Anesthesia and Pain Management
Anesthesia and pain management tests pharmacology, block selection, sedation depth, and rescue from local anesthetic systemic toxicity (LAST). The exam favors dose calculations and 'what went wrong' scenarios over drug lists.
Core rules
- Lidocaine 2% contains 20 mg/mL; a standard 1.8 mL cartridge holds 36 mg. With 1:100,000 epinephrine, maximum is 7 mg/kg (absolute cap often 500 mg); without vasoconstrictor, 4.4 mg/kg.
- Mepivacaine 3% plain is 30 mg/mL; bupivacaine 0.5% with epi is long-acting and useful for postoperative pain but has greater cardiotoxicity.
- Successful block depends on pKa and lipid solubility; inflamed tissue lowers pH, trapping anesthetic in the ionized form and reducing efficacy.
- Sedation is a continuum. Moderate sedation requires independent rescue of airway and ventilation; capnography detects hypoventilation earlier than pulse oximetry.
- LAST presents with tinnitus, metallic taste, agitation, then seizures and cardiovascular collapse. Stop injection, call for help, airway/100% oxygen, benzodiazepine for seizures, and lipid emulsion 1.5 mL/kg bolus followed by infusion.
Worked example
A 70 kg patient needs 2% lidocaine with 1:100,000 epi. Maximum lidocaine = 70 × 7 = 490 mg. Each 1.8 mL cartridge = 36 mg, so 490/36 = 13.6 cartridges. But epinephrine limit (about 0.2 mg healthy adult; 0.04 mg cardiac) may be reached first, so calculate both and use the lower limit.
Traps
- Using mg/kg for lidocaine without converting percent concentration.
- Giving epinephrine IM for simple syncope.
- Calling deep sedation 'moderate' when the patient cannot maintain ventilation.
- Ignoring LAST because the patient only feels 'odd.'