Basic Sciences Applied to Anesthesiology: Physics, Equipment, and Monitoring
Physics and equipment
Start with flow and pressure models, because exam questions often disguise simple relationships. Laminar flow follows Poiseuille's law, so resistance rises with the fourth power of radius and depends on viscosity; turbulent flow depends on density and the square of velocity. The Bernoulli principle explains pressure drops at a stenosis, while the Venturi effect explains entrainment of air or nebulized drug. A variable-bypass vaporizer splits fresh gas between a vaporizing chamber and a bypass channel; its output depends on fresh gas flow, temperature, and carrier gas, and it can deliver an unintended concentration if filled with the wrong agent. Desflurane uses an electrically heated, pressurized vaporizer. Oxygen cylinders contain compressed gas; nitrous oxide and carbon dioxide contain liquid plus vapor, so their contents are not linearly proportional to pressure.
Monitoring
Pulse oximetry compares red and infrared absorbance and assumes two species, oxyhemoglobin and deoxyhemoglobin. Carboxyhemoglobin falsely elevates SpO2, methemoglobin drives SpO2 toward about 85%, and poor perfusion, venous congestion, and nail polish cause artifact. Capnography has four phases; the alveolar plateau approximates alveolar PCO2, and end-tidal CO2 underestimates arterial CO2 when dead space or low cardiac output is present. Arterial line accuracy depends on damping and resonance; an underdamped trace exaggerates systolic and underestimates diastolic pressure.
Worked example: An E-cylinder contains 660 L of oxygen at full pressure. At 3 L/min, 660/3 = 220 minutes, but a safe reserve of about 15% leaves roughly 187 minutes. Rule of thumb: cylinder time = usable gas volume divided by flow; never plan to use the last 500 psi.
Trap: do not calculate nitrous oxide cylinder time from pressure alone, because pressure reflects vapor pressure, not remaining liquid volume.