Applied Physiology, Pharmacology and Physics in Anaesthesia
This domain tests the physical principles behind monitors, breathing systems and vaporisers, together with applied physiology and clinical pharmacokinetics. Questions are often calculation-based: an alveolar gas equation, a cylinder duration, a MAC fraction, or a context-sensitive half-time.
Physiology
- The oxygen cascade runs from humidified inspired gas at about 20 kPa, to alveolar 13.3 kPa, to arterial 12.6 kPa, to mitochondria at 0.5–3 kPa. Alveolar oxygen is calculated as FiO2 × (Patm − 47) − PaCO2/0.8, and the A–a gradient is that value minus PaO2.
- Laminar flow obeys Poiseuille: flow is proportional to radius to the fourth power and inversely proportional to viscosity. Turbulent flow appears above a Reynolds number of 2000 and its pressure drop is proportional to gas density and the square of velocity, which is why heliox relieves upper airway obstruction.
Equipment physics
- Modern vaporisers are variable-bypass, temperature-compensated and plenum-driven. Desflurane requires a heated (39 °C) and pressurised (2 atmospheres) vaporiser because of its high vapour pressure.
- An E-cylinder holds 680 L of oxygen as gas; a nitrous oxide cylinder holds liquid, so the pressure gauge reads constant until the liquid is exhausted.
Pharmacology
- MAC values in young adults: sevoflurane 2, isoflurane 1.15, desflurane 6, nitrous oxide 105 vol%. MAC values are additive, so 0.5 MAC sevoflurane plus 0.5 MAC nitrous oxide is approximately 1 MAC.
- Context-sensitive half-time increases with infusion duration for propofol and fentanyl but remains short and constant at 3–4 minutes for remifentanil.
Worked example
A patient breathing room air has PaCO2 5.3 kPa and PaO2 10 kPa. Alveolar oxygen = 0.21 × (101.3 − 6.3) − 5.3/0.8 = 19.95 − 6.6 = 13.3 kPa. The A–a gradient is 13.3 − 10 = 3.3 kPa, which is raised for a young adult (normal under about 2 kPa, widening with age) and points to shunt or ventilation–perfusion mismatch.
Traps
Confusing MAC with MAC-awake; forgetting to subtract water vapour pressure (47 mmHg) in the alveolar gas equation; assuming nitrous oxide cylinder pressure reflects remaining contents; and calculating doses on actual body weight in the obese.