Anesthesia for Surgical Subspecialties
This domain tests whether you can adapt a standard anaesthetic to the physiology the surgeon is about to create: bypass, one-lung ventilation, raised intracranial pressure, the pregnant uterus, the small child, irrigation fluid, or a tourniquet. Expect single-best-answer scenarios built on subspecialty detail.
High-yield specifics
- Cardiac: anticipate hypotension after protamine, vasoplegia on bypass, and low cardiac output during weaning; transoesophageal echo guides volume and contractility.
- Thoracic: double-lumen tubes are sized 35 to 41 French (37 for an average man); hypoxaemia during one-lung ventilation is managed by checking tube position, 100% oxygen, PEEP to the dependent lung, and CPAP to the non-dependent lung.
- Neuro: maintain cerebral perfusion pressure (MAP minus ICP, target 60 to 70 mmHg); total intravenous anaesthesia preserves motor evoked potentials.
- Obstetric: sodium citrate and an H2 blocker for aspiration prophylaxis, left uterine displacement, and phenylephrine for spinal hypotension; magnesium toxicity is treated with calcium gluconate.
- Paediatric: airway resistance is high and FRC low, so desaturation is rapid; uncuffed tube size is (age/4) + 4 mm.
- Urology: TURP syndrome is dilutional hyponatraemia from glycine absorption with confusion, hypertension, and bradycardia; stop surgery and correct the sodium.
- Orthopaedic: tourniquet pain appears after 60 to 90 minutes; bone cement implantation syndrome causes hypoxia and hypotension at cementing.
Worked example
A 4-year-old, 16 kg child for hernia repair. Uncuffed tube size = (4/4) + 4 = 5.0 mm internal diameter; insertion depth at the lip = 3 x diameter = 15 cm. Maintenance fluid by the 4-2-1 rule: 4 mL/kg for the first 10 kg (40 mL) plus 2 mL/kg for the next 10 kg (12 mL) plus 1 mL/kg thereafter (0) = 52 mL/hour. Reasoning: paediatric airway and fluid arithmetic must be calculated before induction, not estimated during the case.
Traps that cost marks
- Using adult tidal volumes of 10 mL/kg in children rather than 6 to 8 mL/kg.
- Missing that TURP irrigation fluid is hypotonic, not saline.
- Attributing hypotension at cementing to anaesthetic depth alone.
- Failing to reduce the volatile agent in an obstetric patient, producing a hypotensive, awake mother.