Key Facts

Anaesthesia in pregnancy presents unique challenges due to the physiological changes of pregnancy, the presence of a second patient (foetus), and the potential for rapid deterioration. Obstetric anaesthesia accounts for approximately 25% of all anaesthetic workload.

Epidemiology

Approximately 700,000 births occur annually in the UK. Caesarean section rate is approximately 30% (elective 13%, emergency 17%). Maternal mortality rate is approximately 9-10 per 100,000 maternities (MBRRACE-UK). Anaesthetic-related deaths have declined significantly but remain a focus of the confidential enquiry.

Aetiology

Physiological changes in pregnancy affecting anaesthesia:

• CVS: CO increased 40%, HR increased 15-20 bpm, SVR decreased, aortocaval compression from 20 weeks
• Respiratory: Minute ventilation increased 50% (progesterone), FRC decreased 20%, O2 consumption increased 20% → rapid desaturation
• GI: Delayed gastric emptying, reduced LOS tone → aspiration risk
• Haematological: Dilutional anaemia (Hb ~110g/L), hypercoagulable, increased fibrinogen
• Pharmacological: Increased sensitivity to inhalational agents (MAC reduced 30-40%), increased sensitivity to LA (reduced epidural dose requirement)

Pathophysiology

The pregnant patient at term has minimal respiratory reserve — the combination of decreased FRC and increased O2 consumption means that desaturation occurs approximately twice as fast as in a non-pregnant patient during apnoea. Aortocaval compression by the gravid uterus can reduce cardiac output by up to 40% in the supine position.

Indications for Obstetric Anaesthesia

• Labour analgesia: Epidural (gold standard), remifentanil PCA, Entonox
• Caesarean section: Spinal (elective/Cat 2-4), epidural top-up (Cat 2-3), GA (Cat 1 if no epidural in situ)
• Instrumental delivery: Epidural top-up or spinal
• Non-obstetric surgery in pregnancy: Any trimester — minimise teratogenic risk

Urgency Classification (Caesarean Section)

• Category 1: Immediate threat — decision-to-delivery interval <30 min (aim <15 min)
• Category 2: Maternal or foetal compromise but not immediately life-threatening — <75 min
• Category 3: No compromise but early delivery needed — within hours
• Category 4: Elective — at a time to suit mother and team

Obstetric Emergencies Requiring Anaesthetic Input

• Massive obstetric haemorrhage
• Eclampsia (magnesium sulphate 4g IV loading)
• Maternal cardiac arrest (perimortem caesarean within 5 min)
• Placental abruption, uterine rupture
• Cord prolapse

Red Flags

• Maternal tachycardia >100 bpm or hypotension — suspect haemorrhage or aortocaval compression
• Difficulty with intubation — higher failed intubation rate in pregnancy
• Desaturation during pre-oxygenation — decreased FRC, consider head-up positioning
• Post-partum collapse — consider haemorrhage, amniotic fluid embolism, PE

Bedside

• Airway assessment: Mallampati may worsen during pregnancy (oedema)
• Observations: BP, HR, SpO2 — left lateral position
• CTG: Foetal heart rate monitoring
• Estimated blood loss: Weigh swabs, measure suction volume

Bloods

• FBC: Hb (physiological anaemia of pregnancy; Hb <105g/L in 3rd trimester is abnormal)
• Group and save/crossmatch: For all caesarean sections and high-risk deliveries
• Coagulation: If pre-eclampsia, haemorrhage, or liver disease
• U&Es, LFTs, urate: Pre-eclampsia assessment
• Fibrinogen: <2g/L in obstetric haemorrhage predicts severe PPH

Imaging

• USS: Placental localisation, foetal assessment

Special Tests

• Point-of-care coagulation (ROTEM/TEG): Guide blood product replacement in massive haemorrhage
• Acid-base (cord gases): Post-delivery to assess neonatal condition

Non-pharmacological

• Left lateral tilt (15°): From 20 weeks gestation — mandatory during all procedures
• Pre-oxygenation: Essential — ramped position, 100% O2 for 3 minutes
• Antacid prophylaxis: Ranitidine 150mg PO (night before + morning of CS) + sodium citrate 0.3M 30mL PO immediately pre-operatively
• WHO checklist: Adapted for obstetric theatre

Pharmacological

Spinal for caesarean section:

• Hyperbaric bupivacaine 0.5% 2.2-2.8mL + fentanyl 15mcg + diamorphine 300mcg
• Phenylephrine infusion 25-50mcg/min or boluses to maintain BP

GA for emergency caesarean:

• RSI: Pre-oxygenation → propofol 2-3mg/kg → suxamethonium 1-1.5mg/kg (or rocuronium 1.2mg/kg) → cricoid pressure → intubation
• Avoid excessive volatile concentration before delivery (uterine relaxation)

Labour epidural:

• Bupivacaine 0.1% + fentanyl 2mcg/mL via PCEA
• For Cat 1 CS top-up: Lidocaine 2% 15-20mL or 3% 2-chloroprocaine

Obstetric haemorrhage:

• Uterotonic: Oxytocin 5 IU slow IV → infusion 40 IU in 500mL over 4h
• Ergometrine 500mcg IM (not in hypertension/pre-eclampsia)
• Carboprost 250mcg IM (not in asthma)
• Tranexamic acid 1g IV (WOMAN trial)
• Major haemorrhage protocol: O-neg blood, 1:1:1 ratio

Surgical/Interventional

• Perimortem caesarean section: Within 5 minutes of maternal cardiac arrest — improves both maternal and foetal survival
• Intrauterine balloon tamponade (Bakri balloon): For post-partum haemorrhage
• Interventional radiology: Uterine artery embolisation
• B-Lynch suture: Surgical compression of uterus

Referral Criteria

• High-risk pregnancy — antenatal anaesthetic assessment (obesity, cardiac disease, difficult airway, anticoagulants)
• Category 1 CS — immediate senior anaesthetic and obstetric team activation
• Massive obstetric haemorrhage — major haemorrhage protocol activation

• Maternal mortality (UK): ~9-10 per 100,000 maternities (MBRRACE-UK)
• Anaesthesia-related maternal deaths: Now very rare (<1 per year in UK) — mostly GA-related
• Regional vs GA for CS: Regional anaesthesia associated with significantly lower maternal mortality
• Perimortem CS: Improves maternal survival (autotransfusion from uterus, improved venous return)
• Massive PPH: Accounts for ~10% of direct maternal deaths; multidisciplinary management saves lives

Caesarean Section Categories

Drugs to Avoid in Pregnancy