Fick Calculation Calculator for Cardiac Output

A professional tool for clinicians and researchers to determine cardiac output using the Fick principle.

Fick Cardiac Output Calculator

Formula: CO = VO₂ / ( (CaO₂ - CvO₂) * 10 )

What is the Fick Calculation?

The Fick calculation, based on the Fick principle developed by Adolf Eugen Fick in 1870, is a fundamental method in cardiovascular physiology for measuring cardiac output (CO). It is considered a gold-standard measurement because it directly relates the body's oxygen consumption to blood flow. The principle states that the total uptake of oxygen by the body's tissues is equal to the blood flow (cardiac output) multiplied by the difference in oxygen content between arterial and mixed venous blood. A precise fick calculation is crucial for managing critically ill patients.

This method is most commonly used by intensivists, cardiologists, and anesthesiologists to get a precise measure of a patient's hemodynamic status. Common misconceptions are that the fick calculation is purely theoretical; in reality, it is a highly practical, albeit invasive, procedure performed in intensive care units (ICUs) and cardiac catheterization labs.

Fick Calculation Formula and Mathematical Explanation

The core of the fick calculation is its elegant formula, which ties together metabolic rate and circulatory function. The step-by-step derivation is based on the conservation of mass for oxygen in the body.

1. Oxygen Delivery: The amount of oxygen delivered to the tissues is the cardiac output (CO) multiplied by the arterial oxygen content (CaO₂).
2. Oxygen Return: The amount of oxygen returning from the tissues is the cardiac output (CO) multiplied by the mixed venous oxygen content (CvO₂).
3. Oxygen Consumption (VO₂): The difference between delivered and returning oxygen is the amount consumed by the tissues (VO₂).

This relationship is expressed as: VO₂ = CO × (CaO₂ - CvO₂). To find the cardiac output, we rearrange the formula:

CO = VO₂ / (CaO₂ - CvO₂)

A conversion factor is often needed to align units. If CaO₂ and CvO₂ are in mL/dL, a factor of 10 is used to convert them to mL/L, matching the units for cardiac output in L/min. The full fick calculation becomes: CO (L/min) = VO₂ (mL/min) / [ (CaO₂ - CvO₂) (mL/dL) × 10 ].

Variables Table

Variable	Meaning	Unit	Typical Range (Resting)
CO	Cardiac Output	L/min	4.0 - 8.0
VO₂	Oxygen Consumption	mL/min	200 - 300
CaO₂	Arterial Oxygen Content	mL/dL	18 - 21
CvO₂	Mixed Venous Oxygen Content	mL/dL	13 - 16

Table explaining the variables involved in the Fick calculation for cardiac output.

Practical Examples (Real-World Use Cases)

Example 1: Healthy Individual at Rest

Consider a healthy 70 kg adult resting quietly. Their metabolic rate is stable. A successful fick calculation requires accurate measurements.

• Inputs:
  • Oxygen Consumption (VO₂): 250 mL/min
  • Arterial O₂ Content (CaO₂): 20 mL/dL
  • Mixed Venous O₂ Content (CvO₂): 15 mL/dL
• Calculation:
  • A-vO₂ Difference = 20 - 15 = 5 mL/dL
  • CO = 250 / (5 * 10) = 5.0 L/min
• Interpretation: A cardiac output of 5.0 L/min is well within the normal range, indicating healthy heart function and adequate oxygen delivery for a person at rest. This is a classic example of a normal fick calculation.

Example 2: Patient with Septic Shock

Now, consider a patient in the ICU with septic shock, a condition that can dramatically alter metabolic demands and cardiac function. For more on this, see our guide to hemodynamic monitoring.

• Inputs:
  • Oxygen Consumption (VO₂): 350 mL/min (increased due to fever/stress)
  • Arterial O₂ Content (CaO₂): 19 mL/dL
  • Mixed Venous O₂ Content (CvO₂): 12 mL/dL (tissues are extracting more oxygen)
• Calculation:
  • A-vO₂ Difference = 19 - 12 = 7 mL/dL
  • CO = 350 / (7 * 10) = 5.0 L/min
• Interpretation: Although the cardiac output is numerically "normal",