What is the physiological mechanism and effects of spinal anesthesia?

Physiology of Spinal Anesthesia

Mechanism of Action

Spinal anesthesia works by injecting local anesthetic directly into the cerebrospinal fluid (CSF) within the subarachnoid space, where it blocks nerve conduction by preventing sodium influx through voltage-gated channels in nerve cell membranes. 1, 2

The local anesthetic acts on nerve roots and the spinal cord itself, producing:

• Sensory blockade through inhibition of pain transmission in dorsal nerve roots 1
• Motor blockade via effects on anterior nerve roots 2
• Autonomic blockade by blocking preganglionic sympathetic fibers, which is responsible for most physiological side effects 3, 1

Determinants of Block Characteristics

The total dose of local anesthetic injected into the subarachnoid space is the primary determinant of both therapeutic and unwanted effects of spinal anesthesia. 1, 2

Key factors influencing block spread include:

• CSF volume at the lumbosacral level is the main determinant of block height, though this cannot be estimated clinically 2
• Baricity of the solution (hyperbaric solutions produce more predictable blocks with fewer high blocks compared to isobaric solutions) 4, 5
• Patient positioning after injection affects the distribution of hyperbaric or hypobaric solutions 5
• Needle insertion technique (combined spinal-epidural produces greater sensorimotor anesthesia than single-shot spinal with identical doses due to disruption of CSF-epidural pressure balance) 6

Cardiovascular Effects

Hypotension is the most frequent complication of spinal anesthesia, occurring due to sympathetic blockade that causes vasodilation and decreased systemic vascular resistance. 3, 1

The cardiovascular physiology involves:

• Sympathetic denervation leads to venous pooling, decreased preload, and reduced cardiac output 3
• Bradycardia may occur from unopposed vagal tone or decreased venous return 3
• Patients over 65 years, particularly those with hypertension, are at increased risk for hypotensive effects 3
• Management requires vasopressors and intravenous fluids 7

Respiratory Effects

Respiratory paralysis or underventilation can occur due to cephalad extension of the motor level of anesthesia, potentially leading to secondary cardiac arrest if untreated. 3

Respiratory physiology considerations:

• High spinal blocks (above T4) can impair intercostal muscle function 3
• Total spinal anesthesia causes complete respiratory paralysis requiring immediate intubation and ventilation 7
• The incidence of high/total spinal requiring cardiovascular/respiratory support is approximately 1 in 4367 cases 7, 4

Central Nervous System Effects

Local anesthetic systemic toxicity can produce CNS effects characterized by excitation and/or depression:

• Early signs include restlessness, anxiety, dizziness, tinnitus, blurred vision, or tremors 3
• Progression may lead to convulsions, though excitement may be transient with depression being the first manifestation 3
• Advanced toxicity presents as drowsiness merging into unconsciousness and respiratory arrest 3

Block Onset and Duration

Spinal anesthesia provides deep and fast surgical block through injection of small doses of local anesthetic, with onset typically within 5-10 minutes. 1

Temporal characteristics:

• Peak sensory level is typically reached within 10-20 minutes after injection 6
• Block height should be assessed at least once every 5 minutes until no further extension is observed 7
• Duration depends on the specific local anesthetic used (long-acting agents like bupivacaine provide 2-4 hours of surgical anesthesia) 1, 2
• Small doses of long-acting agents (bupivacaine or ropivacaine) can produce adequately short spinal blocks suitable for outpatient procedures 1, 2

Dose-Response Relationships

Lower doses of intrathecal bupivacaine (<10 mg) can reduce associated hypotension in elderly patients while maintaining adequate anesthesia. 8

The addition of adjuvants modifies the dose requirements:

• Opioids added to local anesthetics allow reduction in local anesthetic dose, improving quality and duration of analgesia without affecting recovery profile 2
• Hyperbaric bupivacaine 10-12.5 mg (2-2.5 ml) is recommended for lower extremity surgery 4

Common Pitfalls

Failure to aspirate CSF from a catheter does not exclude positioning within the subarachnoid space, and inadvertent administration of epidural doses through intrathecal catheters can cause high or total spinal blocks. 7

Critical safety considerations:

• Clear labeling of catheters and good communication between healthcare professionals is essential to prevent dosing errors 7
• Increasing agitation, significant hypotension, bradycardia, upper limb weakness, dyspnoea, or difficulty speaking indicate developing high block requiring immediate intervention 7
• Pencil-point spinal needles should be used instead of cutting-bevel needles to minimize post-dural puncture headache risk 7