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Updated: Aug 9, 2026
07:52
Expired CO2 Measurement in Intubated or Spontaneously Breathing Patients from the Emergency Department
Published on: January 29, 2011
Can alveolar pCO2 exceed pulmonary end-capillary CO2? No
Summary
Alveolar carbon dioxide levels (PCO2) do not exactly match end-capillary blood PCO2 due to incomplete CO2 reactions. However, this difference is negligible in practice, rendering proposed mechanisms for significant disparities unlikely.
Area of Science:
- Physiology
- Respiratory System
- Gas Exchange
Background:
- Carbon dioxide (CO2) exchange and reactions in blood are incomplete during capillary transit.
- Theoretical models suggest alveolar PCO2 may not precisely equal end-capillary PCO2.
Purpose of the Study:
- To evaluate the significance of the difference between alveolar and end-capillary PCO2.
- To assess the plausibility of proposed mechanisms for alveolar PCO2 exceeding end-capillary PCO2 during rebreathing.
Main Methods:
- Analysis of CO2 transport dynamics in pulmonary capillaries.
- Thermodynamic evaluation of proposed ion gradients.
Main Results:
- Technical measurement errors in PCO2 are larger than the physiological difference between alveolar and end-capillary PCO2.
- The large effective solubility of CO2 in blood minimizes this difference.
- Proposed mechanisms involving radial gradients of H+ and HCO3- are thermodynamically unlikely.
Conclusions:
- The difference between alveolar and end-capillary PCO2 can be practically neglected.
- Hypothetical mechanisms requiring significant ion and PCO2 gradients are improbable due to energy constraints.
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