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.
Also called: Fick principle, Thermodilution, Cardiac index
See it in 3D: Pulmonary arteries · Right atrium · Left ventricle
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
- 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.
- ∫C(t)dt
- area under the concentration curve, without recirculation
Thermodilution uses cold saline and blood temperature instead of dye concentration.
- VTI
- velocity–time integral (cm)
- d
- outflow-tract diameter (cm)
Errors in diameter are squared, so the diameter is the weakest point.
- 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.
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.
Connections
See it in 3D3
The structures of Durgical's 3D anatomy where it happens.
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 (MedlinePlus, US National Library of Medicine, 2026); Echocardiogram: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Heart Failure (Congestive Heart Failure) (StatPearls (NCBI Bookshelf), 2026).
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Representative model, for educational purposes only.