Spinal Anaesthesia

Spinal anaesthesia is:

Indications

Relative:

  • Patient factors
    • Difficult airway
      Airway patency is maintained.
    • Respiratory disease
      Less respiratory impairment than GA.
    • Frailty and systemic disease
      Spinal anaesthesia tends to be less physiologically disruptive.
  • Surgical factors
    • Sub-umbilical surgery
    • Muscle relaxation
      Good operating conditions provided.
    • Reduced blood loss

Contraindications

This covers contraindications specific to spinal anesthesia. General contraindications to neuraxial techniques are covered under principles of neuraxial anaesthesia.

  • Operative duration
    Surgery expected to last >2 hours will usually require a different technique.

  • C

    • Volume and haemodynamic state

  • D

    • Anxiety levels

  • E

    • Back anatomy
    • Presence of local infection

Preparation

  • Alert the patient that spinal anaesthesia:
    • Removes pain but some sensation may remain
      This will be weird or uncomfortable but shouldn’t be painful
    • Causes motor weakness
  • Standard ANZCA monitoring
    Regular (e.g. 1 minutely) BP after spinal injection.
  • 18G or greater IVC
  • Vasopressor available
    Consider infusion if it is necessary to defend SBP.
  • LP equipment
  • Local anaesthetic

Factors Affecting Block Spread

With the exception of baricity and patient position, the influence of most of these factors is small and unpredictable

  • Baricity
    Hyperbaric solutions behave more predictably due to the effect of gravity.
  • Patient position
    Block height can be altered for up to 20 minutes post injection.
  • Concentration and volume of LA
    • Volume determines extent of block
    • Dose (mg) determines block density
      Therefore the same dose administered in a lower volume (i.e. higher concentration) causes a denser block.
  • Level of injection
    Blocks achieve a denser block close to the level of injection.
  • Speed of injection
    Slower injections produce more predictable spread.
  • Intraabdominal pressure
    Affects epidural vein volume, which in turn affects CSF volume.

Dosing

Classically bupivacaine is used: * Typically lasts 2-2.5 hours * Can be either: * Hyperbaric/heavy bupivacaine
Mixed with 8% dextrose. Heavy bupivacaine: * Dosing of heavy bupivacaine * Adults: 2-4ml (10-20mg) * <5kg: 0.4-0.5mg/kg * 5-15kg: 0.3-0.4mg/kg * 15-40: 0.25-0.3mg/kg * Plain bupivacaine
Mixed with saline online. Plain bupivacaine: * Tends to rise slightly * Cannot be adjusted * Dosing * Adults: 2-3.5ml 0.5% plain bupivacaine

Additives:

  • 8% dextrose
    Forms hyperbaric “heavy” bupivacaine, which:
    • Allows block height and spread to be adjusted by position
    • Can be adjusted by adjusting patient position
      For a block at L3/4:
      • Sitting patients will tend to achieve an upper block height on T7-T10
        Allows a dense saddle block to be achieved with relatively small volumees.
      • Supine patients will tend to achieve an upper block of T4-T7
  • Opioid
    Opioid may be added to prolong the duration of analgesia and reduce the dose of LA required, at the risk of respiratory depression. Dosing:
    • Fentanyl 10-25ug
      Lasts 1-4 hours.
    • Morphine 100-300ug
      Lasts 8-24 hours.

Induction

Performance of lumbar puncture is covered here.

Assessing Block

  • By temperature
    Using ice, relative to a reference standard (e.g. the forehead). Temperature sensation will be lost 1-2 dermatomal levels above pain, which is useful for determining adequate surgical anesthesia.
  • By pain
    Assess response to pain (e.g. pinching with toothed forceps) prior to commence operation. Pain sensation is lost 1-2 dermatomal levels above touch.
  • By touch
  • By motor function
    Objective motor weakness, using the Bromage score:
    • 0: No motor block
    • 1: Inability to raise extended leg; able to move knees and feet
    • 2: Inability to raise extended leg or knee; able to move feet
    • 3: Complete motor block

Troubleshooting Block

  • Block too low
    • With a hyperbaric solution
      Tilt the patient head down.
    • With a plan solution
      Turn the patient supine and then prone again.
  • Block too high

Intraoperative

Postoperative

Complications

Can result either from:

  • Dural puncture
    • Post Dural Puncture Headache
  • Spinal anaesthesia
    • Bradycardia
      Due to blockade of cardioaccelerator fibres.
      • If symptomatic, hypotensive, or excessively bradycardic
        Consider atropine or glycopyrrolate.
    • Hypotension
      Generally due to drop in SVR.
      • Vasopressors
    • High/Total Spinal

References

  1. Aspen. Marcain Heavy, 0.5% solution for injection. 2018.
  2. Hocking G, Wildsmith JA. Intrathecal drug spread. Br J Anaesth. 2004 Oct;93(4):568-78. Epub 2004 Jun 25.