Fick principle: how to calculate cardiac output
The Fick principle uses oxygen as a tracer to find cardiac output from oxygen consumption and the arteriovenous oxygen content difference. Learn the formula, the sampling sites and the units trap.
Chapters (12)
Calculate cardiac output with the Fick principle from oxygen consumption and the arteriovenous oxygen content difference, using the correct sampling sites and units.
The short version
- Cardiac output = oxygen consumption ÷ arteriovenous (AV) oxygen content difference.
- The venous sample must be mixed venous blood from the pulmonary artery; the arterial sample can come from a systemic artery.
- Catheter reports often give oxygen content in mL/dL: multiply by 10 to get mL/L before you divide.
- Tissues set oxygen consumption, so at a constant oxygen consumption a wide AV difference means low output and a narrow one means high output.
- Direct Fick (measured oxygen consumption) is the gold standard; indirect Fick (assumed oxygen consumption) can be inaccurate.
Cardiac output and the Fick principle
Cardiac output is the volume of blood the heart pumps per minute. It equals heart rate × stroke volume, and at rest a healthy adult's output is about 5 L/min.[1][3][7]
You cannot watch blood leave the heart and time it. In 1870 Adolf Fick proposed using oxygen as the tracer instead. The Fick principle links oxygen consumption to blood flow and to the arteriovenous oxygen content difference.[2][6]
Picture the lungs as a loading dock and the body as one big customer. Every minute the lungs load oxygen onto the blood, and in a steady state the tissues take exactly that much off again. So oxygen consumption equals blood flow × the oxygen each litre of blood gives up.
The Fick formula for cardiac output
Rearrange that statement and you have the Fick cardiac output formula.[2][4]
VO₂ = oxygen consumption (mL/min); CaO₂ = arterial oxygen content; CvO₂ = mixed venous oxygen content (both in mL/L)
Oxygen consumption, VO₂, is normally about 250 mL/min at rest.[3]
Which blood to sample: arterial and mixed venous
The arterial content is the oxygen in blood leaving the lungs. In practice a systemic artery is used.[2][4]
The venous content must be mixed venous blood, sampled from the pulmonary artery. Different organs return blood with different oxygen content, and by the pulmonary artery the venous return is fully mixed.[2][4]
Fick cardiac output worked example and the units trap
A patient consumes 250 mL of oxygen per minute. Arterial content is 200 mL/L and mixed venous content is 150 mL/L. These are illustrative teaching numbers, not patient data.
- CaO₂ − CvO₂ = 200 − 150 = 50 mL of oxygen taken from every litre
- CO = 250 / 50 = 5 L/min
- Check: 5 L/min × 50 mL/L = 250 mL/min, which matches VO₂
- At a heart rate of 70/min, stroke volume = 5000 / 70 ≈ 71 mL per beat
The last step reuses cardiac output = heart rate × stroke volume.[1]
Why a wide AV difference means low cardiac output
The heart does not set oxygen consumption. The tissues do. If output falls, each litre has to give up more oxygen to meet the same demand, so venous content drops and the difference widens.[6][7]
| AV oxygen content difference | Cardiac output |
|---|---|
| Wide | Low |
| Narrow | High |
Direct vs indirect Fick
Measuring oxygen consumption is awkward, so it is often assumed from tables. That is indirect Fick, and it can be inaccurate. Direct Fick, with measured oxygen consumption, is the gold standard.[4][5]
Practice question
Original · USMLE styleA 61-year-old woman is evaluated for progressive breathlessness. During right heart catheterisation, her oxygen consumption is measured by expired-gas analysis at 240 mL/min. Blood sampled simultaneously gives:
| Systemic arterial blood, oxygen content | 18 mL/dL |
|---|---|
| Pulmonary artery blood, oxygen content | 12 mL/dL |
Common questions
What is the Fick principle formula for cardiac output?
