Closing capacity — FRCA Primary MCQ

Correct answer: B — Closing capacity exceeding FRC causes airway closure

When closing capacity exceeds FRC, airway closure occurs during tidal breathing. Hypocapnia causes cerebral vasoconstriction is less appropriate because it would fit a different clinical clue or a separate anaesthetic problem. Reduced haemoglobin lowers arterial oxygen content is less appropriate because it would fit a different clinical clue or a separate anaesthetic problem. Raised PaCO2 lowers alveolar oxygen tension is less appropriate because it would fit a different clinical clue or a separate anaesthetic problem. Hypoxic pulmonary vasoconstriction diverts blood flow is less appropriate because it would fit a different clinical clue or a separate anaesthetic problem. In Primary FRCA physiology, the correct answer is usually the variable that best explains the observed change in the anaesthetised patient.

Reference: West's Respiratory Physiology; Ganong Review of Medical Physiology; Primary FRCA textbooks