Regional Anesthesia and Acute Pain Management
Regional anaesthesia questions test three things at once: the anatomy of the target, the dose of local anaesthetic that is safe, and what to do when it goes wrong. Acute pain overlaps heavily, so learn multimodal and opioid-sparing strategies as one topic.
Local anaesthetic arithmetic
- 1% solution = 10 mg/mL; 0.5% = 5 mg/mL; 0.25% = 2.5 mg/mL.
- Maximum doses in a healthy adult: lidocaine 3 mg/kg plain and 7 mg/kg with adrenaline; bupivacaine 2 mg/kg; levobupivacaine 2.5 mg/kg; ropivacaine 3 mg/kg. Account for every compartment you inject, including surgeon infiltration.
- Additives: adrenaline prolongs block and marks intravascular injection; dexmedetomidine and dexamethasone extend duration.
- Lipid emulsion 20% is the antidote for systemic toxicity: 1.5 mL/kg bolus, then 15 mL/kg/hour with repeat boluses, alongside airway management and seizure control.
Neuraxial rules
- Spinal anaesthesia with hyperbaric bupivacaine allows control of block height; hypotension is expected, so prevent it with fluids and a vasopressor.
- Epidural haematoma presents as new back pain with progressive motor and sensory loss and is a surgical emergency requiring urgent MRI.
- Antithrombotic timing: after prophylactic low molecular weight heparin wait 12 hours before neuraxial placement, and 4 hours after needle placement before the next dose.
- Post-dural puncture headache is postural; treat with hydration, caffeine, and an epidural blood patch after 24 to 48 hours of failed conservative care.
Worked example
A 70 kg patient needs a brachial plexus block and you draw 30 mL of 0.5% bupivacaine. Dose = 30 x 5 = 150 mg. Maximum = 2 x 70 = 140 mg. This exceeds the ceiling by 10 mg, so reduce to 28 mL or switch to a lower concentration for a volume-dependent block. Reasoning: convert percentage to mg/mL first, then divide the calculated maximum by the concentration to get the permitted volume.
Traps that cost marks
- Forgetting local anaesthetic already given by the surgeon.
- Assuming a negative aspiration excludes intravascular injection.
- Treating lidocaine with adrenaline as if it were plain lidocaine.
- Mistaking bilateral leg weakness after epidural placement for anxiety rather than haematoma.