Cardiac output = oxygen consumption ÷ (arterial oxygen content − mixed venous oxygen content). Use consistent units, with content in mL/L.
Why must the venous sample come from the pulmonary artery?
Different organs return blood with different oxygen content, and by the pulmonary artery the venous return is fully mixed. A sample from one peripheral vein describes only one organ.
How do you convert mL/dL to mL/L for the Fick calculation?
Multiply by 10. For example, 5 mL/dL is 50 mL/L. Do this before dividing oxygen consumption by the difference.
What is the difference between direct and indirect Fick?
Direct Fick uses measured oxygen consumption and is the gold standard. Indirect Fick assumes oxygen consumption from tables and can be inaccurate.
Transcript
Read the full narration, by chapter641 words
0:00Three numbers, one cardiac output
Your patient burns 250 mL of oxygen every minute. Blood leaves the lungs carrying 200 mL of oxygen per litre, and comes back carrying only 150. From those three numbers alone, can you work out how many litres the heart pumps each minute? By the end of this lesson, you'll calculate cardiac output with the Fick principle, from oxygen consumption and the arteriovenous oxygen content difference.
0:27What cardiac output is
First, what are we measuring? Cardiac output is the volume of blood the heart pumps per minute. It equals heart rate times stroke volume, the stroke volume you read off the pressure-volume loop. At rest, a healthy adult's output is about five litres per minute.
0:45Adolf Fick and oxygen as the tracer
But you can't watch blood leave the heart and time it. So in 1870, Adolf Fick proposed using oxygen as the tracer instead. Picture the lungs as a loading dock, and the body as one big customer. Every minute, the lungs load oxygen onto the blood. In a steady state, the tissues take exactly that much off again.
1:08The Fick formula
So oxygen consumption equals blood flow times the oxygen each litre of blood gives up. Rearrange that, and cardiac output equals oxygen consumption, divided by the arteriovenous oxygen content difference. Oxygen consumption, VO two, is normally about 250 mL per minute at rest.
1:29Which blood to sample: arterial and mixed venous
The arterial content is the oxygen in blood leaving the lungs. In practice, a systemic artery is used. The venous content must be mixed venous blood, sampled from the pulmonary artery. Why there? Different organs return blood with different oxygen content, and only the pulmonary artery holds the true mixture. A sample from one peripheral vein only describes one organ.
1:57Worked example
Now let's work our patient. Arterial content, 200 mL per litre. Mixed venous content, 150. The difference is 50 mL of oxygen taken from every litre. Two 150 divided by 50 gives five litres per minute. Check it: five litres, each giving up 50 mL, delivers 250. Divide by a heart rate of 70, and each beat ejects about 71 mL.
2:28The units trap (mL/dL to mL/L)
Now, a units trap. Catheter reports give oxygen content in mL per decilitre. Five mL per decilitre is 50 mL per litre. Multiply by 10 before you divide.
2:43The twist: wide AV difference = low output
And here's the twist. The heart doesn't set oxygen consumption. The tissues do. If output falls, each litre has to give up more oxygen to meet the same demand. So venous content drops, and the difference widens. A wide arteriovenous difference means low output. A narrow one means high output.
3:05Direct vs indirect Fick
One last point. Measuring oxygen consumption is awkward, so it's often assumed from tables. That's indirect Fick, and it can be inaccurate. Direct Fick, with measured oxygen consumption, is the gold standard.
3:21Practice question
Your turn. Pause if you want to work it out. A 61-year-old woman with breathlessness has a right heart catheterisation. Her measured oxygen consumption is 240 mL per minute. Arterial blood contains 18 mL of oxygen per decilitre. Pulmonary artery blood contains 12. What is her cardiac output? Let's strike the wrong ones first. A, 1.3, divides by the arterial content alone. You need the difference. B, two, divides by the venous content alone. Same mistake. D, six, is the arteriovenous difference itself, in mL per decilitre. It isn't a flow. E, 40, forgets to convert. Divide 240 by six, and you've mixed decilitres with litres. So the answer is C: four litres per minute. Six mL per decilitre is 60 per litre, and 240 divided by 60 is four. A wider difference than our first patient's, so a lower output, at the low end of normal.
