Fick cardiac output: Formula, worked example, and free worksheet

Key takeaways

Fick cardiac output is VO2 divided by the arteriovenous oxygen difference multiplied by 10, which returns liters per minute.

The multiplier matters because oxygen content is reported per 100 mL of blood, while VO2 is reported per minute.

Normal cardiac output is 4 to 8 L/min, and normal cardiac index is 2.5 to 4.0 L/min/m².

Direct Fick measures VO2 with a metabolic cart, while indirect Fick estimates it and carries 15 to 25% error.

Recording whether VO2 was measured or estimated is what makes the result reviewable weeks later.

Fick cardiac output is oxygen consumption divided by the arteriovenous oxygen difference, corrected for units. Written out, that is CO = VO2 / [(CaO2 − CvO2) × 10], and the answer arrives in liters per minute. The same relationship sits behind every cardiac output formula used at the bedside.

Cardiologists, intensivists, and advanced practice clinicians use it during right heart catheterization to quantify perfusion. This guide covers the equation, a worked example, normal ranges, and the errors that quietly distort a result.

The worksheet above collects every variable in one place, so the calculation is documented rather than worked out on a scrap of paper.

What is the Fick method?

The Fick method measures blood flow using oxygen as a tracer. The principle states that oxygen taken up each minute equals blood flow multiplied by the arteriovenous oxygen difference. Rearrange that and you have cardiac output.

Cardiac output is the volume the left ventricle pumps per minute, typically 4 to 8 L/min in a resting adult. When VO2 is measured rather than estimated, Fick is the reference standard in the cath lab, as detailed in StatPearls’ overview of the calculation.

Why clinicians use it: the calculation guides management of cardiogenic shock, heart failure, valvular disease, and congenital shunts. In the US, the right heart catheterization that supplies the blood samples is billed under 93451.

The Fick equation and its variables

The equation itself is short. The units are what catch people out.

The equation: CO = VO2 / [(CaO2 − CvO2) × 10]

That multiplier is not optional. Oxygen content is reported per 100 mL of blood, while VO2 is reported in mL per minute. Multiplying the difference by 10 rescales it to a per-liter basis, so cardiac output lands in L/min.

Oxygen content comes from hemoglobin, oxygen saturation, and dissolved oxygen. Draw CaO2 from an arterial blood gas sample and CvO2 from the tip of the pulmonary artery catheter.

How to calculate cardiac output step by step

Five steps take you from blood samples to a cardiac index at the bedside or in the cath lab.

1. Measure or estimate VO2. A metabolic cart measures it directly. Without one, the LaFarge-Miettinen formula estimates VO2 from age, sex, height, and weight. The common resting shortcut is body surface area multiplied by 125 mL/min.
2. Draw arterial blood for CaO2. Take a simultaneous arterial blood gas from a radial or femoral artery. Record hemoglobin, arterial oxygen saturation, and arterial PO2.
3. Draw mixed venous blood for CvO2. Withdraw 1 to 2 mL from the distal port of the pulmonary artery catheter. That sample represents venous blood from the whole systemic circulation. Record hemoglobin, venous saturation, and venous PO2.
4. Calculate oxygen content for both samples. Use O2 content = (Hgb × 1.34 × SatO2) + (0.003 × PO2). The result is in mL O2 per 100 mL of blood.
5. Apply the Fick equation. Divide VO2 by the arteriovenous difference multiplied by 10. Then divide cardiac output by body surface area for cardiac index, in L/min/m².

A worked example makes the units concrete. Take an adult with a body surface area of 1.75 m² and hemoglobin of 14 g/dL. Estimated VO2 is 250 mL/min, SaO2 is 98%, and SvO2 is 75%. The arteriovenous difference works out at 5 mL O2 per 100 mL of blood.

CO = 250 / (5 × 10) = 5.0 L/min. Cardiac index is 5.0 / 1.75 = 2.9 L/min/m², which sits inside the normal range. Drop the ×10 and the same figures read as 50, which is the unit slip worth catching before anyone acts on the number.

Direct vs indirect Fick

The two variants differ only in where VO2 comes from. Direct Fick needs a metabolic cart or spirometry, equipment that is standard in exercise labs where sports medicine practices measure oxygen uptake during testing.

The limitation of indirect Fick: estimation formulas assume a resting metabolic state. Fever, sepsis, hyperthyroidism, and the post-operative period all push oxygen consumption well above baseline. An estimate used in those conditions underestimates cardiac output.

A rising temperature is the easiest warning sign to check, so compare the reading against a temperature chart before you trust an estimated VO2. Flagging the non-steady state in the note tells the next reader how much weight the number deserves.

Fick vs thermodilution

Thermodilution injects cold saline and reads the temperature decay curve. It is faster, which is why it dominates bedside measurement. Fick still wins in two specific situations.

Where Fick wins: thermodilution curves distort in atrial fibrillation and the other rhythm disturbances grouped under I49.9. Oxygen consumption is unaffected by rhythm, so the Fick result holds. Fick also exposes shunts, because a saturation step-up in the right heart shows up in the numbers.

