Anesthesiology: Core Intraoperative Physiology and Pharmacology
What the exam expects
Candidates must reason mechanistically about how anesthetic drugs and ventilation strategies interact with cardiovascular and respiratory physiology. Core concepts include the alveolar gas equation, oxygen delivery (DO2 = CO × CaO2), the oxyhemoglobin dissociation curve and its shifts, volatile agent MAC and its modifiers (age, temperature, pregnancy, ethanol, opioids), and the pharmacodynamics of induction agents and neuromuscular blockers.
Concepts to master
Know that MAC decreases roughly 6% per decade after age 40, that hypothermia and pregnancy lower MAC, and that chronic ethanol raises it. Understand context-sensitive half-time for propofol and remifentanil, and why remifentanil's is constant. For paralytics, distinguish depolarizing from nondepolarizing mechanisms and the pseudocholinesterase deficiency phenotype (prolonged block after succinylcholine, normal after other agents). Ventilation: dead space, shunt, and the effect of PEEP on preload.
Traps
Questions love to invert relationships. A higher MAC means less potency. Hypoxia from shunt does not correct with supplemental oxygen as readily as hypoxemia from hypoventilation. A low SvO2 suggests increased extraction, not necessarily low delivery. Watch units: CaO2 in mL/dL, DO2 in mL/min.
Worked rule of thumb
For a patient with Hb 10 g/dL, SaO2 98%, PaO2 100 mmHg: CaO2 ≈ (1.34 × 10 × 0.98) + (0.003 × 100) = 13.1 + 0.3 = 13.4 mL/dL. Multiply by cardiac output (×10 to convert L to dL) to get DO2. If CO = 5 L/min, DO2 ≈ 670 mL/min. This stepwise calculation is a common multi-step item.
Exam strategy
Anchor each answer in a physiologic equation or receptor mechanism; eliminate options that violate a conservation principle before choosing.