4:37Recap
Quick recap. One: cardiac output equals oxygen consumption, divided by the arteriovenous oxygen content difference. Two: venous blood must come from the pulmonary artery, and decilitres must become litres. Three: a wide difference means low output.
4:58Next lesson
Next, we put this into failing hearts: reduced against preserved ejection fraction, two failing loops.
Sources
Numbers in the text, e.g. [1], point here- Cardiac output (definition, resting range). Wikipedia: en.wikipedia.org/wiki/Cardiac_output
- Fick principle (formula, pulmonary artery sampling, assumed VO2). Wikipedia: en.wikipedia.org/wiki/Fick_principle
- Right heart catheterisation case report with normal haemodynamic ranges and resting VO2 of about 250 mL/min. pmc.ncbi.nlm.nih.gov/articles/PMC8820485/
- Review of Fick cardiac output measurement (VO2 divided by the arteriovenous oxygen content difference; mixed venous sampling; assumed VO2). pmc.ncbi.nlm.nih.gov/articles/PMC13331493/
- Direct vs indirect Fick (direct is the gold standard; indirect can be inaccurate). pmc.ncbi.nlm.nih.gov/articles/PMC13150460/
- Fick 1870 and the arteriovenous oxygen difference. Sci Rep 2026: pmc.ncbi.nlm.nih.gov/articles/PMC13500711/
- Cardiac output as heart rate x stroke volume; ScvO2 and Fick. Front Cardiovasc Med 2025;12:1628380: pmc.ncbi.nlm.nih.gov/articles/PMC12657444/
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Illustrations: Servier Medical Art (smart.servier.com), CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), adapted (recoloured, outlined): pulmonary circulation. "lung" by Servier via Bioicons (bioicons.com), CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/), adapted (recoloured, outlined). 3D heart: "Anatomical Human Heart" by Nexal 3D (BlenderKit), used under the BlenderKit Royalty Free licence (https://www.blenderkit.com/asset-gallery-detail/45c5c47c-c1e6-4f98-b82c-b25cbfa675a2/). Charts, the pressure-volume loop sketch, the catheter report and all handwriting are drawn in code (original). Previous: EP 07 · Valve disease on pressure-volume loops · Next: EP 09 · HFrEF vs HFpEF, two failing loops
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-CARD-007.
For exam study only, not medical advice. Read the medical disclaimer.
In this series
Playlist on YouTube- EP 01Pressure-volume loop explained: four phases, four valve events5:00
- EP 02Preload on the pressure-volume loop: EDV, stroke volume and the EDPVR5:04
- EP 03Afterload on the pressure-volume loop4:49
- EP 04Contractility on the pressure-volume loop: the ESPVR explained5:17
- EP 05Wiggers diagram explained: the cardiac cycle step by step5:30
- EP 06The Frank-Starling curve: preload, inotropy and heart failure4:44
- EP 07Aortic stenosis vs mitral regurgitation on the pressure-volume loop5:41
- EP 08Fick principle: how to calculate cardiac output5:07
- EP 09HFrEF vs HFpEF on the pressure-volume loop5:33
- EP 10Poiseuille's law and vascular resistance: why arterioles matter5:15
- EP 11Venous return and cardiac function curves: where they cross5:11
- EP 12The baroreceptor reflex: from stretch to vagal and sympathetic output5:17
- EP 13Myocardial oxygen supply and demand: why tachycardia causes ischaemia4:50
- EP 14Unstable angina vs NSTEMI vs STEMI: from plaque to ACS4:50
- EP 15Antianginal drugs: nitrates, beta blockers, calcium channel blockers4:56
- EP 16Complications of myocardial infarction, by timeline4:59