Epidural Anaesthesia
Anaesthesia · Regional · lean revision notes
Epidural Anaesthesia
A central neuraxial block in which local anaesthetic is deposited in the potential epidural space, producing segmental analgesia/anaesthesia by acting on spinal nerve roots and the dorsal root ganglia. It is the workhorse of labour analgesia, abdominal/lower-limb surgery, and multimodal postoperative pain control. The single fact that distinguishes it from spinal anaesthesia is the site of injection — outside the dura (epidural) versus the subarachnoid (CSF) space.
Definition & classification
Epidural anaesthesia involves injection of local anaesthetic into the epidural space, which lies between the ligamentum flavum (posteriorly) and the dura mater (anteriorly). The drug does not mix with CSF; instead it diffuses across the dura and acts on nerve roots, and is partly absorbed systemically.
Types/approaches:
- By level: Cervical, thoracic, lumbar, caudal (caudal = epidural via the sacral hiatus, very common in paediatrics).
- By technique: Single-shot epidural vs continuous epidural (catheter-based) — catheter allows top-ups and continuous infusion, the main advantage over spinal.
- Combined Spinal-Epidural (CSE): "Needle-through-needle" technique — rapid onset of spinal plus the flexibility/duration of an epidural catheter. Popular in obstetrics.
High-yield: Epidural can be given at any spinal level (cervical to caudal) because the cord is not entered; spinal anaesthesia is restricted to below L2 (cord ends at L1–L2 in adults) to avoid cord injury.
Anatomy of the epidural space
The epidural (extradural/peridural) space is a true potential space surrounding the dural sac, extending from the foramen magnum to the sacrococcygeal membrane (sacral hiatus, where it is closed by the sacrococcygeal ligament).
Boundaries:
| Boundary | Structure |
|---|---|
| Superior | Foramen magnum (where periosteal & spinal dura fuse — drug cannot ascend intracranially) |
| Inferior | Sacrococcygeal membrane at sacral hiatus |
| Anterior | Posterior longitudinal ligament, vertebral bodies, discs |
| Posterior | Ligamentum flavum, vertebral laminae |
| Lateral | Pedicles & intervertebral foramina (drug leaks out here) |
Contents: Spinal nerve roots, fat, lymphatics, areolar tissue, and the internal vertebral venous plexus (Batson's plexus) — valveless veins that engorge in pregnancy, explaining higher block height and risk of intravascular catheter placement in obstetrics.
Layers pierced (midline approach), outside → in:
Skin → subcutaneous fat → supraspinous ligament → interspinous ligament → ligamentum flavum → (STOP: epidural space) → dura → arachnoid → subarachnoid (the last three are crossed only for spinal).
High-yield: The needle stops at the epidural space after the ligamentum flavum — the dura is NOT punctured. Ligamentum flavum is the dense, "gritty/leathery" landmark resistance felt just before the loss of resistance.
The space is widest in the midline lumbar region (~5–6 mm at L2) and narrowest cervically (~1.5–2 mm), one reason lumbar epidurals are technically easier.
Loss-of-resistance (LOR) technique
Identification of the epidural space relies on the negative-pressure/low-resistance property of the space once the needle tip passes the ligamentum flavum.
Stepwise approach:
- Position: sitting or lateral, with spine flexed ("hunched cat").
- Asepsis, local infiltration, insert a Tuohy needle (curved Huber tip directs the catheter).
- Advance through skin into the interspinous ligament; remove stylet.
- Attach a glass/low-friction LOR syringe filled with saline or air.
- Advance slowly with continuous/intermittent pressure on the plunger — resistance is high while in ligamentum flavum.
- Sudden loss of resistance → tip enters epidural space. Plunger gives way easily.
- Note depth (skin-to-space distance, usually 4–6 cm in adults), thread catheter 3–5 cm into the space, aspirate (no blood/CSF), give test dose, secure.
| LOR medium | Pros | Cons |
|---|---|---|
| Saline | No pneumocephalus, no air embolism, no patchy block | Can be confused with CSF (use glucose/temperature test to differentiate) |
| Air | Fluid at hub = CSF (clear distinction) | Risk of patchy/unilateral block, pneumocephalus, venous air embolism, false LOR |
Other identification aids: hanging drop sign (Gutierrez) — a drop at the needle hub is sucked in by negative pressure of the epidural space; more reliable in the thoracic region.
High-yield: Tuohy needle has a Huber tip (blunt, curved) to deflect the catheter laterally and reduce dural puncture; standard markings are at 1 cm intervals.
The test dose concept
The test dose detects accidental intravascular or intrathecal placement before the full dose is given.
Classic test dose = 3 mL of 1.5% lignocaine with adrenaline 1:200,000 (= 15 micrograms adrenaline).
