Basic Sciences: Physics, Pharmacology, Physiology, Anatomy
Physics and the machine
Gas behavior underlies the anesthesia machine. Boyle's law (P·V constant at fixed temperature) explains bellows and ventilator function; Graham's law explains why CO2 diffuses roughly 20 times faster than O2 across the alveolar membrane; Henry's law explains why a highly blood-soluble gas such as N2O enters air-filled spaces faster than nitrogen leaves them, expanding a pneumothorax, bowel gas, or middle-ear cavity. Laplace's law (P = 2T/r) predicts that small alveoli need more surfactant than large ones to stay open. For volatile agents, potency is quantified by MAC, the alveolar concentration preventing movement in 50% of patients at a standard stimulus. MAC-awake is roughly one third of MAC, and MAC is a population potency measure, not a depth monitor for one patient. Lipid solubility correlates with potency (Meyer–Overton), and potency correlates inversely with vapor pressure. Context-sensitive half-time lengthens with infusion duration for lipophilic drugs (fentanyl, thiopental) but stays short for propofol and remifentanil.
Physiology
Oxygen delivery is DO2 = CO × CaO2 × 10, where CaO2 = (1.34 × Hb × SaO2) + (0.0031 × PaO2). Tissues consume content, not partial pressure.
Common trap
Examiners test whether raising PaO2 fixes anemia. At Hb 8 g/dL, breathing 100% oxygen adds well under 1 mL/dL of dissolved content against a total near 11.
Worked example
Hb 8 g/dL, SaO2 100%, PaO2 100 mmHg: CaO2 = 1.34 × 8 = 10.7, plus 0.31 = about 11.0 mL/dL. At Hb 14 the same saturation yields about 19.1 mL/dL. Hemoglobin, not PaO2, is the dominant lever.