Examqo › Examqo library › Anesthesiology › Special populations · IMLE
Obstetric anaesthesia
Pregnancy reduces the oxygen reserve, raises the risk of aspiration and of a difficult airway and compresses the vena cava — so spinal anaesthesia is preferred for caesarean section and general anaesthesia is kept for emergencies.
Why it matters in the exam
The exam asks about the physiological changes of pregnancy and their meaning for anaesthesia, why regional anaesthesia is preferred for caesarean section, the treatment of hypotension after spinal anaesthesia (phenylephrine), the platelet count that allows a block in pre-eclampsia, and the drugs for postpartum haemorrhage and their contraindications. Other questions deal with labour epidural — does it raise the caesarean section rate (no).
In pregnancy the FRC falls by about 20% and oxygen consumption rises, so desaturation during apnoea is fast; the airway mucosa is oedematous, and the lower oesophageal sphincter is weak. From about week 20 the uterus compresses the inferior vena cava when supine — so the uterus is displaced to the left. MAC falls and sensitivity to local anaesthetics rises. For general anaesthesia at caesarean section an RSI is done with a smaller tube and preparation for a difficult airway.
Remember that obstetric anaesthesia has two patients: every decision is judged also by its effect on the fetus — uterine blood flow depends on maternal blood pressure, so hypotension is treated quickly. A labour epidural gives good analgesia and allows quick extension for an urgent caesarean section, so it is recommended to offer it early to women at risk of surgery or of a difficult airway. In exam questions, caesarean section under general anaesthesia is the right answer mainly when there is no time or when a block is contraindicated.
Key points
- Pregnancy = low FRC, high oxygen consumption, risk of aspiration and a difficult airway.
- From week 20 — left uterine displacement.
- Caesarean section: spinal preferred; general with RSI for emergencies.
- Phenylephrine for hypotension after spinal.
- Platelets of 70,000 or more allow a block in most cases.
Summary table
| Issue | What is done | Exam point |
|---|---|---|
| Hypotension after spinal | Phenylephrine as first line (prophylactic infusion), with fluids | Ephedrine causes more fetal acidosis |
| Aortocaval compression | Left uterine displacement | From about week 20 |
| Pre-eclampsia | A neuraxial block is allowed with platelets of 70,000 or more and no other risk factor | Magnesium prolongs muscle relaxants |
| Postpartum haemorrhage | Oxytocin slowly; carboprost; methylergometrine; tranexamic acid 1 g | Carboprost forbidden in asthma; methylergometrine forbidden in hypertension |
| Aspiration prophylaxis | Sodium citrate, H2 blocker, metoclopramide | In general anaesthesia — RSI |
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, Obstetrics and gynecology, Medical licensing exam (IMLE).
Subscriber? Read the full topic Plans and prices 25 free questions
Sources
- Obstetric Anaesthetists' Association / Difficult Airway Society guidelines for difficult and failed tracheal intubation in obstetrics, 2015 — Algorithms
- International consensus statement on the management of hypotension with vasopressors during caesarean section under spinal anaesthesia, 2018 — Phenylephrine as first line
- SOAP Interdisciplinary Consensus Statement on Neuraxial Procedures in Obstetric Patients With Thrombocytopenia, 2021 — Platelet thresholds
- Miller's Anesthesia, 9th edition (2020) — Obstetric Anesthesia
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.