Pain Medicine and Regional Anesthesia
Pain mechanisms and blocks
Pain medicine questions test mechanism, dose, and complication. Nociception involves transduction, transmission, modulation, and perception; multimodal analgesia combines non-opioids, regional techniques, and opioids to reduce dose-related adverse effects. Local anesthetics block voltage-gated sodium channels. pKa determines onset (closer to physiologic pH means more uncharged drug and faster onset), protein binding affects duration, and maximum dose limits toxicity.
Know maximum doses: bupivacaine about 2–2.5 mg/kg (lower with cardiac risk), ropivacaine about 3 mg/kg, lidocaine 4.5 mg/kg without epinephrine and 7 mg/kg with epinephrine. Add epinephrine to slow absorption and detect intravascular injection, not to rescue an overdose. LAST presents with perioral numbness, tinnitus, seizures, then cardiovascular collapse; stop injection, call for help, secure the airway, and give 20% lipid emulsion per protocol.
Neuraxial and deep blocks require anticoagulation review. Interscalene block risks phrenic nerve palsy; supraclavicular block risks pneumothorax; femoral block causes quadriceps weakness.
Traps
Confusing concentration with total milligrams; ignoring cumulative doses from multiple blocks; attributing LAST to the wrong drug; overlooking anticoagulation guidelines.
Worked example
0.5% bupivacaine contains 5 mg/mL. For a 70 kg patient at 2.5 mg/kg, maximum is 175 mg, or 35 mL. A planned 40 mL would exceed the limit-reduce volume or choose a safer technique.