Anaesthesia: Core Concepts, Traps and a Worked Rule of Thumb
The frame of the day
Candidates must move fluently between pharmacology, physiology, equipment and crisis management. The examiner's favourite trick is to test why a routine is routine: pre-oxygenation, fasting, aspiration prophylaxis, checking suction and a second laryngoscope, and the WHO-style team briefing. Know your airway plan A-D and be able to say when you escalate.
Pharmacology that gets tested
Understand context-sensitive half-time, effect-site versus plasma targeting, and why remifentanil washes out fast while fentanyl does not. For muscle relaxants, know onset, duration, and reversal: sugammadex dose depends on depth of block (2 mg/kg for TOF count 1-2, 4 mg/kg for post-tetanic count 1-2, 16 mg/kg for immediate rescue). Volatiles: MAC, second-gas effect, diffusion hypoxia, and the impact of cardiac output on induction speed.
Physiology and the traps
Trap one: assuming a normal blood pressure means adequate perfusion - think urine output, lactate, mentation, capillary refill. Trap two: confusing hypoxia with hypoxaemia; a low SpO2 with a normal PaO2 suggests poor peripheral perfusion or metHb. Trap three: forgetting that pregnancy, obesity, sepsis and the elderly all change dose requirements and desaturation timelines. Trap four: ignoring the difference between a difficult mask and a difficult intubation.
Worked rule of thumb
For any unexpected desaturation, run 'DOPE' - Displacement, Obstruction, Pneumothorax, Equipment failure - before reaching for a second tube. In a can't-intubate can't-oxygenate scenario, the sequence is: call for help, optimise mask, insert a supraglottic airway, then front-of-neck access. Say the sequence out loud in your answer; examiners reward structured thinking.
Crisis resource language
Use clear, closed-loop communication and declare a plan with time limits ('if SpO2 <90% in 60 seconds, we proceed to front-of-neck').