A smoker with normal spirometry undergoes measurement of dynamic compliance during quiet breathing and then during rapid shallow breathing. Compliance falls markedly at the faster rate. This finding indicates:

• A Uniform time constants throughout the lung
• B Chest wall restriction
• C Reduced surfactant production
• D Early small airway disease with uneven time constants ✓

Explanation

The time constant of a lung unit equals its resistance multiplied by its compliance. When small airways are narrowed unevenly, some units fill slowly and are still inflating when expiration begins, so at high respiratory frequencies they contribute less volume change and measured dynamic compliance falls. Static compliance remains normal because all units eventually equilibrate. Surfactant loss and chest wall restriction lower static and dynamic compliance equally at any rate.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.