A 60-year-old man with severe emphysema undergoes lung volume measurement. Functional residual capacity by helium dilution is 4.2 L, while body plethysmography gives 6.1 L. The best explanation for this discrepancy is:
- A Helium dilution systematically overestimates thoracic gas volume in emphysema
- B Body plethysmography measures only gas communicating with the airway opening
- C Plethysmography detects all compressible thoracic gas, including poorly ventilated bullous spaces that helium cannot reach ✓
- D The difference reflects the volume of pulmonary blood, which only plethysmography excludes
Correct answer: C. Plethysmography detects all compressible thoracic gas, including poorly ventilated bullous spaces that helium cannot reach
Explanation
The helium dilution method depends on complete equilibration of helium with all alveolar gas during the breathing period. In emphysema, large poorly ventilated or bullous regions communicate so slowly with the airway that helium never reaches them, so dilution underestimates true thoracic gas volume. Body plethysmography applies Boyle's law to pressure changes against an occluded shutter and registers every compressible gas pocket in the chest, giving the higher, truer value.
Reference: West's Respiratory Physiology: The Essentials, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.