Dead Space VD/VT — FRCA Primary MCQ

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EasyPhysiology - RespiratoryDead Space VD/VTFRCA Primary

In a healthy adult at rest, with a typical tidal volume of 500 mL, what is the approximate physiological dead-space-to-tidal-volume ratio (VD/VT)?

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Correct answer: A — 0.3

The correct answer is A (0.3). Physiological dead space is the sum of anatomical and alveolar dead space. In a healthy adult, anatomical dead space is approximately 150 mL and alveolar dead space is normally very small, so physiological dead space approximates anatomical dead space. For a typical tidal volume of 500 mL, VD/VT is therefore 150/500 = 0.30, or 30%. A value of 0.2 would underestimate normal dead-space ventilation, whereas 0.5 and 0.7 indicate substantially increased wasted ventilation and may occur with marked ventilation–perfusion inequality or pulmonary vascular disease. A ratio of 0.1 is too low for a normal adult with the stated tidal volume.

Reference: Omole AE, Launico MV. Physiology, Lung Dead Space. StatPearls, updated 15 April 2026. https://pubmed.ncbi.nlm.nih.gov/29494107/