What is the physiological mechanism of spinal and epidural anaesthesia?

Physiology of Spinal and Epidural Anaesthesia

Mechanism of Spinal Anaesthesia

Spinal anaesthesia works by blocking three types of nerve fibers—sensory, motor, and autonomic—which are essential for nerve impulse initiation and conduction, achieved through injection of local anaesthetic into the cerebrospinal fluid within the subarachnoid space. 1

Local Anaesthetic Action and Distribution

• Local anaesthetic drugs injected into the subarachnoid space directly contact nerve roots and spinal cord structures, producing rapid and profound neural blockade 2
• The total dose of local anaesthetic is the primary determinant of both therapeutic and unwanted effects of spinal anaesthesia, more so than concentration or volume alone 3
• The volume of cerebrospinal fluid at the lumbosacral level is the main determinant of block spread, though this cannot be estimated clinically 3
• Drug baricity (density relative to CSF) influences distribution: hyperbaric solutions settle dependently with gravity, while hypobaric solutions rise 4

Differential Nerve Fiber Blockade

• Small nerve fibers (autonomic and pain fibers) are blocked before larger motor fibers, creating a differential block pattern 2
• This explains why sympathetic blockade typically extends 2-6 dermatomes higher than sensory blockade, which in turn extends higher than motor blockade 2

Cardiovascular Effects of Spinal Anaesthesia

Sympathetic Blockade and Hypotension

• Sympathetic nerve blockade is the most frequent side effect, causing vasodilation, venous pooling, decreased venous return, and hypotension 2, 3
• Lower doses of intrathecal bupivacaine (less than 10 mg) significantly reduce the incidence and severity of hypotension while maintaining adequate surgical anaesthesia 1, 5
• Hypotension is successfully treated with volume expansion and vasoactive drugs 2, 3

Monitoring Requirements

• Standard monitoring must include continuous pulse oximetry, ECG, and blood pressure 5
• Supplemental oxygen should always be provided during spinal anaesthesia 5

Mechanism of Epidural Anaesthesia

Anatomical Considerations

• Epidural anaesthesia involves injection of local anaesthetic into the epidural space, outside the dura mater 6
• The epidural space contains fat, blood vessels, and nerve roots exiting the spinal cord 6
• Loss of resistance technique identifies the epidural space by detecting the change from ligamentous resistance to the negative pressure of the epidural space 6

Drug Distribution and Block Characteristics

• Epidural anaesthesia requires higher doses of local anaesthetic compared to spinal, is slower in onset, and provides poorer sacral blockade 7
• Local anaesthetic spreads through the epidural space to reach nerve roots as they exit the spinal cord 6
• Epidural catheters allow incremental dosing and prolongation of anaesthesia, providing flexibility for extended procedures 7

Combined Spinal-Epidural (CSE) Technique

Physiological Differences from Single-Shot Spinal

• CSE produces greater sensorimotor anaesthesia and more prolonged recovery compared to single-shot spinal, despite identical intrathecal doses 8
• Loss of resistance to air during epidural needle placement disrupts the balance between CSF and subatmospheric epidural pressure, potentially enhancing cephalad spread of intrathecal drugs 8
• CSE results in higher peak sensory levels (median T3 versus T5) and more frequent hypotension requiring vasoconstrictors 8

Clinical Advantages

• The needle-through-needle CSE technique is more reliable than epidural alone, as free flow of CSF confirms correct midline placement in the epidural space 7
• CSE allows small-dose spinal initiation with epidural catheter backup for extension or prolongation 7

Critical Safety Considerations

High and Total Spinal Block

• Inadvertent high neuraxial block requiring cardiovascular and/or respiratory support occurs in approximately 1 in 4,367 spinal anaesthetics 7
• Signs include increasing agitation, significant hypotension, bradycardia, upper limb weakness, dyspnoea, or difficulty speaking 7
• Management requires vasopressors, fluids, supplemental oxygen, and potentially tracheal intubation and ventilation 7

Intrathecal Catheter Risks

• Failure to aspirate CSF from a catheter does not exclude positioning within the subarachnoid space, and catheters may migrate over time 7
• Inadvertent administration of epidural doses through intrathecal catheters has caused high/total spinal blocks, respiratory arrest, and cardiac arrest 7
• Block height must be assessed at least every 5 minutes until no further extension is observed 7

Epidural Failure Recognition

• Epidural failure is characterized by no sensory block, no motor block, and negative hemodynamic response, indicating catheter misplacement outside the epidural space 6
• Test the epidural before induction with cold/pinprick testing to confirm bilateral sensory block to at least T10 level for abdominal surgery 6

Contraindications

• Coagulopathy or anticoagulation are standard contraindications that must be verified before proceeding 5
• Patient refusal is an absolute contraindication 5
• Infection at the injection site, raised intracranial pressure, and hypovolemia are relative contraindications 5