Regional Anesthesia – High-Yield ITE Review Notes

1. Patient-Controlled Analgesia (PCA)

• Components: Basal rate, bolus/demand dose, lockout interval, patient limits.

• Routes: IV or epidural.

• Basal infusion: Only for cancer pain or opioid-tolerant patients. ↑ total opioid use, constipation, respiratory depression.

• Benefits: Better analgesia, higher satisfaction vs. nurse-administered.

• Risks: ↑ opioid consumption, ↑ pruritus.

• Special populations:

  • Geriatric → ↓ doses due to ↑ opioid sensitivity.

  • Pediatric → Safe if patient understands use; basal rate more acceptable (though no proven benefit).

2. Pharmacology of Local Anesthetics

• Structure: Lipophilic benzene ring + hydrophilic amine, linked by amide or ester.

• Classes:

  • Amides: Lidocaine, mepivacaine, ropivacaine.

    • Metabolized by liver (hepatic carboxylases).

    • Allergies often due to preservative (methylparaben).

  • Esters: Benzocaine, cocaine, chloroprocaine, procaine, tetracaine.

    • Metabolized by plasma cholinesterases.

    • Contraindicated in atypical pseudocholinesterase.

    • Byproduct: PABA (common allergen).

• Special toxicity: Methemoglobinemia → prilocaine, benzocaine (also tetracaine, lidocaine).

• Mechanism: Bind intracellular voltage-gated Na⁺ channels → ↓ sodium influx → block neuronal transmission. Prefer open/inactive states.

3. Properties of Local Anesthetics

• pKa:

  • Closer to physiologic pH (7.4) → faster onset.

  • Low pKa = fast onset (lidocaine, mepivacaine).

  • Exception: Chloroprocaine → rapid onset due to high concentration used.

• Lipid solubility: ↑ solubility → ↑ potency (bupivacaine, ropivacaine strongest).

• Protein binding: ↑ binding → ↑ duration (bupivacaine, ropivacaine > lidocaine).

4. Differential Blockade

• Order of loss: Autonomic → sensory → motor.

• Fiber susceptibility: Smaller myelinated > larger/unmyelinated.

  • B fibers (autonomic) → blocked first.

  • A-delta (pain/temp/touch).

  • A-alpha, beta, gamma (motor/proprioception) → harder to block.

  • C fibers (unmyelinated, dull pain) → resistant.

5. Local Anesthetic Adjuncts

• Epinephrine: ↓ absorption, prolongs block, some analgesia (α2 action).

• Bicarbonate: ↑ pH, ↑ non-ionized fraction, faster onset (esp. lidocaine).

• α2-agonists (clonidine): Prolong block (~+2 hrs), risk hypotension/bradycardia.

• Steroids (dexamethasone): ↑ block duration (up to +50%).

• Opioids: Synergistic, faster onset, ↑ quality of block.

6. Systemic Absorption (Site-Dependent)

Mnemonic: ICE Head-to-Toe

• Intercostal (highest) > Caudal > Epidural > Brachial plexus > Lower extremity.

7. Local Anesthetic Systemic Toxicity (LAST)

• Neuro signs: AMS, agitation, tinnitus, perioral numbness, seizures → coma.

• CV signs: Hypotension, AV block, ↓ contractility, arrhythmias. Pediatrics may show peaked T waves.

• Treatment:

  • Stop drug.

  • Benzodiazepines for seizures. Avoid propofol (↓ SVR, contractility).

  • ACLS: epinephrine <1 mcg/kg. Avoid vasopressin, CCBs, lidocaine. Amiodarone for refractory arrhythmias.

  • Lipid therapy: Bolus 1.5 mL/kg → infusion 0.25 mL/kg/min. Repeat bolus ×2, double infusion if collapse persists.

8. Neuraxial Anesthesia – Approaches

• Midline: Skin → SQ → supraspinous → interspinous → ligamentum flavum → epidural.

• Paramedian: Skin → SQ → ligamentum flavum → epidural.

• Needles:

  • Spinal → smaller, cutting (Quincke) vs. pencil-point.

  • Epidural → larger, curved, allows catheter.

9. Epidural Space Anatomy

• Posterior: Ligamentum flavum.

• Anterior: Posterior longitudinal ligament.

• Lateral: Pedicles.

• Superior/Inferior: Foramen magnum → sacrococcygeal ligament.

• Epidural not continuous with CNS → no total spinal possible.

10. Pediatric vs Adult Differences

• Cord end: Adult L2; child L3.

• Dural sac: Adult S2; child S3.

• CSF volume: Child ~4 mL/kg vs adult ~1.5 mL/kg.

• Myelination: Looser → faster onset.

• Duration: Shorter in children.

• Risk: Higher cephalic spread → apnea is early sign (<5 yrs).

11. Systemic Effects of Neuraxial Block

• CV: Vasodilation → hypotension; CO often maintained/increased. HR ↑ (reflex) or ↓ (T1–T4 block).

• Pulmonary: Dyspnea from blocked proprioception; apnea if phrenic blocked. Minor ↓ FVC, FEV1 if T6+.

• GI: N/V (opioids, hypoperfusion); ↑ motility from sympathectomy.

• GU: Urinary retention (afferent inhibition).

12. Complications

• Backache: Multiple attempts, dural puncture.

• Post-dural puncture headache (PDPH): Due to CSF loss → venodilation. Risk: female, pregnant, young, low BMI, large/cutting needles. Positional headache, resolves <1 wk. Tx: hydration, analgesics, epidural blood patch (definitive).

• Other: Low-frequency hearing loss, high/total spinal (rare, 1:4000), permanent injury (trauma, abscess, hematoma, hypoperfusion).

13. Neuraxial Opioids

• Hydrophobic (fentanyl, sufentanil): Rapid onset, short duration, less cephalic spread, less late respiratory depression.

• Hydrophilic (morphine, hydromorphone): Slow onset, long duration, more cephalic spread, ↑ side effects.

• Side effects:

  • Pruritus (dose-dependent, not histamine; treat with naloxone/naltrexone).

  • N/V (trigger zone stimulation, esp. morphine).

14. Spread of Subarachnoid Block

• Drug factors:

  • Baricity:

    • Hyperbaric (with dextrose) → flow with gravity (dependent regions).

    • Hypobaric (with water) → float to non-dependent regions.

    • Hyperbaric = more consistent block.

  • Dose/volume: ↑ → ↑ spread.

• Patient factors:

  • Position during/after injection.

  • Pregnancy: ↑ spread (↓ CSF volume).

  • Age: ↑ → slower onset, longer duration (no effect on extent).

15. Anticoagulation (ASRA Guidelines)

• Hold/restart times for common anticoagulants must be observed for neuraxial procedures & catheter removal (reference table provided in lecture).

Summary:
High-yield regional anesthesia review covered PCA, local anesthetic pharmacology, key physiologic properties (pKa, solubility, protein binding), adjuncts, systemic toxicity, neuraxial techniques, anatomy, pediatric/adult differences, systemic effects, complications, neuraxial opioids, subarachnoid block spread, and anticoagulation safety.