Calculates cardiac output and cardiac index by the Fick principle, from a measured or an assumed oxygen consumption.
Result
Enter hemoglobin and both saturations.
Sources
2 studies
2
studies
31
citations
970
PubMed papers
Evidence
Known limitations
In two studies with Douglas bag measurements at rest, VO2 estimated as 125 mL/min/m² × body surface area averaged 259.9 mL/min against a measured 307.2 mL/min. The mean absolute difference was 52.9 mL/min, and 20% of estimates were more than 25% off; error grew with BMI and was larger in men. The authors conclude VO2 should be measured, not estimated.
Am J Cardiol: Journal Metrics Score 2.2, Q2 · cited 31 times
In 535 adults having right heart catheterization with measured VO2 (mean 241 mL/min), the Dehmer, LaFarge and Bergstra formulas all differed significantly from measured values, by more than 25% in 17 to 25% of patients, with larger errors at a BMI above 40.
Circulation: Journal Metrics Score 2.3, Q2 · cited 100 times
The key study has been cited 31 times, and 970 PubMed articles mention Fick cardiac output in their title or abstract. Citation counts from OpenAlex and article counts from PubMed, retrieved November 9, 2026. Journal Metrics Scores and quartiles are from this site’s current release; follow the journal name for details.
After the result
Low cardiac index · When VO2 was assumed · High cardiac index
Confirm it, then read it with pressures and perfusion. Measure VO2 when the decision depends on it. Think of high-output states and sampling errors.
Next steps
Avoid common errors
6 common errors to avoid
and 4 more
When the Fick result misleads
1Assumed VO2
An assumed VO2 is usually the largest source of error: in 535 catheterized patients, estimated and measured VO2 differed by more than 25% in 17 to 25% depending on the formula (Narang 2014).
2Wrong venous sample
A superior vena cava or right atrial saturation is not mixed venous. Draw from the pulmonary artery without wedging.
3Shunts
Any left-to-right shunt raises the pulmonary artery saturation and inflates the output, as in the anomalous venous return example below.
4Not simultaneous
Hemoglobin and both saturations should come from the same moment. Oxygen therapy changes or a transfusion between samples distort the difference.
5Small saturation difference
When arterial and venous saturations are close, a 1 to 2 point error in either changes the result a lot.
6Dissolved oxygen
The formula leaves out dissolved oxygen (0.003 mL/dL per mm Hg). On room air this is a few percent; with a very high arterial PO2, as on 100% oxygen, the output can be overstated by 20 to 30%.
Formula
VO2 / [1.34 × Hb × (SaO2 − SvO2)/100 × 10]; VO2 measured or assumed at 125 mL/min/m².
VO2 / [1.34 × Hb × (SaO2 − SvO2)/100 × 10]
VO2 / [1.34 × Hb × (SaO2 − SvO2)/100 × 10]; VO2 measured or assumed at 125 mL/min/m².
Formula and ranges
| Value | Formula |
|---|---|
| Oxygen content difference | CaO2 − CvO2 (mL/dL) = 1.34 × Hb (g/dL) × (SaO2 − SvO2) / 100, saturations in % |
| Cardiac output | VO2 (mL/min) / [(CaO2 − CvO2) × 10] in L/min |
| Assumed VO2 | 125 mL/min/m² × body surface area |
| Body surface area (Mosteller) | √(height cm × weight kg / 3600) |
| Cardiac index | cardiac output / body surface area |
| Result | L/min/m² | Meaning |
|---|---|---|
| Very low cardiac index | below 1.8 | A cardiac index below 1.8 L/min/m²: very low. With low blood pressure and signs of poor perfusion, this fits cardiogenic shock. |
| Low cardiac index | 1.8 to 2.1 | A cardiac index of 1.8 to 2.1 L/min/m²: low, as in heart failure with low output. In a patient on inotropes, vasopressors or mechanical support, an index up to 2.2 with low blood pressure and poor perfusion also fits cardiogenic shock. |
| Not low | 2.2 to 4.0 | A cardiac index of 2.2 to 4.0 L/min/m²: not low. The usual resting range is about 2.5 to 4.0, so 2.2 to 2.4 is borderline. |
| High cardiac index | above 4.0 | A cardiac index above 4.0 L/min/m²: high output, as in sepsis, anemia, pregnancy, thyrotoxicosis, a shunt or an arteriovenous fistula, or an overestimated oxygen consumption. |
The Fick principle
Assumed VO2
Constant
Units
Test cases
3 published cases, each also a test case
3 published cases; the calculator reproduces every result.
Worked examples
A 29-year-old man with biventricular failure and constrictive pericarditis on home milrinone had right and left heart catheterization. With a measured VO2 of 267 mL/min, hemoglobin 13.5 g/dL, arterial saturation 95.5% and pulmonary artery saturation 62.8%, the report gives a Fick cardiac output of 4.46 L/min (thermodilution 3.70).
“VO2 267 mL/min … PA saturation 62.80% Hgb 13.50 Arterial saturation 95.50% on room air CO (Fick) 4.46 L/min”
The calculator gives 4.51 with 1.34 mL/g (4.45 with 1.36), shown as 4.5, as is 4.46. Fick and thermodilution differed by 0.8 L/min in the same patient. The paper reports a Fick cardiac index of 2.08 L/min/m² (thermodilution 1.73), which is low; without height and weight the calculator can judge only the output against adult ranges, where 4.5 L/min looks usual.
