Describe the types of dead space in the respiratory system (50% marks). Explain the consequences of increased dead space on gas exchange (50% mark).
A definition of dead space incorporating subtypes (anatomical, apparatus, alveolar,
physiological) was expected. Explanation of measurement methods attracted
additional marks. Changes to End-tidal CO2 relative to PaCO2 were relevant to the
question. The Bohr equation was stated and variables defined in better answers.
Causes of an increased dead space should have been described including
hypoperfusion and increased alveolar pressure.
Increased dead space primarily results in CO2 retention, unless minute ventilation is
increased commensurately. The physiological effects of increased PaCO2,
increased respiratory rate and work of breathing are central to the question.
Many candidates predicted severe hypoxemia; however the alveolar gas equation
was not stated to explain this observation. Hypoxemia is a relatively late effect of
significant hypo-ventilation, especially if the patient is breathing supplemental O2.
Syllabus: B1e 2c
Reference: Nunn’s Applied Respiratory Physiology p310-311. Principles of
Physiology for the Anaesthetist, Power & Kam p84-87