Spinal anaesthesia

Definition and key principles

• Injection of local anaesthetic into CSF in the subarachnoid space to produce reversible block of nerve roots and spinal cord pathways
• Produces differential block: sympathetic > sensory > motor (onset and regression differ)
• Block height relates to baricity, dose, volume, patient position, pregnancy, spinal anatomy, and injection characteristics

Relevant anatomy

• Layers traversed (midline): skin → subcut tissue → supraspinous ligament → interspinous ligament → ligamentum flavum → epidural space → dura → arachnoid → subarachnoid space (CSF)
• Adult spinal cord ends ~L1 (range T12–L3), dural sac to ~S2, spinal performed below L2 (commonly L3/4 or L4/5)
• Tuffier’s line (iliac crests) approximates L4 spinous process / L4/5 interspace (variable)
• Dermatomes (useful targets): T4 nipple, T6 xiphisternum, T10 umbilicus, L1 inguinal, S2–4 perineum
  • LSCS typically requires sensory block to ~T4 (and adequate sacral spread)

Physiology of spinal block

• Sympathetic block (T1–L2) causes vasodilation (venous >, arterial) → ↓ venous return and ↓ SVR → hypotension
  • Magnitude depends on block height, baseline tone (e.g. hypovolaemia), and aortocaval compression in pregnancy
• Bradycardia: blockade of cardioaccelerator fibres (T1–T4) and reduced venous return (Bezold–Jarisch reflex)
• Respiratory effects: usually minimal if block below T4, high block can impair intercostals, dyspnoea may be due to blocked chest wall sensation with preserved diaphragmatic function
  • Test phonation/ability to speak, consider EtCO2 if sedated
• GI/GU: increased gut motility, urinary retention common (sacral block)
• Thermoregulation: vasodilation and impaired shivering responses below block → hypothermia risk

Contraindications (absolute and relative)

• Absolute: patient refusal, infection at site, uncorrected severe hypovolaemia/shock, true LA allergy (rare), raised ICP due to mass lesion/obstructive hydrocephalus, coagulopathy/unsafe anticoagulation, inability to cooperate (context-dependent)
• Relative: sepsis/bacteraemia, fixed cardiac output lesions (e.g. severe AS), severe mitral stenosis, severe pulmonary hypertension, major spinal deformity/previous surgery, neurological disease (risk/benefit), thrombocytopenia (threshold depends on trend/cause), aortic stenosis is not absolute but requires senior planning and haemodynamic strategy

Anticoagulation/antiplatelets (high-yield principles)

• Neuraxial techniques require assessment of bleeding risk: drug, dose, timing, renal function, other agents, traumatic needle pass
• Aspirin alone is usually not a contraindication, dual antiplatelet therapy and recent P2Y12 inhibitors increase risk—follow local/ASRA/ESAIC guidance
• LMWH: avoid neuraxial puncture too close to dosing, ensure appropriate interval before and after, higher-dose regimens require longer intervals
• Warfarin: ensure INR acceptable, DOACs require adequate cessation based on agent/renal function

Local anaesthetic choices and baricity

• Common agent: bupivacaine (0.5%) as hyperbaric (in glucose) or isobaric, levobupivacaine/ropivacaine alternatives
• Baricity = density of solution / density of CSF (at 37°C): hyperbaric sinks, hypobaric rises, isobaric relatively position-independent
• Hyperbaric bupivacaine: more predictable spread with positioning, commonly used for LSCS and lower limb surgery
• Adjuvants: intrathecal opioids (fentanyl, diamorphine/morphine) improve analgesia, clonidine can prolong block but increases hypotension/sedation, preservative-free only
  • Intrathecal morphine/diamorphine: excellent prolonged analgesia but risk of pruritus, PONV, urinary retention, delayed respiratory depression

Technique (practical steps)

