Spinal Anaesthesia

Anatomy and Mechanism

1. Sympathetic (first, widest spread - up to 2 dermatomes higher than sensory)
2. Sensory (second)
3. Motor (last, most restricted)

Indications

• Lower limb surgery (joint arthroplasty, orthopaedic procedures)
• Urological procedures (transurethral resection of the prostate - an awake patient helps detect TURP syndrome)
• Obstetric surgery (most elective Caesarean sections are performed under spinal; saddle block for operative vaginal delivery)
• Lower abdominal and pelvic surgery
• Perineal and anorectal procedures
• Gynaecological surgery
• Sabiston Textbook of Surgery, p. 348
• Morgan and Mikhail's Clinical Anesthesiology, 7e

Contraindications

• Lack of patient consent
• Sepsis or bacteraemia
• Infection at the injection site
• Coagulopathy / anticoagulation (significant)
• Severe hypovolaemia
• Raised intracranial pressure

• Pre-existing neurological disease (controversial - limited data)
• Severe cardiovascular disease / severe aortic stenosis
• Abnormal spinal anatomy, previous spinal surgery
• Obesity (alters spread prediction)
• Patient refusal or inability to cooperate
• Morgan and Mikhail's Clinical Anesthesiology, 7e, p. 1785
• Sabiston Textbook of Surgery, p. 348

Technique

1. Position: Patient placed sitting (most common - good for hyperbaric solutions targeting lumbar/sacral levels) or lateral decubitus.
2. Level: L3-L4 or L4-L5 interspace in adults (below the conus medullaris at L1-L2).
3. Needle: A small-gauge (22-27G), pencil-point needle (Whitacre, Sprotte, or Gertie Marx) is preferred to minimise the dural hole and reduce post-dural puncture headache (PDPH).
4. Endpoint: Freely flowing, clear CSF confirms correct placement.
5. Injection: Local anaesthetic injected slowly; rapid injection causes turbulent flow and unpredictable spread.

• Morgan and Mikhail's Clinical Anesthesiology, 7e, p. 1693

Drugs Used

Local Anaesthetics

Adjuvants

• Opioids (fentanyl 20 µg, sufentanil 5-7.5 µg, or morphine 0.25 mg): prolong analgesia, improve quality of block, help with tourniquet pain.
• Epinephrine (0.1-0.2 mg): prolongs duration, especially with short-acting agents.
• Clonidine: prolongs duration of block.

Factors Affecting Block Level and Spread

1. Baricity of solution relative to CSF:
   • Hyperbaric: sinks - patient position controls spread. Sitting = saddle block; head-down = higher spread.
   • Isobaric: stays at level of injection regardless of position.
   • Hypobaric: floats - rises from injection site.
2. Dose and volume: higher dose = greater cephalad spread and longer duration.
3. Speed of injection: slow injection = predictable spread.
4. Patient factors: obesity, pregnancy, raised intra-abdominal pressure, scoliosis, previous spinal surgery, elderly age - all tend to increase spread or sensitivity.
5. Vasoconstrictors: epinephrine or phenylephrine added to short-acting agents prolongs duration.

• Sabiston Textbook of Surgery, pp. 347-348

Physiological Effects

Cardiovascular

• Hypotension is the most common side effect, resulting from sympathetic blockade causing vasodilation and reduced venous return. Risk is highest when block exceeds T10.
• Bradycardia occurs if cardioaccelerator fibres (T1-T4) are blocked.
• Management: left uterine displacement (in obstetrics), IV fluids (co-loading preferred over pre-loading), phenylephrine (now first-line vasopressor for spinal hypotension - less fetal acidosis than ephedrine), atropine for bradycardia.

International consensus recommends prophylactic phenylephrine infusion + crystalloid co-loading + left lateral uterine displacement for Caesarean section under spinal. - Miller's Anesthesia, 10e, p. 8863

Respiratory

• Usually maintained unless high/total spinal occurs (loss of intercostal and phrenic nerve function).

Neurological

• Motor block below level of injection; resolves as drug wears off.

Advantages Over General Anaesthesia

• Avoids airway manipulation and risk of failed intubation
• No postoperative nausea/vomiting from volatile agents
• Awake patient (valuable in TURP - monitors for hyponatraemia and bladder perforation)
• Better postoperative analgesia (with intrathecal opioids)
• Lower maternal and neonatal morbidity in obstetrics
• Regional anaesthesia fails rate <1% for single-shot spinal
• Sabiston Textbook of Surgery, p. 348

Complications

Post-Dural Puncture Headache (PDPH) - Detail

• Postural headache worsened by sitting/standing, relieved by lying flat
• Due to traction on pain-sensitive intracranial structures from CSF loss
• Risk increased by: female sex, younger age, larger/cutting-tip needles
• Prevention: use 25-27G pencil-point needles, limit puncture attempts
• Conservative treatment: bed rest, oral hydration, NSAIDs, caffeine, paracetamol
• Definitive treatment: epidural blood patch (EBP) - 15-20 mL autologous blood into epidural space; success rate ~84%, may require a second patch in ~17%
• Miller's Anesthesia, 10e, pp. 8878-8879

Single-Shot vs. Continuous Spinal

• Single-shot (most common): simple, fast, reliable, but fixed duration.
• Continuous spinal (via catheter): allows incremental titration and re-dosing; used in selected elderly patients and high-risk obstetric cases. Avoided in young patients because of high PDPH risk. Small-bore micro-catheters are largely abandoned due to local anaesthetic neurotoxicity (cauda equina syndrome).
• Sabiston Textbook of Surgery, p. 348

Spinal vs. Epidural - Key Differences

• Morgan and Mikhail's Clinical Anesthesiology, 7e, p. 1786
• Bailey and Love's Short Practice of Surgery, 28e

Special Situations

Obstetrics - Caesarean Section

Saddle Block

Combined Spinal-Epidural (CSE)

• Miller's Anesthesia, 10e (Miller)
• Morgan and Mikhail's Clinical Anesthesiology, 7e
• Sabiston Textbook of Surgery
• Bailey and Love's Short Practice of Surgery, 28e