Anaesthesiology
The anaesthesiology domain tests applied physiology, pharmacology, equipment, airway and regional techniques, and crisis management at final MCQ level. Most stems turn on one dose, contraindication, or algorithm step rather than broad theory.
Core rules and numbers
- Potency: MAC is the end-tidal concentration preventing movement to a standard stimulus in 50% of patients. MAC awake is roughly 0.3–0.4; MAC values are additive, and nitrous oxide alone cannot reliably produce surgical anaesthesia.
- Pharmacokinetics: context-sensitive half-time rises with infusion duration for propofol and fentanyl; remifentanil is context-insensitive because nonspecific esterases hydrolyse it.
- Local anaesthetics: lidocaine 3 mg/kg plain or 7 mg/kg with adrenaline; bupivacaine 2 mg/kg; ropivacaine 3 mg/kg. Use lean or adjusted body weight in obesity. 1% = 10 mg/mL.
- Airway: follow ASA/DAS algorithms; optimise oxygenation, use videolaryngoscopy and a supraglottic airway for predicted difficulty, and proceed to front-of-neck access in cannot-intubate-cannot-oxygenate.
- MH: volatile agents and suxamethonium trigger malignant hyperthermia; stop triggers, give dantrolene 2.5 mg/kg IV, hyperventilate with 100% oxygen, cool, and treat hyperkalaemia.
- LAST: stop injection, secure airway, give 20% lipid emulsion 1.5 mL/kg bolus then 0.25 mL/kg/min, and use small adrenaline doses if needed.
Worked example
A 70 kg woman is booked for a fascia iliaca block with ropivacaine 0.75%. Convert the strength: 0.75% = 7.5 mg/mL. The maximum dose is 3 mg/kg = 210 mg. Maximum volume is 210 ÷ 7.5 = 28 mL. A planned 30 mL would deliver 225 mg, 15 mg above the limit. Correct options are 28 mL of 0.75% or 30 mL of 0.5% (150 mg) if a larger volume is needed.
Traps that cost marks
- Confusing percentage strength with mg/mL.
- Using total body weight in obesity for local anaesthetic or propofol infusions.
- Giving suxamethonium in hyperkalaemia, burns, or denervation.
- Missing LAST because seizures are absent and onset is delayed.
- Continuing attempts at intubation instead of declaring CICO and performing front-of-neck access.
- Treating Mallampati class as a reliable predictor of difficult mask ventilation.