• Preparation: consent (including failure/GA conversion, PDPH, nerve injury, infection/bleeding), check anticoagulation, IV access, resus drugs ready, baseline observations
• Position: sitting or lateral, optimise flexion, maintain left uterine displacement in pregnancy
• Asepsis: hat/mask, sterile gloves, skin prep (chlorhexidine in alcohol—allow to dry), sterile drapes, avoid contamination of needle
• Landmark and approach: midline or paramedian, identify L3/4 or L4/5, infiltrate skin with LA
• Needle choice: pencil-point (Whitacre/Sprotte) reduces PDPH vs cutting (Quincke), introducer often used
• Confirm CSF flow (free flow), aspirate gently if needed, inject dose slowly, avoid repeated aspiration with very fine needles if traumatic
• Post-injection: position appropriately (esp hyperbaric), monitor BP closely, assess block (cold/light touch, motor Bromage), document level and haemodynamics

Assessment of block and adequacy

• Sensory testing: cold (A-delta) and light touch (A-beta), surgical anaesthesia correlates better with light touch than pinprick alone
• Motor: Bromage scale, note that motor block may be dense even if sacral sensory sparing exists
• For LSCS: aim T4 to light touch with stable haemodynamics, check ability to tolerate uterine exteriorisation/peritoneal traction (often needs opioid adjunct)

Haemodynamic management (core FRCA)

• Prevention: left uterine displacement (pregnancy), avoid aortocaval compression, judicious fluid loading (co-load often preferred), early vasopressors
• Treat hypotension promptly: vasopressor (phenylephrine or ephedrine depending on HR), fluids, reduce block height if possible (position), consider atropine for bradycardia
  • In obstetrics, phenylephrine is commonly first-line to maintain uteroplacental perfusion, ephedrine associated with more fetal acidosis in some studies
• Severe bradycardia/asystole: call for help, 100% O2, treat as peri-arrest, consider adrenaline early, consider high spinal as cause

Complications and management

• Failure/patchy block: wrong space, inadequate dose/spread, catheter/needle issues, options—wait/reassess, repeat spinal (caution), convert to GA, supplement with sedation/analgesia or peripheral blocks
• High/total spinal: hypotension, bradycardia, nausea, dyspnoea, upper limb tingling, loss of consciousness/apnoea
  • Management: call help, left uterine displacement if pregnant, airway support/ventilation, vasopressors (phenylephrine/ephedrine) and adrenaline if severe, IV fluids, treat bradycardia (atropine) and consider early intubation
• Local anaesthetic systemic toxicity (rare with spinal but possible with wrong drug/route): seizures, arrhythmias
  • Management: ABC, stop injection, intralipid per guidelines, treat seizures, ALS modifications
• Post-dural puncture headache (PDPH): postural headache ± neck stiffness, photophobia, tinnitus, risk reduced with pencil-point small gauge
  • Management: exclude other causes, hydration, simple analgesia, caffeine (selected), antiemetics, epidural blood patch for severe/persistent symptoms
• Neurological injury: transient neurological symptoms, nerve root trauma, cauda equina syndrome (rare), spinal cord injury (avoid high levels)
  • Red flags: progressive weakness, saddle anaesthesia, sphincter dysfunction—urgent MRI/neurosurgical input
• Spinal/epidural haematoma (rare, catastrophic): severe back pain, motor/sensory deficit, bladder/bowel dysfunction
  • Management: urgent MRI and decompression ideally within hours, treat as emergency
• Infection: meningitis, epidural abscess (rare), strict asepsis, investigate fever/back pain/neuro signs
• Hypothermia/shivering, nausea/vomiting (often hypotension-related), urinary retention

Special situations

• Obstetrics: reduced CSF volume and engorged epidural veins → higher spread, dose often reduced, prophylactic vasopressor strategy important
• Elderly: increased sensitivity and reduced physiological reserve, higher hypotension risk, consider lower dose and careful titration (CSE/epidural) for frail patients
• Aortic stenosis/fixed output: spinal may cause profound hypotension, if used, require senior plan, invasive monitoring, vasopressors ready, consider graded neuraxial (CSE/epidural) or GA
• Sepsis: relative contraindication, consider source control, haemodynamic stability, coagulation status, risk of hypotension and neuraxial infection