A mechanically ventilated patient is placed on 100 percent oxygen. Several hours later, chest imaging shows collapse of poorly ventilated dependent lung regions. The physiological mechanism of this absorption atelectasis is:

• A Direct oxygen toxicity destroying type II pneumocytes and abolishing surfactant production
• B High inspired oxygen causes reflex bronchoconstriction through vagal C-fibre activation
• C Hypoxic pulmonary vasoconstriction diverts blood away and starves the alveoli of nutrient supply
• D Nitrogen is washed out, so gas in low V/Q units is absorbed into blood faster than fresh gas can replace it ✓

Explanation

Normally alveolar nitrogen acts as a splint, remaining in the alveolus and limiting net gas absorption. Breathing 100 percent oxygen removes this inert reservoir, so in units with low ventilation-perfusion ratios the entire alveolar gas content is taken up by the blood faster than inspired gas can replenish it, and the unit collapses. Surfactant production and hypoxic vasoconstriction are not the operative mechanisms here, which removes both tempting distractors.

Reference: West's Respiratory Physiology: The Essentials, 11th ed.

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Written and medically reviewed by the StethoPrep medical team.