The Fick method provides a highly regarded and accurate way to determine cardiac output, especially in clinical and research settings. This page offers a user-friendly Fick cardiac output calculator, alongside a detailed explanation of its principles, formula, and applications. Understanding the Fick method cardiac output principle is essential for anyone involved in monitoring cardiac performance accurately, particularly when needing to calculate cardiac output from oxygen consumption.

Fick Method Cardiac Output Calculator

Results:

Cardiac Output (CO): – L/min

Cardiac Index (CI): – L/min/m²

Fick Principle Formula:

CO = VO₂ / (CaO₂ – CvO₂) × 10

Where:

  • CO = Cardiac Output (L/min)
  • VO₂ = Oxygen Consumption (mL/min)
  • CaO₂ = Arterial Oxygen Content (mL/dL)
  • CvO₂ = Venous Oxygen Content (mL/dL)

Oxygen Content Formula (if not provided directly):

CaO₂ or CvO₂ = (1.34 × Hb × SaO₂ or SvO₂) + (0.003 × PaO₂)


Unit Conversion Note: If CaO2 and CvO2 are expressed in mL O2/dL, the denominator (CaO2 – CvO2) needs to be multiplied by 10 to convert it to mL O2/L before dividing VO2 (in mL O2/min) to get CO in L/min. Our calculator handles this conversion automatically. For more on formulas, see our cardiac output formulas guide.

Understanding the Fick Principle

The Fick principle, developed by Adolf Fick in 1870, states that blood flow to an organ (or the entire body) can be calculated if one knows the rate at which a marker substance is taken up by the organ, and the difference in concentration of that marker substance in the arterial blood supplying the organ and the venous blood leaving it. For systemic cardiac output, oxygen is the most commonly used marker substance.

Essentially, the total uptake of (or release of) a substance by an organ is the product of the blood flow to the organ and the arteriovenous concentration difference of the substance. If you need a refresher on what cardiac output itself is, please see “What is Cardiac Output?”.

How to Obtain Values for the Fick Calculation

Step-by-Step Measurement and Calculation Example:

  1. Determine Oxygen Consumption (VO2):
    • Direct Measurement (Respirometry): Using a metabolic cart to measure inspired and expired oxygen concentrations and minute ventilation. This is the most accurate.
    • Estimation:
      • Assumed VO2: ~125 mL O2/min/m2 of Body Surface Area (BSA). BSA can be calculated from height and weight.
      • Weight-based: ~3.0-3.5 mL O2/min/kg of body weight.
  2. Determine Arterial Oxygen Content (CaO2):

    CaO2 = (Hemoglobin [g/dL] × 1.34 mL O2/g Hb × SaO2) + (PaO2 [mmHg] × 0.0031 mL O2/dL/mmHg)


    Requires an arterial blood gas (ABG) sample for SaO2 (arterial oxygen saturation) and PaO2 (partial pressure of arterial oxygen), and a complete blood count (CBC) for hemoglobin.


  3. Determine Mixed Venous Oxygen Content (CvO2):

    CvO2 = (Hemoglobin [g/dL] × 1.34 mL O2/g Hb × SvO2) + (PvO2 [mmHg] × 0.0031 mL O2/dL/mmHg)


    Requires a blood sample from the pulmonary artery (via a Swan-Ganz catheter) for SvO2 (mixed venous oxygen saturation) and PvO2 (partial pressure of mixed venous oxygen).


  4. Input Values into the Calculator: Use the measured or estimated values in the calculator above.

Example: VO2 = 250 mL/min, CaO2 = 20 mL O2/dL, CvO2 = 15 mL O2/dL.
CO = 250 / ((20 – 15) × 10) = 250 / 50 = 5 L/min.

Clinical Applications and Significance

The Fick method is invaluable in specific clinical contexts:

  • Critical Care/ICU: For patients with severe heart failure, shock, or complex hemodynamic instability where precise CO is needed to guide therapy. Understanding the clinical significance of cardiac output is paramount here.
  • Cardiac Catheterization Labs: Used during diagnostic procedures, especially for assessing valvular heart disease or shunts.
  • Research: Often used as a reference method to validate newer, less invasive CO monitoring technologies like Doppler echo. Our Doppler vs Fick comparison discusses this.
  • High-Risk Surgical Patients: Perioperative monitoring in select cases.

Limitations and Considerations

  • Invasiveness: Requires arterial line and pulmonary artery catheterization for blood sampling, carrying risks.
  • Accuracy of VO2: Direct VO2 measurement is complex and not always available. Estimated VO2 can introduce significant error.
  • Steady State: Assumes a stable metabolic and circulatory state during the measurement period. Rapid changes in patient condition can affect accuracy.
  • Technical Skill: Requires careful blood sampling and analysis.
  • Lung Disease: In patients with significant V/Q mismatch or shunts, VO2 measurement via respired gases might not accurately reflect tissue oxygen uptake.

Despite these, its strong physiological basis makes the Fick method a cornerstone of cardiac output measurement theory. For further reading on oxygen transport and Fick’s principle, consult physiology textbooks or resources like The American Physiological Society.

For alternative or less invasive methods, explore our main calculators hub. You might also be interested in the Cardiac Index Calculator to normalize CO for body size.