Fick Method for Calculating Cardiac Output
The Fick method calculates cardiac output by dividing oxygen consumption (V̇O₂) by the arteriovenous oxygen content difference [C(a-v)O₂], expressed as: Cardiac Output = V̇O₂ / [C(a-v)O₂]. 1 This principle states that the amount of oxygen consumed by tissues equals the difference between oxygen delivered in arterial blood and oxygen remaining in mixed venous blood, multiplied by blood flow. 2
Key Components Required
- Oxygen consumption (V̇O₂): Measured directly through respiratory gas analysis or estimated (though direct measurement is more reliable) 2
- Arterial oxygen content (CaO₂): Obtained from arterial blood sampling 1
- Mixed venous oxygen content (CvO₂): Requires pulmonary artery catheterization to sample mixed venous blood 1, 2
- The arteriovenous difference [C(a-v)O₂] represents oxygen extraction by tissues, with maximal extraction approximately 75% of arterial oxygen content in healthy individuals 1
Clinical Considerations
- The direct Fick method (measuring actual oxygen consumption) is the gold standard but requires invasive monitoring and is technically demanding 2, 3
- The indirect Fick method uses estimated oxygen consumption values and is more widely available but less reliable than direct measurement 2
- This method is particularly recommended for patients with severe tricuspid regurgitation where thermodilution underestimates cardiac output 2
- Caution is warranted in patients with abnormal oxygen extraction or when the assumption of linear relationship between V̇O₂ and cardiac output may not hold (e.g., cardiovascular disease) 1