Why End-Tidal CO2 is Elevated in Hypoventilation
End-tidal CO2 (ETCO2) rises in hypoventilation because inadequate alveolar ventilation prevents sufficient elimination of CO2 from the lungs, leading to CO2 retention in both alveolar gas and arterial blood. 1
Fundamental Mechanism
Low alveolar minute ventilation is by far the most common cause of hypercapnia. 1 When ventilation is insufficient relative to CO2 production, CO2 accumulates in the alveoli and equilibrates with pulmonary capillary blood due to CO2's high solubility and diffusibility. 1 This results in:
- Elevated alveolar PCO2 (PACO2) that directly reflects inadequate CO2 clearance 1
- Minimal alveolar-arterial gradient for CO2 because CO2 equilibrates rapidly across the alveolar-capillary membrane 1
- Increased arterial PCO2 (PaCO2) that mirrors the elevated ETCO2 1
Clinical Contexts Where This Occurs
Absolute Hypoventilation
Reduced minute ventilation occurs when: 1
- Respiratory center depression from drugs (opioids, sedatives), head injury, or intracerebral hemorrhage 1
- Respiratory muscle weakness from neuromuscular diseases 1
- Major airway obstruction 1
- Chest wall restriction 1
In these conditions, ETCO2 increases progressively as ventilation decreases. 1, 2 ETCO2 values >50 mmHg indicate significant hypoventilation and potential respiratory compromise. 1, 2
Relative Hypoventilation (Ineffective Ventilation)
In COPD and other obstructive diseases, patients may paradoxically have increased total minute ventilation yet still retain CO2. 1 This occurs because:
- Rapid shallow breathing increases dead space-to-tidal volume ratio (VD/VT) 1
- More ventilation is "wasted" on anatomical dead space with each small breath 1
- V/Q mismatch increases physiological dead space 1
- Alveolar ventilation remains inadequate despite apparent hyperventilation 1
The respiratory muscles cannot overcome the mechanical load, resulting in alveolar hypoventilation and CO2 retention even when the patient appears to be breathing vigorously. 1
Important Clinical Pitfalls
Supplemental oxygen masks hypoventilation detection by pulse oximetry alone. 3 Patients receiving oxygen may maintain adequate SpO2 despite progressive hypercapnia and rising ETCO2. 3 This is why:
- ETCO2 monitoring can detect hypoventilation before oxygen desaturation occurs 1, 2
- An absolute change from baseline ETCO2 >10 mmHg signals respiratory depression 1, 2
- Capnography is essential when supplemental oxygen is administered 3
Hypoventilation is defined by elevated PCO2, not by the appearance of respiratory effort. 1 The European Respiratory Journal guidelines emphasize that hypoventilation means a "high PCO2 set-point" regardless of whether minute ventilation appears normal or elevated. 1