Gas Collection Systems for Calculating Cardiac Output via Fick Principle
The primary gas collection systems that can be used to calculate cardiac output via the Fick principle are CO2 rebreathing methods (using a rubber bag with low or high CO2 concentrations) and foreign gas rebreathing methods (using soluble gases like acetylene or nitrous oxide). 1
CO2 Rebreathing Methods (Indirect Fick)
The indirect Fick method uses CO2 as the indicator gas, with blood concentrations estimated from partial pressures in the gas phase 1. Two specific approaches exist:
Low CO2 Concentration Rebreathing
- System: Rubber bag containing low concentration of CO2
- Mechanism: Patient rebreaths from the bag while the CO2 tension curve gradually increases toward a limit
- Measurement: Mixed venous PCO2 is determined from this equilibration curve
- Formula: Cardiac output = veno-arterial CO2 output ÷ somatic CO2 difference 1
High CO2 Concentration Equilibration Method
- System: Rubber bag containing high concentration of CO2 in oxygen (e.g., 5% CO2 and 95% O2)
- Mechanism: Uses equilibration principle with the lungs acting as a tonometer
- Measurement: End-tidal CO2 represents arterial blood CO2; CO2 output determined from expired air sample 1
Foreign Gas Rebreathing Methods
Foreign gas methods using soluble inspired gases such as acetylene or nitrous oxide have been shown to be reliable and safe for noninvasive cardiac output assessment 1.
Key Characteristics:
- Assumption: Mixed venous content of the foreign gas is zero
- Measurement: Arterial partial pressure assumed equal to end-tidal air
- Traditional technology: Respiratory mass spectrometer for measuring low gas concentrations
- Newer technology: Portable infrared absorption spectrometer facilitates point-of-care monitoring 1
Reproducibility:
The coefficient of variation for rebreathing methods ranges from 7% to 11% in patients with and without heart failure 1
Important Clinical Caveats
Limitations of CO2 Rebreathing:
- Patient cooperation required - may be difficult in some patients
- Side effects: High inhaled CO2 concentrations can cause lightheadedness or feelings of suffocation
- Accuracy concerns: Multiple confounding variables and underlying assumptions can compromise results
- Advanced pulmonary disease: Large errors can occur in this population 1
Advantages of Foreign Gas Methods:
- More reliable in patients where CO2 rebreathing may be problematic
- Newer portable technology makes clinical implementation more feasible
- Does not require assumptions about mixed venous CO2 content 1
Clinical Application Context
While these gas collection systems provide noninvasive cardiac output determination, the American Heart Association notes that additional research is needed before noninvasive cardiac output becomes standard practice in clinical cardiopulmonary exercise testing 1. However, these methods may enhance the prognostic value of peak VO2 measurements, particularly in heart failure patients where two patients could have identical low peak VO2 values but different peak cardiac outputs 1.