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Egyptian Board Anesthesia study materials
Egyptian Board Anesthesia study resources

Egyptian Board Anesthesia: Basic Sciences Applied to Anesthesia: Physiology, Pharmacology, and Equipment Physics

Egyptian Board Anesthesia · Work through your exam, one topic at a time.

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Basic Sciences Applied to Anesthesia: Physiology, Pharmacology, and Equipment Physics

This domain tests the physiology, pharmacology, and physics that explain why anaesthetic interventions work or fail, from the oxygen cascade to MAC and the behaviour of gases in cylinders and vaporisers. Questions are usually clinical vignettes requiring an equation or a mechanism rather than recall of isolated numbers.

Respiratory and oxygen physiology

  • Alveolar gas equation: PAO2 = FiO2 x (PB - 47) - PaCO2/R, with R about 0.8.
  • Oxygen delivery: DO2 = CO x CaO2, where CaO2 = (1.34 x Hb x SaO2) + (0.003 x PaO2). The dissolved fraction is negligible at atmospheric pressure.
  • Shunt is the only cause of hypoxaemia that does not improve with increasing FiO2; V/Q mismatch does.
  • Functional residual capacity falls with supine position, obesity, pregnancy and induction, while closing capacity rises with age; when closing capacity exceeds FRC, airway closure occurs during tidal breathing.

Pharmacology

  • Meyer-Overton: potency tracks lipid solubility, so MAC is inversely proportional to potency; a low MAC means a potent agent.
  • pKa determines the unionised fraction and therefore onset speed; the closer the pKa to tissue pH, the faster the block.
  • Amide local anaesthetics are hepatically cleared; ester agents are hydrolysed by plasma esterases.
  • Offset of a single propofol or thiopentone dose follows redistribution, not metabolism; context-sensitive half-time explains slow emergence after prolonged infusions.

Physics and equipment

  • Poiseuille law: laminar flow is proportional to r to the fourth power divided by viscosity and length, so halving the radius cuts flow 16-fold.
  • Bernoulli and Venturi principles underlie jet ventilation, nebulisers, and oxygen mask air entrainment.
  • A variable-bypass vaporiser splits fresh gas between a vaporising chamber and a bypass; a plenum vaporiser requires positive upstream pressure.

Worked example

A patient at sea level (PB 760 mmHg) breathing room air has PaCO2 40 mmHg. PAO2 = 0.21 x (760 - 47) - 40/0.8 = 0.21 x 713 - 50 = 149.7 - 50 = about 100 mmHg. Reasoning: subtract water vapour pressure first, then correct for CO2 production. With a normal A-a gradient of 10 mmHg, expected PaO2 is about 90 mmHg; a much lower value suggests shunt or V/Q mismatch.

Traps that cost marks

  • Omitting the 47 mmHg water-vapour factor, or dividing by R = 1.
  • Confusing alveolar (PAO2) and arterial (PaO2) symbols.
  • Assuming a larger MAC means a stronger drug.
  • Attributing rapid propofol offset to fast hepatic clearance.