Basic Sciences Applied to Anesthesia
What the exam tests
This domain asks you to connect anatomy, physiology, physics, and pharmacology to safe anesthesia practice. Expect airway anatomy, oxygen delivery, ventilation, gas laws, flow, vaporizers, and MAC.
Core rules and concepts
- The adult trachea is about 10–12 cm long; the right main bronchus is wider and more vertical, so a deep tube tends to enter the right lung.
- Oxygen delivery is DO2 = cardiac output × arterial oxygen content; haemoglobin and saturation matter more than dissolved oxygen.
- Functional residual capacity is roughly 30 mL/kg and is the main oxygen reservoir during apnea; obesity, pregnancy, and supine posture reduce it.
- Laminar flow follows the Hagen–Poiseuille relationship, so halving tube radius greatly increases resistance; turbulent flow depends on density and velocity.
- MAC is the alveolar concentration at which 50% of patients do not move to a surgical stimulus. MAC values are not strictly additive, and nitrous oxide has a MAC above 100%.
- Vaporizer output depends on saturated vapor pressure, fresh-gas flow, temperature, and splitting ratio; desflurane needs a heated, pressurized vaporizer.
Worked example
An E-cylinder contains 660 L of oxygen. At 5 L/min, 660 ÷ 5 = 132 minutes of theoretical use. Because you must preserve a reserve and account for gauge inaccuracy, plan to change the cylinder well before 132 minutes-often at about 30% remaining, giving roughly 90 usable minutes.
Traps
Do not assume a cylinder is full because the gauge is in the green; pressure falls only when liquid is exhausted. Do not add MAC percentages arithmetically without adjusting for potency. Do not confuse minute ventilation with alveolar ventilation.