Basic Sciences Relevant to Anesthesia
What the exam tests
The basic-science domain is not an isolated physics paper; questions usually embed a clinical stem and ask for the principle that explains it. Expect airway and neuraxial anatomy, respiratory and cardiovascular physiology, pharmacokinetics and dynamics, and equipment physics.
Core rules
- Oxygen delivery: DO2 = CO × CaO2, where CaO2 = 1.34 × Hb × SaO2 + 0.003 × PaO2. Dissolved oxygen is small but can matter in anemia.
- MAC is the alveolar concentration preventing movement in 50% of patients to a noxious stimulus; it is a potency index, not an anesthetic-depth monitor. MAC values are additive.
- Clearance and context-sensitive half-time explain recovery: propofol and remifentanil differ markedly from thiopental.
- Cisatracurium undergoes Hofmann elimination, so it is useful in organ failure.
- Poiseuille flow varies with radius to the fourth power; turbulent flow depends on density and velocity. A halved tube radius increases resistance 16-fold for laminar flow.
- In a variable-bypass vaporizer, fresh gas splits between a vaporizing chamber and bypass; desflurane uses a heated, pressurized vaporizer.
Worked example
A patient has Hb 10 g/dL, SaO2 98%, PaO2 100 mmHg, and CO 5 L/min. CaO2 = (1.34 × 10 × 0.98) + (0.003 × 100) = 13.13 + 0.30 = 13.43 mL/dL. DO2 = 50 dL/min × 13.43 = about 672 mL/min. Reasoning: convert L to dL before multiplying; do not omit dissolved oxygen when PaO2 is high.
Traps
Confusing MAC with awareness monitoring; using 1.36 instead of 1.34 (minor but can change an answer); assuming all neuromuscular blockers are organ-dependent; forgetting that a smaller ETT raises resistance dramatically.