilution-doppler/