Regional Anaesthesia and Pain Medicine
This domain examines local anaesthetic pharmacology and dose calculation, block selection and safety, neuraxial technique and its contraindications, and multimodal acute and cancer pain management. PGIM questions are typically numerical - a dose that exceeds the maximum, or a patient who deteriorates minutes after injection.
Local anaesthetics
- All are weak bases that block voltage-gated sodium channels from the internal pore. Onset depends on pKa (a pKa closer to 7.4 means more unionised base and faster onset), potency on lipid solubility, and duration on protein binding and vasodilatation.
- Maximum safe doses in a 70 kg adult: lidocaine 3 mg/kg plain or 7 mg/kg with adrenaline; bupivacaine 2 mg/kg; ropivacaine 3 mg/kg. Adrenaline slows systemic absorption and prolongs the block.
- Local anaesthetic systemic toxicity: stop injecting, call for help, secure the airway with 100% oxygen, control seizures with a benzodiazepine, and give 20% lipid emulsion 1.5 mL/kg bolus followed by 15 mL/kg/h.
Neuraxial block
Spinal anaesthesia for caesarean section uses hyperbaric 0.5% bupivacaine 2.2–2.5 mL; block height depends on dose, baricity, patient height and position. Contraindications are patient refusal, coagulopathy, local sepsis, hypovolaemia and raised intracranial pressure.
Acute pain
Multimodal analgesia combines paracetamol, an NSAID where safe, opioid-sparing agents (ketamine, dexmedetomidine, lidocaine infusion) and a regional block wherever the site allows. Perineural adjuvants include dexamethasone, dexmedetomidine and adrenaline.
Worked example
An axillary brachial plexus block is performed with 40 mL of 0.5% bupivacaine in a 70 kg patient. That is 200 mg, or 2.9 mg/kg - above the 2 mg/kg limit. Ten minutes later she reports tinnitus and a metallic taste, then seizes. This is LAST. Stop the injection, secure the airway with 100% oxygen, give midazolam 2–4 mg, and start lipid emulsion at 1.5 mL/kg bolus with a 15 mL/kg/h infusion.
Traps
Not aspirating before incremental injection; exceeding the dose because the percentage looks small; dismissing early perioral tingling as anxiety; believing an adrenaline-containing solution has no ceiling dose; and assuming a negative epidural test dose excludes later intravascular migration of a catheter.