Interpreting Cardiac Output from CO2 Measurements
Cardiac output can be determined from CO2 using the indirect Fick method, which calculates: Cardiac Output = CO2 production ÷ (mixed venous CO2 content - arterial CO2 content), though this approach requires patient cooperation and has significant limitations in patients with advanced pulmonary disease. 1
Understanding the CO2 Rebreathing Method
The CO2 rebreathing technique uses carbon dioxide as the indicator gas instead of oxygen, with the lungs functioning as a tonometer to estimate mixed venous and arterial CO2 concentrations from gas phase measurements. 1
Two primary approaches exist for measuring mixed venous PCO2 during exercise: 1
- Low CO2 concentration rebreathing: Patient rebreathes from a bag containing low CO2 concentration, and mixed venous CO2 content is determined as the CO2 tension gradually increases toward a limit
- High CO2 equilibration method: Uses a bag containing high CO2 concentration in oxygen for equilibration
- End-tidal CO2 measurement: Serves as the surrogate for arterial blood CO2 partial pressure 1
- CO2 output determination: Calculated from expired air sample analysis 1
Clinical Accuracy and Validation
The modified CO2 Fick method shows strong correlation with invasive thermodilution measurements in mechanically ventilated patients (r=0.939, P<0.001), with no significant difference in cardiac output values. 2
Research demonstrates that CO2-based cardiac output measurements correlate well with pulmonary artery catheter methods (mean difference 0.03 ± 1.3 L/min, 95% CI -0.19 to 0.25 L/min), though the largest discrepancies occur in low cardiac output states (<5 L/min). 3
A modified approach using VCO2 and oximetry saturations (Cardiac Output = VCO2/[k(SaO2-SvO2)]) demonstrates excellent correlation with thermodilution (r²=0.96, standard error 0.59 L/min), offering a simpler alternative that avoids difficulties with accurate VO2 measurement. 4
Critical Limitations and When to Avoid
The American Heart Association emphasizes that CO2 rebreathing accuracy is compromised by numerous confounding variables, and large errors occur in patients with advanced pulmonary disease. 1, 5
Patient-Related Limitations:
- Patient cooperation requirement: Technique demands active patient participation, which may be impossible in critically ill, confused, or uncooperative patients 1, 5
- Symptomatic side effects: High inspired CO2 concentrations cause lightheadedness and feelings of suffocation, limiting tolerability 1, 5
- Advanced pulmonary disease: Ventilation-perfusion mismatch fundamentally undermines the assumptions required for accurate CO2-based calculations 1, 5
Technical Considerations:
- Reproducibility: Coefficient of variation ranges from 7-11% in patients with and without heart failure, which is acceptable but not ideal 1
- Not standard practice: Additional research is needed before noninvasive cardiac output measurement becomes routine in clinical cardiopulmonary exercise testing 1
- Equipment demands: While less invasive than pulmonary artery catheterization, the technique remains technically demanding and is not routinely performed in most clinical exercise laboratories 1
Alternative Noninvasive Methods
Foreign gas rebreathing methods using acetylene or nitrous oxide represent a more reliable and safer alternative for noninvasive cardiac output assessment during cardiopulmonary exercise testing. 1, 5
These methods assume zero mixed venous content of the foreign gas and that arterial partial pressure equals end-tidal air measurements, with newer portable infrared absorption spectrometry facilitating point-of-care monitoring. 1
When CO2 Methods Are Most Appropriate
CO2-based cardiac output determination is most suitable for mechanically ventilated patients in steady state without severe pulmonary disease, where continuous noninvasive monitoring is desired and pulmonary artery catheterization is unavailable or contraindicated. 2, 3
The technique may enhance prognostic assessment in heart failure patients when combined with peak VO2 measurements during cardiopulmonary exercise testing, though this remains investigational rather than standard practice. 1