Spinal Anaesthesia

Indications

  • Surgical procedures to the lower body
  • Analgesia for upper abdominal surgery (used in combination with GA)

Contraindications

  • Relative
    • Aortic/mitral stenosis
      • Because the spinal block also blocks the sympathetic nerves, vasodilation occurs below the level of block, causing hypotension.  In stenosis, the heart may not be able to compensate (fixed output).
    • Systemic sepsis
  • Absolute
    • Localised sepsis
    • Anticoagulated patient

Anatomy

  • Spinal cord terminates at L1/2 in adults (lower in children).  The dura/subarachnoid space ends at S2 in adults.
    • Spinal anaesthesia is infiltrated in the CSF surrounding the spinal cord.  To avoid injury to the spinal cord, needles are usually inserted between these levels.
  • Surface anatomy
    • Iliac crests -> L3/4
    • PSIS -> S2
  • Skin – subcutaneous tissue – supraspinous ligament – ligamentum flavum – dura

Technique

  • The patient should be sitting or lying on their side
    • Back flexion opens the intervertebral spaces.
  • Clean the back using antiseptic solution.  Adopt an aseptic approach to the procedure.
  • Aim to identify the L3/4, L4/5 or L5/S1 interspace (use the iliac crest as a landmark).
  • The spinal needle is inserted in the midline, aiming slightly cranially.
    • Resistance increases as the ligamentum flavum is entered and when the dura is encountered, with a sudden “give” as the dura is pierced.
    • Correct placement of the needle is confirmed by cerebrospinal fluid at the hub.
  • Inject the local anaesthetic
    • Note only a small amount is required (usually Bupivacaine 0.5% 1.2mls or 15 micrograms of fentanyl)
  • Monitoring
    • Make sure to monitor BP and HR closely, as well as any blood loss (usually requires fluid resuscitation)
    • Rarely, if the anaesthetic spreads to the brainstem, the patient may experience decrease in conscious level, dysphonia, dyspnoea etc.
    • More commonly, headache may occur due to transient decrease in the ICP after puncture of the dura.
    • Patients should also be catheterised (urinary retention is not uncommon)

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