π« Cardiac Output Calculator
Fick principle: oxygen consumption over AV oxygen difference.
Educational only. Hemodynamic decisions belong to the clinical team.
The Fick principle is the most physiologically honest way to measure cardiac output: the heart must pump exactly enough blood to deliver the oxygen the body consumes. Divide oxygen consumption by the arterial-venous oxygen difference, and out comes flow. Adolf Fick described it in 1870, and it remains the gold standard against which thermodilution and echo methods are judged. This calculator runs the full computation including oxygen contents.
How to Use This Calculator
Enter oxygen consumption, hemoglobin, and arterial and mixed venous saturations. The calculator returns cardiac output plus the intermediate oxygen contents.
Enter VO2 in mL/min: measured by metabolic cart, or roughly 250 at rest (about 3.5 mL/kg/min).
Enter hemoglobin in g/dL and SaO2 from pulse oximetry or arterial blood gas.
Enter SvO2 from a pulmonary artery catheter sample (mixed venous). ScvO2 from a central line runs a few points higher.
Read cardiac output and the extraction ratio. Normal resting CO is 4 to 8 L/min; normal extraction about 25 percent.
The Fick Equation
The logic is mass balance: every milliliter of oxygen the tissues consume must have been carried there by blood, so flow equals consumption divided by how much oxygen each unit of blood gave up. The 1.34 is Huffner's constant, milliliters of oxygen per gram of hemoglobin. Dissolved oxygen (0.003 x PaO2) is omitted as negligible at normal pressures. Units need care: VO2 in mL/min divided by AV difference in mL/dL gives deciliters per minute, times 10 for the textbook L/min, which the calculator handles.
The extraction ratio is the sleeper variable. Normal tissues extract about a quarter of delivered oxygen, leaving SvO2 near 75 percent. Rising extraction (falling SvO2) means delivery is failing to meet demand: the tissues wring more from each unit of blood. An SvO2 under 60 percent with a rising lactate is the physiology of shock, whatever the cardiac output number says. That is why ICU teams watch SvO2 trends as closely as the output itself.
Worked Example
VO2 250 mL/min, hemoglobin 15 g/dL, SaO2 98 percent, SvO2 75 percent. CaO2 = 1.34 x 15 x 0.98 = 19.7 mL/dL. CvO2 = 1.34 x 15 x 0.75 = 15.1 mL/dL. AV difference = 4.6 mL/dL. CO = 250 / (4.6 x 10) = 5.4 L/min, textbook normal. Extraction ratio = 4.6 / 19.7 = 23 percent, normal. Now drop SvO2 to 55 percent: CvO2 = 13.5, AV difference = 6.2, CO = 4.0 L/min, extraction 32 percent. Same oxygen consumption, lower flow, harder-working tissues: the pattern of early shock or severe anemia compensation.
Fick vs Thermodilution vs Echo
Direct Fick with measured VO2 is the reference standard, but metabolic carts are rare outside cath labs, so most "Fick" outputs use assumed VO2 from formulas (LaFarge, Dehmer), which introduces real error: assumed VO2 can miss by 20 percent or more. Thermodilution is the ICU workhorse, fast and repeatable but thrown off by tricuspid regurgitation and low flow states. Doppler echo is noninvasive and excellent for trends; our echo CO calculator runs that method. In practice, clinicians pick the method the patient already has access to and follow the trend, because all methods agree that direction matters more than the digit.
To compare the result across body sizes, divide by BSA with our cardiac index calculator: 5.4 L/min in a 2.0 m2 adult is a CI of 2.7, normal. And remember the clinical frame: cardiac output is adequate when tissues are perfused (warm, mentating, making urine, normal lactate), not when the number crosses a line. The equation is 150 years old; the judgment around it is the actual medicine.
Common Pitfalls
Using ScvO2 (central venous, from a regular central line) in place of SvO2 (mixed venous, needs a PA catheter): ScvO2 runs about 5 points higher, which understates the AV difference and overstates output. Forgetting that assumed VO2 is a guess: label it as estimated Fick, not Fick. Anemia breaks intuition: low hemoglobin shrinks both contents proportionally, so the difference narrows and the computed output rises, correctly reflecting the compensatory high-output state of severe anemia. Intracardiac shunts invalidate the whole method, since mixed venous blood is not truly mixed. Know the method's assumptions and it serves you well; forget them and it lies politely.
For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
What is the Fick principle?
Cardiac output equals oxygen consumption divided by the arterial-venous oxygen content difference (CO = VO2 / (CaO2 - CvO2)). Described by Adolf Fick in 1870, it derives flow from mass balance: the heart must pump enough blood to deliver the oxygen tissues consume.
How do you calculate cardiac output by Fick?
Measure or estimate VO2 (mL/min), compute CaO2 = 1.34 x Hb x SaO2 and CvO2 = 1.34 x Hb x SvO2 (mL/dL), then CO = VO2 / ((CaO2 - CvO2) x 10) in L/min. This calculator performs all steps.
What is normal cardiac output?
About 4 to 8 L/min at rest in adults, varying with body size. During maximal exercise it can exceed 20 L/min in trained athletes. For cross-patient comparison, divide by BSA to get cardiac index (normal 2.5-4.0).
What is a normal SvO2?
Mixed venous saturation around 70-80%, typically ~75%. Below 60% suggests delivery failing to meet demand (shock, severe anemia, hypoxia). ScvO2 from a central line runs a few points higher than true mixed venous SvO2.
What is oxygen extraction ratio?
The fraction of delivered oxygen tissues consume: (CaO2 - CvO2) / CaO2, normally about 25%. Rising extraction means tissues are compensating for falling delivery, an early warning sign before blood pressure drops.
Is Fick more accurate than thermodilution?
Direct Fick with measured VO2 is the gold standard. But most clinical "Fick" uses assumed VO2, which can err by 20%+, making it potentially less accurate than thermodilution. Assumed-VO2 Fick should be labeled estimated.
What is the difference between SvO2 and ScvO2?
SvO2 is mixed venous blood from the pulmonary artery (true whole-body average); ScvO2 comes from a central venous catheter in the SVC (upper-body weighted). ScvO2 reads ~5 points higher. Substituting one for the other biases the Fick calculation.
Can Fick be used with a shunt?
No, not reliably. Intracardiac or intrapulmonary shunts mean the "mixed venous" sample is not truly mixed, invalidating the mass-balance assumption. Shunt quantification uses oximetry runs instead.
Why does anemia raise cardiac output?
Low hemoglobin narrows the arterial-venous oxygen difference (less oxygen per unit blood), so the heart must pump more volume to deliver the same oxygen. The Fick equation shows this directly: smaller denominator, larger flow.
Who was Fick and when did he publish this?
Adolf Fick, a German physiologist, published the principle in 1870. It predates every measurement device used with it today, which speaks to the durability of reasoning from first principles.