Basic Sciences: Physics, Physiology, Pharmacology, Pathophysiology
Physics and equipment
Laminar flow varies with radius to the fourth power (halving a tube's diameter cuts flow 16-fold), while turbulent flow depends on gas density and is quantified by the Reynolds number -- the principle behind helium-oxygen mixtures lowering resistance in upper airway obstruction. Variable-bypass vaporizers split fresh gas between a bypass channel and a vaporizing chamber; desflurane requires a heated, pressurized canister. A nitrous oxide cylinder contains liquid, so its gauge reads a constant pressure until the liquid is exhausted, whereas oxygen pressure falls linearly with content.
Equations to be able to manipulate
Alveolar gas equation: PAO2 = FiO2(Pb - 47) - PaCO2/0.8. Oxygen delivery: DO2 = CO x CaO2 x 10, where CaO2 = 1.34 x Hb x SaO2 + 0.003 x PaO2. Cerebral perfusion pressure = MAP - ICP. Shunt, dead space (Bohr), and the Fick principle explain hypoxemia that does not correct with supplemental oxygen.
Pharmacology
MAC is the alveolar concentration at one atmosphere preventing movement in 50% of subjects: a potency measure, not an efficacy measure, and MAC values are additive (0.6 + 0.4 = 1 MAC). Blood:gas partition coefficient governs speed of equilibration, not strength. Context-sensitive half-time describes redistribution-driven offset after infusions. Weak bases (local anesthetics) are ion-trapped in acidotic fetal or abscess fluid. Neonates and the elderly have lower MAC and larger water-soluble volumes of distribution per kg.
Traps
Confusing partial pressure with concentration; assuming a high partition coefficient means greater potency; forgetting nitrous oxide's second-gas and cuff-diffusion effects; omitting the 47 mmHg of water vapor; applying adult mg/kg dosing to neonates.
Worked example
FiO2 0.6, PaCO2 40, sea level: PAO2 = 0.6(760 - 47) - 50 = 378 mmHg. A PaO2 of 90 with that alveolar value implies substantial shunt or V/Q mismatch, not hypoventilation.