Fick Principle: The gold standard for measuring cardiac output based on oxygen consumption and arteriovenous oxygen difference.
Guidelines & Evidence
Clinical Details
Section 1
When to Use
When to Use
Evaluation of hemodynamics during Right Heart Catheterization (RHC).
Quantification of cardiac output (CO) and cardiac index (CI).
Assessing shock states and guiding inotropic/vasopressor therapy.
Section 2
Formula & Logic
The Fick Principle
The Fick principle states that the uptake of a substance by an organ is the product of the blood flow to the organ and the arteriovenous concentration difference of the substance. In cardiology, this is applied using oxygen consumption (VO2).
Formula
CO = VO2 / [13.4 × Hgb × (SaO2 − SvO2)]
Where VO2 is oxygen consumption (typically estimated as 125 mL/min/m2 or using the LaFarge formula).
Section 3
Pearls/Pitfalls
Estimated vs. Measured VO2
Most labs use "Estimated Fick" where oxygen consumption is assumed based on age and sex. This can be inaccurate in patients with severe obesity or thyroid disease. "Measured Fick" (using a metabolic hood) is the gold standard.
Section 4
Next Steps
Complementary Calculators
Cardiac Output Index
SVR Calculator
PVR Calculator
MAP Calculator
Stroke Volume Index
Section 5
Evidence Appraisal
Foundational Principle
Uber die Messung des Blutquantums in den Herzventrikeln.
Fick A. • Sitzungsberichte der Physikalisch-Medizinischen Gesellschaft zu Würzburg.. 1870;The original description of the Fick principle.
