Chart — Fundamentals
Five Causes of Hypoxemia
Every cause of hypoxemia falls into one of five mechanisms. Two questions separate them at the bedside: is the A-a gradient normal or elevated, and does the hypoxemia correct with supplemental oxygen? Use this grid to reason from the gas to the mechanism.
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
The Five Mechanisms
| Mechanism | A-a Gradient | Corrects with O₂? | PaCO₂ | Typical Examples |
|---|---|---|---|---|
| V/Q mismatch | Elevated | Yes | Normal or variable | COPD, asthma, pneumonia, atelectasis (the most common cause overall) |
| Shunt | Elevated | No (refractory) | Normal or low | ARDS, lobar pneumonia / consolidation, pulmonary edema, atelectasis, intracardiac shunt |
| Hypoventilation | Normal | Yes | Elevated (high) | Opioid overdose, neuromuscular disease, oversedation, obesity hypoventilation |
| Diffusion limitation | Elevated | Yes | Normal or low | Interstitial lung disease / pulmonary fibrosis, exercise at altitude |
| Low inspired PO₂ (FiO₂) | Normal | Yes | Normal or low | High altitude, a low-FiO₂ gas-source error |
How to Use This Chart
- The two normal-A-a causes (hypoventilation and a low inspired PO₂) are told apart by the PaCO₂ — high in hypoventilation — and the history (altitude or a gas-source problem).
- The “does it correct with oxygen?” test isolates shunt — the one mechanism that resists supplemental oxygen and instead needs recruitment and PEEP.
- Most everyday clinical hypoxemia is V/Q mismatch.
- An elevated A-a gradient with refractory hypoxemia on a high FiO₂ is shunt until proven otherwise.
Related Resources
Sources
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. Gas exchange and respiratory failure chapters.
- West JB, Luks AM. West's Respiratory Physiology: The Essentials. 11th ed. Wolters Kluwer; 2021.