Obstetric Anaesthesia - A Comprehensive Overview

1. Physiologic Changes of Pregnancy Relevant to Anaesthesia

Airway & Pulmonary Changes

• Capillary engorgement of nasal, oropharyngeal, and laryngeal mucosa occurs throughout pregnancy - worsened by preeclampsia, second-stage pushing, and tocolytic therapy. Always use small ETT (6.0-6.5 mm) and have a difficult airway plan ready.
• Mallampati score increases progressively throughout labor.
• Breast engorgement may impair laryngoscopy - use a short-handled laryngoscope.
• Minute ventilation increases by 50% (tidal volume up 45%, small RR increase) driven by progesterone.
• FRC falls by 30% (RV down 25%, ERV down 20%) - this dramatically reduces the apnea-safe window during induction; preoxygenation is mandatory.
• Progesterone causes bronchodilation; FEV1/FVC ratio is unchanged.

Cardiovascular Changes

• Cardiac output rises 40-50% by term (increased HR + stroke volume).
• Systemic vascular resistance falls (progesterone-mediated vasodilation).
• Aortocaval compression by the gravid uterus when supine: causes decreased venous return, hypotension, and fetal compromise. Always use left lateral tilt (>15° wedge under right hip) after 20 weeks.
• Blood pressure normally falls in mid-pregnancy and returns to baseline at term.

Gastrointestinal Changes

• Lower oesophageal sphincter tone reduced by progesterone.
• Gastric emptying delayed; intragastric pressure elevated by gravid uterus.
• All obstetric patients are treated as having a full stomach regardless of time of last oral intake.
• Aspiration prophylaxis: sodium citrate 0.3M (15-30 mL orally), ranitidine/omeprazole, metoclopramide 10 mg IV.

Haematologic Changes

• Hypercoagulable state: fibrinogen doubles, factors VII, VIII, X, XII elevated.
• Platelets may fall slightly. Minimum platelet count for neuraxial: typically >70,000-80,000/μL.
• Pseudocholinesterase activity decreases by ~25% - prolonged succinylcholine effect.

Neurological Changes

• MAC for inhalational agents reduced by 30-40% (progesterone and endorphin effects).
• Epidural and spinal drug requirements are reduced (engorged epidural veins reduce epidural space volume; increased sensitivity of nerve roots).

2. Placental Transfer of Drugs

• The driving force for placental drug transfer is the concentration gradient of free (unbound) drug between maternal and fetal blood.
• Lipid solubility, low molecular weight (<500 Da), low protein binding, and un-ionized form all favor placental transfer.
• Fetal blood is more acidic (pH ~7.35) than maternal blood - ion trapping of weak bases (e.g., local anaesthetics, opioids) can occur in fetal distress (fetal acidosis), increasing fetal drug levels.
• Nearly all parenteral opioids and sedatives readily cross the placenta. Regional techniques are preferred to minimize fetal drug exposure.
• Neuromuscular blockers: highly ionized, large molecules - do NOT cross in clinically significant amounts.

3. Pain Pathways in Labour

• Visceral afferents travel with sympathetic fibers via the uterovaginal plexus → inferior hypogastric plexus → T10-L1 nerve roots.
• Perineal pain (second stage) is carried by the pudendal nerve (S2-S4).

4. Labour Analgesia

Non-Pharmacologic Methods

• Psychoprophylaxis (Lamaze), hydrotherapy, TENS, intradermal water injections, acupuncture.

Systemic Medication

• Opioids (IV/IM): pethidine (meperidine), morphine, fentanyl, remifentanil PCA. All cross placenta and can cause neonatal respiratory depression.
• Remifentanil PCA: increasingly popular; ultra-short acting but requires close monitoring.
• Nitrous oxide (50% N2O/O2 - Entonox): self-administered during contractions; moderate analgesia, minimal fetal effect.
• Ketamine (sub-dissociative IV doses 0.25 mg/kg): useful supplement in second stage.

Neuraxial Analgesia (Gold Standard)

• Initiated when patient requests it; does not increase operative delivery rate with dilute LA-opioid mixtures.
• Catheter placed at L2-L3 or L3-L4.
• Test dose: bupivacaine 5 mg or lidocaine 45 mg + epinephrine 15 μg (monitors for intrathecal or intravascular placement).
• Typical maintenance: bupivacaine 0.0625-0.125% + fentanyl 2-3 μg/mL or sufentanil 0.3-0.5 μg/mL.
• PCEA (patient-controlled epidural analgesia) allows parturient control.

• Spinal component: intrathecal fentanyl 25 μg ± bupivacaine 2.5 mg → rapid onset ("walking epidural").
• Epidural catheter: maintained for prolonged analgesia, escalation to surgical block.
• Especially useful for severe early labor pain or when delivery is imminent.

