Basic Sciences Applied to Anesthesia: Anatomy, Physiology, Pharmacology, Physics and Equipment
The basic-science domain tests whether you can derive a clinical decision from first principles rather than recall isolated facts. Expect calculation items and 'why' questions linking physiology, pharmacology and physics to the machine on your left.
Physiology you must be able to quantify
- Oxygen delivery: DO₂ = CO × CaO₂ × 10; CaO₂ = 1.34 × Hb × SaO₂ + 0.003 × PaO₂. Critical DO₂ is about 330 mL/min.
- Closing capacity rises with age and exceeds FRC in the supine position from the third decade, producing dependent atelectasis and shunt.
- Hypoxic pulmonary vasoconstriction diverts blood from poorly ventilated lung; it is blunted by volatile agents above 1 MAC and by vasodilators.
Pharmacology anchors
- MAC: isoflurane 1.15%, sevoflurane 2.0%, desflurane 6.0%, nitrous oxide 105% - so N₂O cannot deliver 1 MAC at one atmosphere.
- Succinylcholine 1–1.5 mg/kg gives 5–10 minutes of block; prolonged block suggests pseudocholinesterase deficiency (dibucaine number below 20 in homozygotes).
- Sugammadex 2 mg/kg reverses a train-of-four count of 2 or more, 4 mg/kg for deep block, 16 mg/kg after a 1.2 mg/kg rocuronium intubating dose.
Physics and equipment
- Poiseuille: flow is proportional to radius to the fourth power, so halving radius cuts flow 16-fold; turbulence (Reynolds above 2000) makes flow pressure-dependent.
- An E-cylinder holds about 625 L of oxygen at 13,700 kPa; the gauge reads pressure, which does not fall linearly as volume is used.
Worked example
A 70 kg man with Hb 10 g/dL, SaO₂ 98%, PaO₂ 100 mmHg and cardiac output 5 L/min. CaO₂ = (1.34 × 10 × 0.98) + (0.003 × 100) = 13.1 + 0.3 = 13.4 mL/dL. DO₂ = 5 × 13.4 × 10 = 670 mL/min, roughly 60% of normal - so tolerance to further anaemia or hypotension is small.
Traps
Confusing MAC with MAC-awake (about 0.3–0.4 MAC); applying 6 mL/kg to actual rather than predicted body weight; assuming a cylinder pressure gauge shows remaining volume in a liquid-containing cylinder.