Entered as: Hemoglobin: 13.5 g/dL; Arterial saturation (SaO2): 95.5 %; Mixed venous saturation (SvO2): 62.8 %; Measured oxygen consumption (VO2): 267 mL/min.
Source: Wood T et al., Eur Heart J Case Rep 2020 (PubMed · Free full text · DOI)
A review of invasive assessment in heart failure works through the Fick equation with an oxygen consumption of 305 mL/min, hemoglobin 12.4 g/dL, arterial saturation 92% and venous saturation 45%, giving a cardiac output of 3.86 L/min and an index of 2.3 L/min/m² for a body surface area of 1.73 m².
“Oxygen consumption is given at 305 ml/min, hemoglobin is given at 12.4 g/dL, arterial oxygen saturation was 92%, and venous oxygen saturation was 45%.”
The review uses 1.36 mL/g and rounds the oxygen contents (15.5 − 7.6); the calculator gives 3.91 with 1.34, shown as 3.9, as the review rounds it for the index.
Entered as: Hemoglobin: 12.4 g/dL; Arterial saturation (SaO2): 92 %; Mixed venous saturation (SvO2): 45 %; Measured oxygen consumption (VO2): 305 mL/min.
Source: Wan SH et al., Cardiol Res Pract 2019 (PubMed · Free full text · DOI)
A 62-year-old woman with ischemic cardiomyopathy had repeated right heart catheterizations before transplant. Twenty months before, hemoglobin was 12.3 g/dL, arterial saturation 93% and pulmonary artery saturation 71.5%, and the Fick cardiac index was 3.48 L/min/m². Fick calculations kept showing a normal index while she worsened; after transplant, partial anomalous pulmonary venous return was found.
“Fick method calculations consistently demonstrated a normal cardiac index. However, the patient experienced worsening symptoms.”
The paper does not say how VO2 was obtained, but its 5.93 L/min and 3.48 L/min/m² back-calculate to about 125 mL/min/m² with 1.36 mL/g. The calculator gives 3.53 with 1.34 mL/g (3.48 with 1.36), shown as 3.5. The authors do not discuss the Fick error, but the left superior pulmonary vein drained into the innominate vein, so oxygenated blood reached the pulmonary artery. A Fick output from that sample measures pulmonary rather than systemic flow, and overstates systemic output.
Entered as: Hemoglobin: 12.3 g/dL; Arterial saturation (SaO2): 93 %; Mixed venous saturation (SvO2): 71.5 %.
Source: Salas de Armas IA et al., BMC Cardiovasc Disord 2021 (PubMed · Free full text · DOI)
FAQ
6 questions and answers
and 4 more answers
Questions
Cardiac output (L/min) = oxygen consumption (VO2, mL/min) / [(CaO2 − CvO2) × 10], where oxygen content (mL/dL) = 1.34 × hemoglobin (g/dL) × saturation. With a VO2 of 305 mL/min, hemoglobin 12.4 g/dL and saturations of 92% and 45%: 1.34 × 12.4 × 0.47 = 7.81 mL/dL, and 305 / 78.1 = 3.9 L/min.
Direct Fick uses a measured oxygen consumption, from expired gas analysis. Indirect (assumed or estimated) Fick replaces it with an estimate, most often 125 mL/min per m² of body surface area, or a formula such as LaFarge’s based on age, sex and heart rate. The estimate is the weak point: in Narang and colleagues’ studies it differed from the measured value by more than 25% in about one patient in five.
When VO2 is assumed to be 125 mL/min/m², the body surface area cancels out: cardiac index = 125 / [(CaO2 − CvO2) × 10]. You only need height and weight to turn the index into an output in L/min, or to index a measured VO2.
A resting adult’s cardiac output is about 4 to 8 L/min, and the cardiac index about 2.5 to 4.0 L/min/m²; an index below about 2.2 is commonly taken as low. Interpret it with blood pressure, filling pressures, lactate and the clinical picture.
All are in use for the oxygen bound per gram of hemoglobin. This calculator uses 1.34; 1.36 gives a result about 1.5% lower. The difference is small next to the error from an estimated VO2 or a poorly timed blood sample.
Blood in the pulmonary artery is mixed from the whole body. A central venous sample from the superior vena cava misses blood from the lower body and the heart itself, and its saturation is usually different. A left-to-right shunt raises the pulmonary artery saturation and makes the Fick output falsely high, as in the anomalous venous return example below.
Further reading
7 references with PubMed and DOI links
7
references
3
free full text
From Narang N et al., Am J Cardiol 2012 to the newest case reports.
References
Contributed by Dr. Meng Zhao (MD · Former Neurosurgeon · Medical AI Researcher), from the original publications listed here. Last updated November 9, 2026. LinkedIn
The calculator code is tested against every worked example on this page before each release.
This calculator supports, and does not replace, clinical judgment. It is meant for healthcare professionals and is not medical advice for patients. Report an error through the contact page.
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Which published the 2014 Narang study, with its current metrics.