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EDAIC: Relevant basic sciences appropriate for a specialist anaesthesiologist

EDAIC · Work through your exam, one topic at a time.

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Relevant basic sciences appropriate for a specialist anaesthesiologist

Domain 1 of the EDAIC covers applied physiology, pharmacology, physics and equipment, anatomy and clinical measurement as they underpin anaesthetic practice. Part I tests them as single-best-answer items inside a scenario - a change in cardiac output during inhalational induction, a dose calculation, a vaporiser or circuit fault - and Part II revisits the same principles orally, expecting you to justify rather than merely recognise the answer.

Principles that generate the most questions

  • Oxygen delivery. DO2 = CO × CaO2, and CaO2 = (1.34 × Hb × SaO2) + (0.003 × PaO2). At sea level dissolved oxygen adds only ~0.3–1.5 mL/dL, so anaemia and low cardiac output matter more than moderate desaturation. Extraction rises before mixed venous saturation falls; lactate is a late sign.
  • Pharmacokinetics. k = Cl/Vd and t1/2 = 0.693 × Vd/Cl. Context-sensitive half-time lengthens with infusion duration for thiopentone, fentanyl and propofol but stays nearly flat for remifentanil.
  • Potency. MAC is the steady-state alveolar concentration at 1 atm that abolishes movement to a standard stimulus in 50% of patients; ~1.3 MAC is the ED95, MAC-awake is ~0.3–0.4, and MAC values are additive as fractions of 1.
  • Inhalational uptake. Lower blood:gas solubility (desflurane 0.42, sevoflurane 0.65, isoflurane 1.4) means faster induction; more alveolar ventilation speeds it, while higher cardiac output slows it by removing agent. Nitrous oxide adds concentration and second-gas effects.
  • Respiratory physiology. PAO2 = FiO2(PB − 6.3 kPa) − PaCO2/R. A raised A–a gradient indicates V/Q mismatch or shunt, not hypoventilation alone. Poiseuille's law (R ∝ 1/r⁴) explains why halving a tube's radius raises resistance 16-fold.

Worked example

A 70 kg man receives an infusion of a drug with Vd 140 L and clearance 2.1 L/min. What is the elimination half-life, and how long until ~94% of steady state?

k = Cl/Vd = 2.1 ÷ 140 = 0.015 min⁻¹; t1/2 = 0.693 ÷ 0.015 ≈ 46 min. Four half-lives (~184 min) give ~94% of steady state - the reason a loading dose (Vd × target concentration ÷ bioavailability) is given when an immediate effect is needed.

Traps that cost marks

Unit mismatch (mL/min/kg against L/min) is the commonest arithmetic loss. MAC is not a global measure of potency: it says nothing about cardiovascular or analgesic effects. Higher cardiac output slows, not accelerates, inhalational induction. Do not omit the water-vapour term or R in the alveolar gas equation, and do not dose hydrophilic drugs on total body weight in the obese.