Physiological Consequences of Central Neuraxial Blocks

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