- Intrathecal (subarachnoid) injection → rapid, dense motor + sensory block within ~3–5 min (a "spinal" appears).
- Intravascular injection → adrenaline causes ↑ heart rate ≥20 bpm and/or ↑ SBP ≥15 mmHg within ~30–60 s; patient may report palpitations/perioral tingling.
High-yield: The adrenaline component of the test dose is unreliable in patients on beta-blockers (blunted tachycardia) and in labouring women (contraction-related HR variability). Always aspirate + give the full dose in fractionated increments regardless of test dose result.
Agents & dosing
Local anaesthetics for epidural use, often with adjuvants:
| Drug | Concentration (typical) | Onset | Notes |
|---|---|---|---|
| Lignocaine (lidocaine) | 1.5–2% | Fast (10–15 min) | Intermediate duration; surgical block |
| Bupivacaine | 0.0625–0.5% | Slow (15–20 min) | Long-acting; low conc (0.0625–0.125%) for labour |
| Ropivacaine | 0.1–0.75% | Slow | Less cardiotoxic, more sensory-motor "differential" block |
| Levobupivacaine | 0.1–0.5% | Slow | S-enantiomer; less cardiotoxic than racemic bupivacaine |
| 2-Chloroprocaine | 3% | Very fast | Rapid onset/offset; obstetric top-up for emergency LSCS |
Dose principle: roughly 1–2 mL of LA per segment to be blocked (volume determines spread/height; concentration determines density/intensity of block). A typical lumbar surgical epidural needs 10–20 mL.
Adjuvants: Opioids (fentanyl, morphine — morphine is hydrophilic → wide rostral spread and delayed respiratory depression), adrenaline (prolongs block, marks intravascular injection), clonidine, dexmedetomidine, bicarbonate (speeds onset).
High-yield: Concentration = density (motor block); Volume = spread (number of segments). Labour analgesia uses low-concentration LA + opioid to spare motor function ("walking epidural").
Labour analgesia & obstetric use
- First stage pain (T10–L1) and second stage (S2–S4) are both covered by a lumbar epidural — its key obstetric advantage.
- Modern regimens: PCEA (patient-controlled epidural analgesia) or programmed intermittent epidural boluses with dilute bupivacaine/ropivacaine + fentanyl.
- Epidural does NOT significantly increase Caesarean rate; may modestly prolong second stage and increase instrumental delivery.
- For emergency LSCS, an in-situ labour catheter can be rapidly "topped up" to a surgical block (saves time vs spinal).
High-yield: Maintain left uterine displacement and treat hypotension with IV fluids and a vasopressor — phenylephrine is now the preferred vasopressor in obstetric neuraxial hypotension (less fetal acidosis than ephedrine).
Epidural vs Spinal — the classic comparison
| Feature | Epidural | Spinal (subarachnoid) |
|---|---|---|
| Space | Epidural (extradural) | Subarachnoid (in CSF) |
| Level limit | Any level (incl. thoracic, cervical, caudal) | Below L2 only |
| Needle | Tuohy (16–18G) | Pencil-point (Whitacre/Sprotte) 25–27G |
| Drug volume | Large (10–20 mL) | Small (1.5–3.5 mL) |
| Onset | Slow (15–20 min) | Rapid (~5 min) |
| Endpoint | Loss of resistance | Free flow of CSF |
| Catheter/continuity | Yes — continuous infusion/top-up | Usually single-shot |
| Block quality | Segmental, titratable, less dense | Dense, profound |
| Hypotension | Gradual | Sudden, profound |
| PDPH risk | Low (unless dural tap) | Higher (needle gauge dependent) |
| Systemic toxicity (LAST) | Higher (large dose) | Low |
High-yield: "Epidural = segmental, slow, large volume, catheter, LOR. Spinal = below L2, fast, small volume, dense, CSF flow." This single line answers most MCQs comparing the two.
Complications
Immediate / technical:
- Accidental dural puncture ("wet tap") → CSF in hub; risk of post-dural puncture headache (PDPH) — postural, fronto-occipital, relieved on lying down; treat with hydration, caffeine, analgesics; epidural blood patch is definitive for severe/persistent PDPH.
- Inadvertent intrathecal injection of an epidural-sized dose → total/high spinal: rapid hypotension, bradycardia, apnoea, unconsciousness, dilated pupils. Manage ABC — secure airway/ventilate, vasopressors, atropine, fluids.
- Intravascular injection / LAST (local anaesthetic systemic toxicity): perioral tingling, tinnitus, metallic taste → seizures → cardiovascular collapse (bupivacaine especially cardiotoxic). Treatment: 20% Intralipid (lipid emulsion) 1.5 mL/kg bolus + infusion, stop LA, ACLS (avoid vasopressin, reduce adrenaline doses).
