Basic and Applied Sciences: Physics, Pharmacology, and Physiology
Core concepts
Anesthesia science questions reward linking mechanism to a number. For drugs, separate volume of distribution (sets the loading dose) from clearance (sets the maintenance infusion rate). Context-sensitive half-time is the time for a 50% fall in effect-site concentration after an infusion stops; it lengthens with infusion duration for lipophilic drugs such as fentanyl, but is nearly constant for remifentanil because plasma esterases clear it independently of hepatic and renal function.
For volatile agents, MAC is the minimum alveolar concentration preventing movement in 50% of patients to a standardized noxious stimulus. Potency is inversely related to MAC, and oil:gas solubility correlates with potency; blood:gas solubility determines speed of induction - low solubility (desflurane, sevoflurane) gives faster equilibration. MAC-awake is lower than MAC.
Physics: Henry's law (partial pressure and dissolved gas), Poiseuille's law (flow proportional to radius to the fourth power), and the Fick principle. Oxygen delivery, DO₂ = CO × CaO₂ × 10, and CaO₂ = 1.34 × Hb × SaO₂ + 0.0031 × PaO₂. A shunt does not correct fully with a higher FiO₂.
Traps
Confusing MAC with an awareness threshold; assuming a long elimination half-life always means slow offset; forgetting that Vd and clearance are independent.
Worked example
Hb 10 g/dL, SaO₂ 100%, PaO₂ 100 mmHg: CaO₂ = 13.4 + 0.31 ≈ 13.7 mL/dL. With CO 5 L/min, DO₂ = 5 × 13.7 × 10 ≈ 685 mL/min. Rule of thumb: each gram of hemoglobin carries about 1.34 mL O₂.