Fick Cardiac Output Calculator

Calculate cardiac output using Fick principle from oxygen consumption and arteriovenous O2 difference.

O2 consumption VO2 (mL O2/min) (mL/min)

Arterial O2 content CaO2 (mL O2/100mL) (mL/dL)

Mixed venous O2 content CvO2 (mL O2/100mL) (mL/dL)

Hemoglobin (g/dL) for O2 content (g/dL)

SaO2 % and SvO2 % (%)

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Why the a–v Difference Widens in Heart Failure

The Fick principle rearranges oxygen consumption into cardiac output. The clinically important consequence is that when output falls, tissues extract more oxygen from each unit of blood to maintain delivery.

CO = VO₂ / [(CaO₂ − CvO₂) × 10]

State	a–v difference	Mixed venous saturation
Normal rest	4–5 mL/dL	70–75%
Heart failure	8–10 mL/dL	< 60%
Exercise	12–15 mL/dL	25–35%
Sepsis	Narrow	Often > 75%

This makes mixed venous oxygen saturation a direct index of the adequacy of cardiac output. A low SvO₂ means tissues are extracting hard because delivery is inadequate.

Sepsis inverts the pattern, which is diagnostically valuable. Microcirculatory shunting and impaired mitochondrial oxygen use mean tissues cannot extract normally, so SvO₂ stays high despite tissue hypoxia. A high SvO₂ in a shocked patient points to distributive rather than cardiogenic shock.

The Factor of 10, and Where VO₂ Comes From

The multiplier converts mL O₂ per dL into mL per L, since oxygen content is reported per decilitre and cardiac output in litres. Omitting it gives an answer ten times too large.

Direct measurement of VO₂ requires expired gas collection, which is cumbersome. Most clinical applications use the indirect Fick, estimating VO₂ from body surface area — typically 125 mL/min/m². That assumption is the method's largest error source, since real VO₂ varies with sedation, fever and agitation.

Method	Advantage	Limitation
Direct Fick	Reference standard	Requires expired gas collection
Indirect Fick	Practical	VO₂ assumed, not measured
Thermodilution	Quick, repeatable	Overestimates in tricuspid regurgitation and low output

Fick is preferred over thermodilution precisely in low output states and significant tricuspid regurgitation, where thermodilution systematically overestimates.

Worked Examples

Example 1: Normal: VO2=250, CaO2=20, CvO2=15

CO=250/(5×10)

Result: CO=5.0 L/min — normal

Normal resting cardiac output

Example 2: Heart failure: VO2=200, CaO2=20, CvO2=12

CO=200/(8×10)

Result: CO=2.5 L/min — severely reduced

Wide a-v difference = high O2 extraction

Example 3: Interpreting a high SvO₂ in shock

Shocked patient, SvO₂ 82%

Result: Suggests sepsis, not cardiogenic shock

Tissues cannot extract oxygen because of microcirculatory shunting and mitochondrial dysfunction. Cardiac output may even be raised.

Example 4: Why indirect Fick has wide error

VO₂ estimated at 125 mL/min/m²

Result: Real value varies substantially

Fever, agitation and sedation all shift oxygen consumption. The assumed value, not the arithmetic, is the dominant error source.

Common Mistakes

⚠️

Omitting the factor of 10

Oxygen content is per decilitre and cardiac output in litres. Without the conversion the answer is ten times too high.

⚠️

Assuming a low SvO₂ always means cardiac failure

Anaemia, hypoxaemia and raised oxygen demand all lower it. The a–v difference reflects the balance of delivery and consumption, not output alone.

⚠️

Missing the inverted pattern in sepsis

Impaired extraction keeps SvO₂ high despite tissue hypoxia. A high value in a shocked patient suggests distributive rather than cardiogenic shock.

⚠️

Trusting thermodilution in low output or tricuspid regurgitation

It systematically overestimates in both. Fick is the preferred method in those situations.

Frequently Asked Questions

Why wide a-v O2 difference in heart failure?

Low CO: tissues extract more O2 from each dL of blood (compensate for less flow). CvO2 drops → wide a-v difference. High CvO2 = poor extraction (cyanide poisoning, sepsis, high flow state).

Fick vs thermodilution?

Fick (gold standard): accurate but requires VO2 measurement (metabolic cart or assumed). Thermodilution (PA catheter): indicator dilution method, less accurate in low CO or tricuspid regurgitation. Both used in cardiac catheterization.

Why does the a–v difference widen in heart failure?

Because tissues extract more oxygen from each unit of blood to compensate for reduced flow. Mixed venous saturation falls accordingly.

Why is SvO₂ high in sepsis?

Microcirculatory shunting and impaired mitochondrial oxygen use prevent normal extraction, so venous blood returns still well oxygenated despite tissue hypoxia.

What is the factor of 10 for?

Converting oxygen content in mL per decilitre to mL per litre, matching the units of cardiac output.

When is Fick preferred over thermodilution?

In low cardiac output states and significant tricuspid regurgitation, where thermodilution systematically overestimates.