A 55-year-old man with severe emphysema and giant upper-lobe bullae undergoes lung volume measurement. Helium dilution gives a TLC of 5.8 L, while body plethysmography gives a TLC of 7.9 L. Which explanation best accounts for this discrepancy?
- A Helium dilution underestimates true volume because tracer cannot equilibrate with poorly ventilated trapped gas ✓
- B Body plethysmography underestimates volume because thoracic gas is compressed during panting
- C Helium dilution overestimates volume because helium is absorbed into pulmonary blood
- D Plethysmography measures only communicating gas volumes
Correct answer: A. Helium dilution underestimates true volume because tracer cannot equilibrate with poorly ventilated trapped gas
Explanation
Helium dilution requires the tracer to distribute evenly throughout all communicating gas. In bullous emphysema, large pockets of poorly ventilated gas equilibrate slowly or not at all, so helium dilution yields a falsely low volume. Plethysmography uses Boyle's law on all compressible thoracic gas, including noncommunicating spaces, and thus reads higher and closer to the true value here. The reverse error occurs when abdominal gas makes plethysmography overestimate volume.
Reference: West's Respiratory Physiology: The Essentials, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.