5. Anesthesia for Caesarean Section

Preoperative Preparation

• Aspiration prophylaxis as above.
• Large-bore IV access (at least 16G), baseline BP monitoring.
• Left lateral tilt maintained throughout.
• Surgical level T4 dermatome required (to upper sternum/"nipple line").

Neuraxial Anaesthesia (Preferred)

• Hyperbaric 0.75% bupivacaine 12-13.5 mg (1.6-1.8 mL) - provides 90-120 min surgical anaesthesia.
• Additives: fentanyl 10-20 μg (rapid, short-acting adjuvant) or morphine 0.1-0.15 mg (12-18 hours postoperative analgesia).
• Advantages: simple, fast, reliable, dense block; small drug dose.
• Disadvantages: fixed duration, cannot be extended, higher rate of hypotension.

• Slower onset, larger drug volumes; can be titrated and extended.
• 2% lidocaine + epinephrine (1:200,000) OR 3% 2-chloroprocaine - typically 15-25 mL in divided doses over 5-10 min.
• Useful when pre-existing labor epidural can be converted: add bicarbonate to alkalinize solution, add fentanyl, increase concentration.
• Test dose essential; incremental dosing in 5 mL aliquots.

• Spinal provides rapid dense block; epidural allows extension if surgery prolonged.
• Especially useful for complex or anticipated long surgeries.

General Anaesthesia for CS

1. Pre-oxygenation for minimum 3 min (or 4 vital capacity breaths at 100% O2).
2. Rapid sequence induction (RSI) with cricoid pressure.
3. Induction: propofol 2 mg/kg or thiopental 4-5 mg/kg (if available).
4. Succinylcholine 1.5 mg/kg (remember reduced pseudocholinesterase; if contraindicated, rocuronium 1.2 mg/kg with sugammadex reversal available).
5. Intubation with cuffed ETT (6.0-6.5 mm).
6. Maintain with low-dose volatile agent (0.5-1 MAC) + N2O/O2 (50:50) until delivery; avoid >1 MAC to prevent uterine atony.
7. After delivery: increase analgesia (opioids), IV oxytocin.
8. Extubate awake (reversed, following commands) in left lateral position.

6. Anesthetic Complications

Hypotension (Most Common)

• Occurs in up to 80% of spinal anaesthesia for CS without prophylaxis.
• Mechanism: sympathetic blockade → vasodilation + venodilation → decreased preload and afterload.
• Management: left uterine displacement, IV fluid co-loading (1-1.5 L crystalloid or colloid), phenylephrine infusion (preferred over ephedrine as it maintains uteroplacental blood flow better); phenylephrine IV bolus 40-120 μg. Ephedrine if maternal bradycardia present.

Pulmonary Aspiration (Mendelson Syndrome)

• Risk exacerbated by delayed gastric emptying and reduced LOS tone.
• pH <2.5 and volume >25 mL is high risk.
• Prevention: aspiration prophylaxis + RSI + cricoid pressure.
• Treatment: supportive; bronchodilators, CPAP/intubation if needed.

Total Spinal Anaesthesia

• Intrathecal injection of epidural dose → rapid onset of very high block.
• Features: sudden hypotension, bradycardia, apnoea, unconsciousness.
• Management: immediate intubation + IPPV, vasopressors (ephedrine/epinephrine), atropine, CPR if needed.

Local Anaesthetic Systemic Toxicity (LAST)

• From intravascular injection of epidural agent (bupivacaine most cardiotoxic).
• Features: early CNS (tinnitus, perioral numbness, seizures) → cardiac arrhythmias, arrest.
• Management: stop LA injection, call for help, seizure control (midazolam/thiopental), lipid emulsion therapy (20% intralipid 1.5 mL/kg IV bolus then 0.25 mL/kg/min), CPR if needed.

Post-Dural Puncture Headache (PDPH)

• From accidental dural puncture with epidural needle (large bore, 17-18G → incidence ~70-80% if untreated).
• Characteristics: postural, bilateral frontal/occipital, worse sitting/standing, better lying flat; may have neck stiffness, photophobia, diplopia (CN VI palsy).
• Incidence with spinal needles: 1-2% with pencil-point (Whitacre/Sprotte) needles; higher with cutting-tip (Quincke).
• Management:
  • Conservative (24-48h): bed rest, hydration, caffeine (300-500 mg), paracetamol, NSAIDs.
  • Definitive: Epidural blood patch (EBP) - 15-20 mL autologous blood into epidural space; >90% success rate.