Delayed / serious:
- Epidural haematoma — back pain, progressive motor/sensory deficit, bladder dysfunction. Strongly linked to anticoagulation/coagulopathy. MRI is the investigation of choice; surgical decompression (laminectomy) within ~8 hours to preserve neurological function.
- Epidural abscess — fever, back pain, neuro deficit, raised inflammatory markers; Staph aureus commonest; MRI + drainage + antibiotics.
- Hypotension (sympathetic block), urinary retention, shivering, total spinal, neurological injury, catheter migration/shearing.
High-yield: Neuraxial block timing with anticoagulants (ASRA): LMWH prophylactic — wait 12 h; therapeutic LMWH — wait 24 h before puncture/catheter removal. Epidural haematoma + anticoagulation is a favourite exam pairing.
Investigation of choice (situational)
- Suspected epidural haematoma/abscess → urgent MRI spine.
- Differentiating LOR fluid (saline vs CSF) → glucose test strip (CSF +ve), temperature (CSF warm), and protein.
- Confirming catheter position → epidurogram / loss-of-resistance + test dose (clinical).
Contraindications
Absolute: Patient refusal, raised ICP, coagulopathy/therapeutic anticoagulation, infection at puncture site, severe uncorrected hypovolaemia, severe fixed cardiac output states (e.g. critical aortic stenosis — relative/absolute).
Relative: Sepsis/bacteraemia, certain neurological disorders, fixed-output cardiac lesions, anatomical spinal abnormalities, uncooperative patient.
Key differentials / look-alikes in exams
- Epidural vs subdural injection: subdural gives a patchy, delayed, unexpectedly extensive but "spotty" block.
- PDPH vs other headaches: PDPH is postural (worse upright, relieved lying down) — distinguishes it from migraine/meningitis/cortical vein thrombosis.
- High spinal vs LAST: high spinal = neuraxial-pattern (bradycardia, apnoea, fixed pupils) after a "spinal-like" dose; LAST = CNS excitation/seizure → cardiac toxicity from systemic absorption.
Recently asked / exam angle
- Site of needle tip endpoint in epidural = loss of resistance after ligamentum flavum (NOT free flow of CSF — that's spinal).
- Tuohy needle / Huber tip purpose: directs catheter, blunt to reduce dural puncture.
- Test dose composition (3 mL 1.5% lignocaine + adrenaline 1:200,000 = 15 mcg) and what each component detects.
- Volume vs concentration principle — segments per mL; density vs spread.
- Caudal block landmark = sacral hiatus / sacrococcygeal membrane; commonest paediatric regional block.
- Batson's plexus in pregnancy → reduced LA requirement.
- Treatment of LAST = 20% lipid emulsion (Intralipid).
- Epidural haematoma — risk with LMWH/anticoagulants, MRI, decompression within 8 h.
- CSE "needle-through-needle" technique advantages.
- Phenylephrine as preferred vasopressor for obstetric neuraxial hypotension.
- Distinguishing epidural vs spinal in a table-style single-best-answer.
Mnemonics:
- Layers to epidural space — "Some Sailors Sleep In Loose Linen": Skin, Subcutaneous, Supraspinous, Interspinous, Ligamentum flavum (→ epidural).
- LAST treatment — "LIPID": Lipid emulsion, Intubate/airway, Pause LA, Initiate ACLS, Diazepam/benzo for seizures.
Rapid revision
- Epidural space lies between ligamentum flavum and dura; widest at lumbar (~5–6 mm), spans foramen magnum to sacral hiatus.
- Endpoint = loss of resistance after piercing ligamentum flavum; spinal endpoint = free CSF flow.
- Tuohy needle (Huber tip) is used; catheter threaded 3–5 cm into the space.
- Epidural can be done at any level; spinal only below L2.
- Volume → spread (segments); concentration → density (motor block). ~1–2 mL per segment.
- Test dose = 3 mL 1.5% lignocaine + 15 mcg adrenaline; detects intrathecal (rapid block) & intravascular (tachycardia) placement.
- Bupivacaine is most cardiotoxic LA; ropivacaine/levobupivacaine are safer.
- LAST is treated with 20% lipid emulsion + ACLS.
- Accidental dural tap → risk of PDPH (postural headache) → epidural blood patch if severe.
- Epidural haematoma: anticoagulation risk, MRI, decompress within ~8 h.
- Labour analgesia uses dilute LA + opioid ("walking epidural"); covers T10–L1 (1st stage) and S2–S4 (2nd stage).
- Caudal block via sacral hiatus = commonest paediatric regional anaesthetic; phenylephrine preferred for obstetric hypotension.