Examqo › Examqo library › Anesthesiology › Critical care and physiology · IMLE
Respiratory physiology in anaesthesia and hypoxaemia
Hypoxaemia has five causes, and two tools separate them: the A-a gradient (raised or normal) and the response to oxygen — shunt is the only cause that hardly corrects with oxygen.
Why it matters in the exam
The exam asks about calculating the A-a gradient and the alveolar gas equation, the difference between shunt and dead space, what happens to the FRC under anaesthesia and when supine, and the factors that shift the oxygen-haemoglobin curve. It also asks what inhibits hypoxic pulmonary vasoconstriction (HPV) — halogenated agents and vasodilators — and why that matters in one-lung ventilation.
Under general anaesthesia the FRC falls by about 20%, and areas of atelectasis form in the dependent lung within the first minutes, especially with 100% oxygen (absorption atelectasis). Closing capacity rises with age, and when supine it exceeds the FRC at around age 44 — so older patients oxygenate less well. Blood oxygen content depends almost entirely on haemoglobin and saturation; dissolved oxygen is a very small part.
In the recovery room the common causes of desaturation are hypoventilation from opioids or residual paralysis, atelectasis, upper airway obstruction and pain that limits deep breathing. So the assessment includes alertness, neuromuscular monitoring and an examination of breathing, not only extra oxygen. In exam questions a short calculation of the A-a gradient usually separates simple hypoventilation from a problem in the lung itself, and leads to the right answer. Understanding the FRC as an "oxygen store" also explains why preoxygenation matters so much: filling the store with oxygen instead of nitrogen lengthens the safe time during apnoea, and a small store in obesity and pregnancy empties faster.
Key points
- Five causes of hypoxaemia; shunt hardly responds to oxygen.
- Normal A-a gradient = low FiO2 or hypoventilation.
- Under anaesthesia the FRC falls by about 20% and atelectasis forms.
- Right shift: heat, acidosis, CO2, 2,3-DPG; left shift: cold, alkalosis, CO, fetal haemoglobin.
- Halogenated agents and vasodilators inhibit HPV.
Summary table
| Mechanism | A-a gradient | Response to oxygen | Example |
|---|---|---|---|
| Low FiO2 | Normal | Good | High altitude, gas mixture failure |
| Hypoventilation | Normal | Good (high PaCO2) | Opioids, residual paralysis |
| V/Q mismatch | Raised | Good | Partial atelectasis, COPD, embolism |
| Shunt | Raised | Poor when the shunt is large | ARDS, complete atelectasis, endobronchial intubation |
| Diffusion impairment | Raised | Good | Interstitial lung disease, exercise |
In the full version for subscribers
- A simple algorithm
- Memory aid
- Exam traps
- 5 review cards
- Enough for IMLE
The full version is open to subscribers of: Anesthesiology, Internal medicine, Emergency medicine, Medical licensing exam (IMLE).
Subscriber? Read the full topic Plans and prices 25 free questions
Sources
- Miller's Anesthesia, 9th edition (2020) — Respiratory Physiology and Pathophysiology
- West's Respiratory Physiology: The Essentials, 11th edition (2021) — Gas Exchange; Ventilation-Perfusion Relationships
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.