High/Total Neuraxial Block

• Excessive spread of spinal block → C3-C5 involvement → diaphragmatic paralysis.
• Management: immediate intubation + ventilation, vasopressors.

Maternal Mortality

• Leading causes (US CDC 2017): cardiovascular disease (14%), sepsis (13%), cardiomyopathy (12%), haemorrhage (11%), embolism (10%), stroke (8%), hypertensive disorders (7%).
• Anaesthesia-related deaths: only 0.4% of maternal deaths - markedly reduced by shift to neuraxial anaesthesia.

7. High-Risk Conditions

Preeclampsia / Eclampsia

• Avoid aortocaval compression; invasive BP monitoring for severe features.
• Neuraxial anaesthesia is preferred if platelet count allows (check platelets before every block; typically avoid if <70,000-80,000).
• Airway edema is often severe - use smaller ETT.
• Severe hypertension: treat with labetalol, hydralazine, or nicardipine before induction.
• Magnesium sulfate (anticonvulsant): potentiates neuromuscular blockers - reduce dose of NMBDs and monitor carefully.

Obstetric Haemorrhage

• Haemorrhage is the leading cause of maternal mortality worldwide.
• Causes: uterine atony (most common), retained placenta, obstetric lacerations, placenta previa/accreta, uterine rupture.
• Management: large-bore IV access, blood products, oxytocin, ergometrine, carboprost, tranexamic acid; early recourse to general anaesthesia if haemodynamically unstable.
• Neuraxial anaesthesia is generally contraindicated in haemodynamically unstable patients.

Cardiac Disease in Pregnancy

• Cardiovascular conditions now account for >25% of pregnancy-related deaths in the US.
• Decisions regarding neuraxial vs. GA depend on specific lesion:
  • Stenotic lesions (mitral stenosis, aortic stenosis): poorly tolerate tachycardia and decreased SVR.
  • Regurgitant lesions: generally benefit from the decreased SVR of neuraxial block.
  • High-risk lesions (Eisenmenger's, severe pulmonary hypertension, Marfan's): require multidisciplinary team and invasive monitoring.

Obesity

• Higher incidence of gestational HTN, preeclampsia, gestational DM, difficult airway, OSA.
• Neuraxial placement technically more difficult; consider early insertion.
• Accelerated desaturation at induction; careful positioning for RSI.

Diabetes Mellitus

• Gastroparesis may worsen aspiration risk.
• Glucose monitoring throughout labor.
• Insulin requirements change dramatically after delivery.

8. Nonobstetric Surgery During Pregnancy

• Incidence: ~1-2% of pregnancies require surgery.
• Highest risk: 1st trimester (organogenesis), 3rd trimester (preterm labour risk).
• No specific anaesthetic agent is definitively contraindicated or proven safe in all trimesters.
• No anesthetic technique is preferred over another as long as: oxygenation and BP are maintained, hyperventilation is avoided (respiratory alkalosis reduces uteroplacental blood flow), and left uterine displacement is used after 20 weeks.
• Avoid N2O in first trimester (inhibits methionine synthase).
• Fetal monitoring during/after surgery in viable gestations.

9. Neonatal Resuscitation (Apgar Score)

• Assessed at 1 and 5 minutes.
• Score 7-10: normal; 4-6: moderate depression; 0-3: severe depression.
• Intrauterine asphyxia during labour is the most common cause of neonatal depression seen in the delivery room.
• Initial resuscitation: warmth, tactile stimulation, airway positioning, suction if needed; oxygen; PPV with bag-mask; chest compressions + epinephrine if HR <60 bpm.

10. Key Clinical Pearls (Summary)

1. All obstetric patients = full stomach regardless of fasting time; aspiration prophylaxis mandatory.
2. Left lateral tilt after 20 weeks to prevent aortocaval compression.
3. FRC is reduced by 30% - preoxygenate thoroughly; desaturation is rapid.
4. MAC is reduced 30-40% in pregnancy.
5. Neuraxial drug requirements are reduced due to engorged epidural veins and increased nerve root sensitivity.
6. Spinal (hyperbaric bupivacaine 0.75%) is first choice for elective CS.
7. Phenylephrine infusion is preferred vasopressor for spinal hypotension (preserves uteroplacental flow).
8. GA requires RSI with cricoid pressure; use cuffed ETT 6.0-6.5 mm.
9. PDPH is treated definitively with epidural blood patch.
10. Bupivacaine is more cardiotoxic than lidocaine; treat LAST with 20% intralipid.
11. Magnesium potentiates NMBDs in preeclamptic patients.
12. Haemorrhage is the leading cause of maternal mortality worldwide; anaesthesia accounts for only 0.4%.