Asthma and Blood Gases: Understanding Oxygen & Carbon Dioxide Levels
During asthma exacerbation, airway obstruction alters gas exchange:
Pathophysiology
- V/Q mismatch (primary): poorly ventilated units → hypoxemia.
- Hypoventilation (severe): fatigue → ↑PaCO₂ (hypercapnia) — ominous sign.
- Early exacerbation: hyperventilation → respiratory alkalosis (low PaCO₂).
- Life-threatening: PaO₂ <60 mmHg, PaCO₂ >42 mmHg (normal), acidosis (pH <7.3).
ABG Patterns
| Severity | PaO₂ | PaCO₂ | pH |
|---|---|---|---|
| Mild | ↓ (60–80) | ↓ (<35) | ↑ (>7.45) alkalosis |
| Moderate | ↓↓ (<60) | Normal (35–45) | Normal |
| Severe | ↓↓↓ | ↑ (>45) | ↓ (<7.35) acidosis |