On a single-breath nitrogen washout trace, phase IV begins abruptly at low lung volumes in an older long-term smoker but is absent in a healthy young adult tested in the same session. The appearance of a prominent phase IV indicates:

• A Complete emptying of the alveolar dead space
• B Closure of dependent small airways, marking the start of the closing volume ✓
• C Sudden opening of collapsed apical alveoli
• D Transition from tidal to forced expiration

Explanation

During a slow vital capacity exhalation after a 100% oxygen inspiration, phase III is the alveolar plateau. When dependent small airways begin to close, exhaled gas increasingly comes from the oxygen-rich apex, producing the steep phase IV rise. The volume remaining above RV when this starts is the closing volume, which rises with age and early small airway disease such as smoking-related bronchiolitis.

Reference: West's Respiratory Physiology: The Essentials, 10th ed.

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