Obstetric Anaesthesia
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Direct answer
Obstetric anaesthesia manages two patients with one intervention: from about 20 weeks, the gravid uterus compresses the inferior vena cava in supine position (aortocaval compression), so every anaesthetised parturient lies with 15-degree left tilt or a wedge. Caesarean section under spinal anaesthesia uses 12.5-15 mg of 0.5% hyperbaric bupivacaine aiming for a T4 block, with phenylephrine 50-100 microgram boluses plus a 500 mL crystalloid coload as first-line hypotension management; labour analgesia runs on dilute epidural top-ups (0.0625-0.125% bupivacaine with fentanyl 2 microgram/mL) or 50% nitrous oxide (Entonox). General anaesthesia in pregnancy is a rapid-sequence induction by default because progesterone-relaxed sphincters and delayed emptying make aspiration the classic killer, and failed intubation is several-fold commoner in the obstetric airway. Pre-eclampsia modifies everything — platelet count above about 75-100 × 10⁹/L is the usual safety floor for neuraxial blocks, magnesium toxicity is reversed with 1 g calcium gluconate — and postpartum haemorrhage follows the uterotonic ladder: oxytocin, ergometrine, carboprost, misoprostol.
What you must remember
- Aortocaval compression: from 20 weeks, supine position occludes the vena cava, reducing venous return and placental perfusion; left tilt 15 degrees or a right hip wedge is mandatory from induction until delivery.
- Spinal for caesarean: hyperbaric bupivacaine 0.5%, 12.5-15 mg (2.5-3 mL) with fentanyl 15-25 microgram and morphine 100-150 microgram for post-operative analgesia, targeting a T4 sensory level within 5-10 minutes.
- Hypotension protocol: crystalloid coload 500 mL plus phenylephrine 50-100 microgram boluses or 0.25-0.5 microgram/kg/min infusion (international consensus first line); ephedrine 5-10 mg is second line because repeated doses fetal-acidify the baby.
- Labour epidural: low-concentration mixtures such as 0.0625-0.125% bupivacaine with 2 microgram/mL fentanyl in 10-15 mL boluses maintain analgesia with motor sparing; Entonox (50% nitrous oxide in oxygen) and intramuscular pethidine 50-100 mg are the non-epidural options.
- Pre-eclampsia rules: platelets above roughly 75-100 × 10⁹/L and normal coagulation for neuraxial technique; magnesium sulphate toxicity (loss of reflexes, respiratory depression) reversed with 10% calcium gluconate 10 mL (1 g) intravenously; blood pressure control with labetalol or hydralazine.
- PPH drug ladder: oxytocin 5 units slow intravenous bolus then 20-40 units infused over hours in a litre of crystalloid; ergometrine 0.2-0.5 mg (contraindicated in hypertension); carboprost 250 microgram intramuscularly every 15 minutes to a maximum of 8 doses; misoprostol 800 microgram sublingual.
- Post-dural-puncture headache: accidental wet tap with a Tuohy needle during epidural is treated conservatively for 48 hours, then with an epidural blood patch 15-20 mL.
A category-one caesarean at full speed
A category-one caesarean shows the discipline at full speed. A stable parturient with fetal bradycardia reaches theatre with time for a spinal: the technician opens the spinal tray while a 500 mL coload runs, 15 mg of heavy bupivacaine with 25 microgram fentanyl is drawn, the block is established to T4 within eight minutes, a wedge sits under the right hip, and phenylephrine is drawn up before the injection because the pressure will fall within minutes — nausea is often its first signal. Baby delivered, oxytocin 5 units goes in slowly and an infusion of 20-40 units per litre follows, since the atonic uterus, not the anaesthetic, is the next threat. Had she arrived bleeding from a placental abruption, the same urgency would flip to a rapid-sequence general anaesthetic with a reduced propofol dose, O negative blood until the crossmatch returns, and the oxytocic ladder and rapid infuser prepared — the two-patient logic serving mother and fetus with every tilt, vasopressor and unit.
How the exam frames it
Obstetric questions in anaesthesia-technology papers are dosing and safety-threshold questions wearing clinical clothes. The spinal dose (12.5-15 mg hyperbaric bupivacaine, T4 level) and the phenylephrine-first consensus are near-universal; the ephedrine demotion must be explained mechanistically (placental transfer, fetal acidosis) to earn full marks. The pre-eclampsia platelet threshold is asked as a number with a hedge — most units use 75-100 × 10⁹/L with a trend and clinical picture — and the magnesium-calcium gluconate pairing is a reliable MCQ. Indian examiners add two specifics: the classification of caesarean urgency (category 1 to 4, with category 1 meaning immediate threat to life, decision-to-delivery within about 30 minutes) and the PPH uterotonic ladder in exact order with ergometrine's hypertension contraindication spelled out — the order oxytocin-ergometrine-carboprost-misoprostol is the answer script.
Frequently asked questions
What spinal dose is used for caesarean section?
Hyperbaric bupivacaine 0.5%, 12.5-15 mg, usually with fentanyl 15-25 microgram, aiming for a T4 sensory level; onset is complete within 5-10 minutes.
Why is phenylephrine preferred over ephedrine in obstetric spinal hypotension?
Phenylephrine restores maternal blood pressure without crossing to cause fetal acidosis, whereas ephedrine crosses the placenta and repeated doses reduce fetal pH — the basis of the international consensus.
What platelet count is considered safe for neuraxial block in pre-eclampsia?
Commonly a count above 75-100 × 10⁹/L with no other coagulopathy; a falling trend, HELLP syndrome or clinical bleeding prompt full coagulation assessment before spinal or epidural.
Which drug reverses magnesium sulphate toxicity?
Ten percent calcium gluconate 10 mL (1 g) intravenously, for the respiratory depression, loss of reflexes and cardiac depression of supra-therapeutic magnesium; the first step is stopping the infusion.
What is the uterotonic drug ladder for postpartum haemorrhage?
Oxytocin 5 units slowly then infusion, ergometrine 0.2-0.5 mg (avoided in hypertension), carboprost 250 microgram intramuscularly every 15 minutes up to 8 doses, and misoprostol 800 microgram sublingually, alongside uterine massage and escalation to balloon tamponade or surgery.