Bupivacaine for Spinal (Intrathecal) Anesthesia

Overview

• Miller's Anesthesia, 10e
• Katzung's Basic and Clinical Pharmacology, 16e

Classification & Physicochemical Properties

Mechanism of Action

• Sensory fibers - blocking pain, temperature, and touch
• Motor fibers - causing motor block (dose/concentration dependent)
• Autonomic fibers - sympathetic blockade causing vasodilation and potential hypotension

Available Formulations for Spinal Use

1. Hyperbaric Bupivacaine (0.75% in 8.25% dextrose)

• Most commonly used formulation for spinal anesthesia
• Dextrose makes it heavier than CSF (specific gravity >1.003)
• Sinks with gravity - block level is controlled by patient positioning
• Standard for surgical procedures at/below T4-T10 level

2. Isobaric/Plain Bupivacaine (0.5%)

• At room temperature, plain bupivacaine is actually slightly hypobaric compared to CSF
• Less predictable spread than hyperbaric
• Useful when baricity-controlled positioning is not required

Dosing Guidelines

• For ambulatory knee arthroscopy: hyperbaric bupivacaine 4-5 mg combined with unilateral positioning can be adequate.
• Recovery profiles using small doses appear similar to lidocaine.

Factors Affecting Spread in the Subarachnoid Space

1. Baricity - hyperbaric sinks; hypobaric rises; isobaric minimally affected by position
2. Patient position - must be maintained for 10-20 minutes after injection for hyperbaric solutions
3. Dose - higher dose = higher, denser, longer block
4. Injection site - L3/L4 or L4/L5 interspace most common
5. Speed of injection - faster injection may slightly increase spread
6. Patient factors - height, weight (especially obesity can increase spread), age, pregnancy, intraabdominal pressure

Duration of Action

• Surgical anesthesia: ~1.5 to 3 hours (dose-dependent)
• Sensory regression: 2-4 hours
• Motor block regression: may persist slightly longer
• The relatively long duration makes bupivacaine less ideal for ambulatory/outpatient surgery, as it may delay discharge

Spinal Additives Commonly Used With Bupivacaine

Opioids

• Fentanyl 10-25 mcg: rapid onset (10-20 min), duration 4-6 hours; commonly used in ambulatory surgery; may increase side effects (pruritus, nausea)
• Morphine 0.1-0.3 mg: hydrophilic; prolonged analgesia up to 18-24 hours; risk of delayed respiratory depression
• Sufentanil: very lipophilic; rapid onset, short duration

Alpha-2 Agonists

• Clonidine 15-225 mcg: prolongs sensory and motor block by ~1 hour, reduces morphine consumption by up to 40%; may cause hypotension and sedation
• Dexmedetomidine 3-5 mcg: ~10x more alpha-2 selective than clonidine; prolongs sensory block similar to fentanyl

Vasoconstrictors

• Epinephrine and phenylephrine can prolong other local anesthetics (tetracaine, lidocaine), but do NOT significantly prolong bupivacaine spinal anesthesia due to its already long duration and bupivacaine's inherent vasodilatory properties - so epinephrine is generally NOT added to bupivacaine spinal
• Phenylephrine has declined in use due to association with TNS (transient neurologic symptoms)

Toxicity Profile

Neurotoxicity

• Bupivacaine has a relatively favorable record for spinal use - low risk of neurotoxicity
• TNS (Transient Neurologic Symptoms) - bupivacaine is rarely associated with TNS (unlike lidocaine which has a much higher TNS rate)
• This makes it preferred over lidocaine for spinal use despite longer duration

Cardiotoxicity

• Bupivacaine is more cardiotoxic than other amide local anesthetics (e.g., lidocaine, ropivacaine)
• Mechanism: blocks cardiac sodium channels (especially in fast-conducting Purkinje fibers and ventricular muscle), and the block is slow to unbind ("fast-in, slow-out" kinetics)
• Results in: severe reentrant arrhythmias (VT/VF), negative inotropy, cardiac arrest
• Clinical relevance for spinal use: because only small doses are used intrathecally, the risk of systemic toxicity/cardiotoxicity is very low - bupivacaine has a "comparatively unblemished record as a spinal anesthetic"
• If LAST (local anesthetic systemic toxicity) occurs: treat with 20% lipid emulsion (Intralipid)

Compared to Levobupivacaine

• Levobupivacaine (pure S(-) enantiomer) has less cardiotoxicity than racemic bupivacaine
• However, for spinal anesthesia at equivalent doses, there is no significant clinical difference in onset, duration, or block quality between the two

Clinical Uses

Comparison to Other Spinal Local Anesthetics

Contraindications and Precautions for Spinal Bupivacaine

• Patient refusal
• Coagulopathy / anticoagulation (spinal hematoma risk)
• Local infection at injection site
• Raised intracranial pressure
• Severe hypovolemia (sympathetic block will worsen hypotension)
• Pre-existing significant neurologic disease (relative)

• Elderly (reduced dose needed; increased sensitivity to block)
• Obesity (increased spread)
• Pregnancy (enhanced sensitivity; left uterine displacement needed to prevent aortocaval compression)

Key Complications of Spinal Anesthesia With Bupivacaine

1. Hypotension - most common; due to sympathetic blockade; treat with IV fluids, vasopressors (phenylephrine, ephedrine)
2. Bradycardia - especially with high block (T1-T4 cardioaccelerator fiber blockade)
3. High/Total spinal - respiratory arrest, cardiovascular collapse; manage with airway support + vasopressors
4. PDPH (Post-dural puncture headache) - use pencil-point needles (Whitacre/Sprotte) to reduce risk
5. Urinary retention - especially with morphine additives
6. TNS - rare with bupivacaine

Recent Evidence