Adjuvants to Local Anesthetics

Intro & Vasoconstrictors - Squeeze & Seize

• Adjuvants: Enhance LA action: ↑ duration/quality, ↓ onset/dose/toxicity.
• Vasoconstrictors: "Squeeze" vessels, "Seize" LA locally.
  • Mechanism: ↓ blood flow → ↓ LA systemic absorption → ↑ local LA → ↑ duration & intensity. Reduces bleeding.
  • Examples: Epinephrine, Norepinephrine, Phenylephrine, Felypressin.
  • Epinephrine (Adrenaline):
    • Most common. Typical conc: 1:200,000 (5 µg/mL).
    • Max dose (adults): 200-250 µg or 3-5 µg/kg.
    • Effects: Prolongs block (lignocaine by 50-100%), ↓ peak LA plasma.
    • Cautions: CVD, thyrotoxicosis. Avoid end-arterial sites (fingers, toes, penis, nose 📌 FTPN).

⭐ Epinephrine 1:200,000 with lignocaine can double its duration and reduce systemic absorption.

Alpha-2 Agonists - Cool Clonidine Crew

📌 Cool Clonidine Crew!

• Mechanism: Act on presynaptic α2 receptors (↓ Norepinephrine release) & postsynaptic α2 receptors in spinal cord (dorsal horn) for analgesia.
• Examples: Clonidine, Dexmedetomidine.
• Benefits as Adjuvants:
  • ↑ Duration & quality of sensory/motor block.
  • ↓ Local anesthetic dose requirement.
  • Provide sedation.
  • ↓ Shivering.
• Clonidine:
  • Dose (PNB): 1-2 µg/kg; Intrathecal/Epidural: 15-150 µg.
  • Prolongs analgesia by ~2-4 hours.
  • Side effects: Hypotension, bradycardia, sedation.
• Dexmedetomidine (Dexmed):
  • Higher α2 selectivity (α2:α1 ratio ~1600:1 vs. Clonidine ~200:1).
  • Dose (PNB): 0.5-1 µg/kg; Intrathecal: 5-10 µg.
  • Longer analgesia & more profound sedation than clonidine.

  ⭐ Dexmedetomidine provides cooperative sedation with minimal respiratory depression, a key advantage.

  • Side effects: More pronounced hypotension & bradycardia possible; use with caution.

Neuraxial Opioids - Spinal Soothers

• Mechanism: Act on opioid receptors in spinal cord's substantia gelatinosa.
• Benefits: Synergism with LAs (↓ LA dose), prolonged analgesia.
• Types & Doses (Spinal / Epidural):
  • Morphine (Hydrophilic):
    • Onset: 30-60 min; Duration: 12-24 hr.
    • Dose: 0.1-0.3 mg / 2-5 mg.
    • ⚠️ Delayed respiratory depression (monitor 12-24h).
  • Fentanyl (Lipophilic):
    • Onset: 5-10 min; Duration: 2-4 hr.
    • Dose: 10-25 mcg / 50-100 mcg.
    • Early respiratory depression.
• Side Effects: Pruritus (commonest), N/V, urinary retention, respiratory depression.
  • Pruritus Rx: Naloxone (low dose e.g., 0.25-1 mcg/kg/hr), nalbuphine.

⭐ Hydrophilic opioids (e.g., morphine) carry a higher risk of delayed respiratory depression due to slow rostral spread in CSF.

Steroids & Other Agents - Block Boosters

• Dexamethasone:
  • Mechanism: Anti-inflammatory; ↓ ectopic neuronal discharge.
  • Dose: 4-8 mg (IV or perineural).
  • Effect: Prolongs analgesia duration (sensory > motor).

  ⭐ Dexamethasone added to local anesthetics can prolong postoperative analgesia by approximately 6-8 hours.

• Alkalinization (Sodium Bicarbonate):
  • Mechanism: ↑ unionized LA base → faster nerve sheath diffusion → quicker onset.
  • Dose: 1 mEq per 10 mL Lidocaine/Mepivacaine.
  • Caution: Avoid with Bupivacaine (may precipitate). LAs with epinephrine are acidic (pH 3-4).
  • Effect: ↓ onset time.
• Hyaluronidase ("Spreading Factor"):
  • Mechanism: Enzyme; hydrolyzes hyaluronic acid → ↑ LA diffusion.
  • Dose: 150-300 IU added to LA.
  • Uses: Ophthalmic blocks, areas needing wider spread.
  • Effect: ↓ onset, ↑ spread, ↓ duration (faster absorption), ↑ systemic toxicity risk.
• Other Agents:
  • Magnesium: NMDA antagonist; prolongs analgesia.
  • Midazolam (perineural): GABA-A agonist; analgesic, neurotoxicity concerns.
  • Ketamine (low-dose perineural): NMDA antagonist; analgesic, neurotoxic risks.

High‑Yield Points - ⚡ Biggest Takeaways

• Epinephrine: Vasoconstrictor, prolongs duration, reduces systemic toxicity; avoid in end-arterial fields.
• Alpha-2 agonists (Clonidine, Dexmedetomidine): Prolong analgesia, reduce LA dose, cause sedation.
• Opioids (e.g., Fentanyl): Intrathecal/epidural use enhances analgesia; watch for pruritus, respiratory depression.
• Sodium Bicarbonate: Speeds LA onset by ↑ non-ionized form, especially in acidic tissue.
• Dexamethasone: Perineural/IV, prolongs block duration, offers anti-inflammatory effects.
• Hyaluronidase: Facilitates LA spread, useful in ophthalmic blocks for better penetration.