Indirect Fick Method for Cardiac Output Calculation
Overview of the Method
The indirect Fick method calculates cardiac output using CO₂ as the indicator gas, with the formula: Cardiac output = CO₂ output ÷ (mixed venous CO₂ content - arterial CO₂ content difference). 1
The lungs function as a tonometer in this technique, allowing noninvasive estimation of blood gas concentrations from their partial pressures in the gas phase rather than requiring direct blood sampling. 1
Technical Approaches
CO₂ Rebreathing Techniques
Two primary rebreathing approaches exist for estimating mixed venous PCO₂ during exercise: 1
Low CO₂ concentration method: Patient rebreathes from a rubber bag containing low CO₂ concentration, with mixed venous CO₂ content determined from the CO₂ tension curve as it gradually increases toward a limit during rebreathing of a known gas mixture (e.g., 5% CO₂ and 95% O₂). 1
Equilibration method: Uses a bag containing high CO₂ concentration in oxygen. 1
Arterial CO₂ estimation: End-tidal CO₂ serves as the measure of alveolar gas CO₂ partial pressure and represents arterial blood CO₂. 1
CO₂ output determination: Calculated from expired air samples. 1
Foreign Gas Rebreathing Alternative
Foreign gas methods using soluble inspired gases such as acetylene or nitrous oxide provide a reliable and safe noninvasive alternative for cardiac output assessment during cardiopulmonary exercise testing. 1, 2
With this methodology, mixed venous content of the gas is zero, and arterial partial pressure equals end-tidal air values. 1
Foreign gas rebreathing demonstrates a coefficient of variation of 7-11% in patients with and without heart failure. 2
Critical Limitations and Caveats
Patient-Related Constraints
CO₂ rebreathing methods require substantial patient cooperation, which may be difficult or impossible to achieve in some patients, particularly during exercise. 1, 2
High concentrations of inhaled CO₂ may cause lightheadedness or feelings of suffocation. 1, 2
These symptoms can compromise test completion and data quality. 1
Technical Accuracy Issues
The accuracy of the CO₂ rebreathing technique may be compromised by numerous potential confounding variables and underlying assumptions, with large errors occurring in patients with advanced pulmonary disease. 1, 2
The indirect Fick method (using estimated values) is more widely available but less reliable than direct measurement techniques. 2
Clinical Reliability Concerns
Cardiac output is not routinely measured in clinical exercise laboratories because noninvasive techniques (including CO₂ rebreathing) used to estimate it are technically demanding and the reliability of results is questionable. 1
When to Avoid This Method
Exercise caution in interpreting clinical validity in patients where O₂ extraction is not ideal or when the assumption of linearity between V̇O₂ and cardiac output increase during exercise cannot reasonably be made. 1
Specific populations requiring caution include: 1
- Patients with cardiovascular disease where V̇O₂-cardiac output linearity may not hold
- Patients with lung disease where CaO₂ changes during exercise
- Patients with advanced pulmonary disease where technique accuracy is compromised
Preferred Alternative Methods
For patients with advanced pulmonary disease or those unable to cooperate with CO₂ rebreathing, foreign gas rebreathing using acetylene or nitrous oxide shows better reliability and should be considered as the preferred noninvasive approach. 2