Normal values for cardiac output and cardiac index

Reference ranges are what give a calculated number its meaning.

A cardiac index below 2.2 L/min/m² points toward cardiogenic shock. A high-output state above 4.2 suggests sepsis, anemia, or hyperthyroidism. An SvO2 under 60% alongside a low index suggests tissue perfusion is falling short.

Low-output states often reach a practice as R55 long before anyone threads a catheter. Logging the paired observations on a vital signs record makes that trend visible at the next appointment.

Where the calculation is used clinically

Clinicians reach for this calculation in six recurring situations:

• Cardiogenic shock. Separates a low-output presentation, with a cardiac index under 2.2, from a preserved-output one. That distinction drives inotrope and mechanical support decisions.
• Decompensated heart failure. Shows whether a reduced ejection fraction comes with genuinely low output or a compensatory hyperdynamic state.
• Valvular disease workup. Measures true forward output across a stenotic or regurgitant valve before surgical planning.
• Congenital shunt quantification. Compares pulmonary and systemic flow using saturation step-ups to size a left-to-right or right-to-left shunt.
• Transplant assessment. Documents resting and stress output for patients being evaluated for heart or lung transplant.
• Rapid atrial fibrillation. Supplies a usable number when thermodilution fails because of the rhythm.

Sources of error and limitations

Several factors distort the result, and most of them are documentation problems as much as clinical ones:

• Estimated VO2. Demographic formulas carry 15 to 25% error, and more in fever, sepsis, pain, or post-operative agitation. Always record whether VO2 was measured or estimated.
• A broken steady state. The principle assumes stable oxygen consumption and hemodynamics. Measuring during rapid volume shifts or inotrope titration invalidates the result.
• Oxygen content errors. An inaccurate hemoglobin or saturation propagates through the whole calculation. Check blood gas quality when the numbers disagree with the patient in front of you.
• Contaminated venous sample. A wedged or kinked catheter returns blood that is not truly mixed venous. Confirm catheter position before you draw.
• Intrapulmonary shunting. In pneumonia or acute respiratory distress syndrome, arterial saturation may not reflect systemic oxygenation, which compromises CaO2.
• Extreme hemoglobin. Anemia and polycythemia change oxygen carrying capacity, so interpret cardiac index against the actual hemoglobin.

Best practice: document the method, not only the answer. Record the VO2 source, the hemoglobin used, both saturations, and the timing against any intervention. Handing that over in an SBAR report lets the next clinician judge the number instead of inheriting it.

How to use the worksheet

The worksheet at the top of this page follows the same five steps. Enter body surface area, or let the form derive it from height and weight using the DuBois formula.

Then record hemoglobin, the VO2 value and its source, and the four blood gas readings. The form works through oxygen content, applies the arteriovenous difference and the ×10 conversion, and returns output and index. Print it or file it with the rest of the patient’s record.

How Pabau keeps hemodynamic data in one record

In most practices this data ends up in three places. The blood gas printout stays in the lab, the arithmetic sits on paper, and a summary is typed into the record hours later. By then nobody can say whether VO2 was measured or estimated.

Practice management software like Pabau keeps all of it in one client record. Digital intake forms capture hemoglobin, both saturations, and the VO2 source at the point of care. The completed form files itself against the patient, so the figures and the method travel together.

From there, every study sits on one timeline. Structured client records let a cardiology team trend cardiac index across repeat catheterizations, rather than hunting through scanned printouts for last year’s figure.

Pabau Scribe, drafts the procedure note while the numbers are still fresh. Hospital teams and private practices end up with the same auditable record, so nobody retypes a saturation into a second system.

Keep every hemodynamic result in one record

Pabau’s digital forms and structured client records capture VO2, saturations, and the calculated cardiac index at the point of care. Every study stays with the patient, so trends are easy to review later.

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Conclusion

The Fick calculation earns its place when thermodilution cannot be trusted. It stays reliable only when the units and the VO2 source are written down with the result.

Decide before the case which VO2 you will use, then record that decision. A measured value and an estimated one carry very different confidence, even when they produce the same figure. Treat the estimate as a range rather than a reading.

Use the worksheet to keep the inputs, the arithmetic, and the date in one place. Book a demo to see how Pabau files hemodynamic assessments alongside the rest of the patient record.

Continue your research

Nursing a patient with falling output? Decreased cardiac output care plan sets out the interventions and monitoring that follow a low result.

Need the matching pump function figure? Normal ejection fraction by age gives the reference ranges you read alongside cardiac index.

Assessing risk before surgery? Revised cardiac risk index guide walks through scoring perioperative cardiac risk step by step.

Spotting deterioration on the ward? Abnormal vital signs chart lists the thresholds that should trigger escalation before a catheter is considered.

Tracking pressures between visits? Blood pressure monitoring covers how to collect readings that are worth acting on.

Frequently asked questions