Physiological Consequences of Central Neuraxial Blocks

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Cardiovascular Effects

  • Similar to the effects of using combined α1 & β adrenergic blockers - decreased heart rate and arterial blood pressure
  • Sympathectomy extends 2-6 dermatomes above the sensory level in spinal anaesthesia, and at the same level with epidural
  • Causes both venous and arterial vasodilation, however venodilation effect predominates
  • TPR decreases 15-18% in normovolaemic, healthy patients, but up to 25% in elderly patients with concurrent disease
  • Heart rate during high neuraxial block decreases due to blocked cardioaccelerator fibres in T1-T4
  • May also decrease due to a fall in RA filling, causing decreased outflow from intrinsic chronotropic stretch receptors
  • Decrease in arterial blood pressure is generally more gradual and of less magnitude with epidural than with spinal anesthesia of comparable levels

Respiratory Effects

  • Usually of little consequence
  • Tidal volume unchanged, small decrease in vital capacity due to a decrease in expiratory reserve volume due to abdominal muscle paralysis required for forced expiration
  • Respiratory arrest under spinal occurs only rarely, and is due to hypoperfusion of respiratory centres in the brainstem rather than muscle paralysis
  • Partial respiratory muscle paralysis is usually only of significance in patients with underlying respiratory compromise - mainly relating to difficulty coughing/clearing secretions

Gastrointestinal Effects

  • Nausea and vomiting occurs in up to 20% of patients - caused by gastrointestinal hyperperistalsis from unopposed parasympathetic activity
  • Atropine can be used to treat nausea associated with high anaesthesia
  • Blood flow to GI organs falls in tandem with systemic hypotension

Renal Effects

  • Large reserve, therefore decrease in renal blood flow is of little physiological importance
  • Can result in urinary retention

Epidural Anaesthesia

  • Main difference between spinal and epidural anaesthesia is that blood levels can reach toxic concentrations in epidural anaesthesia - causing adverse CNS and cardiovascular effects