TLC measured by the helium dilution method in a patient with advanced bullous emphysema is significantly lower than TLC measured by whole-body plethysmography. The best explanation is:

• A Helium is absorbed into pulmonary capillary blood during the test
• B High airway resistance prevents helium from reaching the alveolar ducts
• C Plethysmography measures only the gas that participates in tidal ventilation
• D Gas trapped in non-communicating bullae does not equilibrate with inspired helium ✓

Explanation

Helium dilution assumes complete equilibration of the marker gas with all intrathoracic gas; bullae and poorly ventilated regions that do not communicate with the airway opening are missed, so dilution methods underestimate true thoracic gas volume. Plethysmography senses compression of all compressible thoracic gas, including trapped gas, and therefore yields the larger, truer value. Helium is nearly insoluble, ruling out option A.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.