RPFT Lung Volume Measurement & Diffusing Capacity 1 — Questions and Answers
Question 1: Which technique measures total lung capacity (TLC) by having the patient breathe a known concentration of helium in a closed circuit until equilibration?
- Body plethysmography
- Helium dilution (Correct answer)
- Nitrogen washout
- Impulse oscillometry
Correct answer: Helium dilution
Helium dilution uses an insoluble tracer gas (helium) breathed in a closed circuit; TLC is calculated from the dilution factor at equilibration.
Question 2: During body plethysmography for FRC measurement, the patient performs panting maneuvers against a closed shutter. This technique measures:
- Only communicating lung volumes
- Both communicating and non-communicating (trapped) gas (Correct answer)
- Alveolar gas only
- Dead space volume exclusively
Correct answer: Both communicating and non-communicating (trapped) gas
Body plethysmography uses Boyle's Law to measure total thoracic gas volume, including trapped (non-communicating) gas that dilution methods miss.
Question 3: In the nitrogen washout technique for measuring FRC, the patient breathes 100% oxygen, and FRC is calculated from:
- The inspired helium concentration at equilibration
- The total volume of nitrogen exhaled until end-tidal N2 reaches <2% (Correct answer)
- The change in box pressure during panting
- The peak expiratory flow rate achieved
Correct answer: The total volume of nitrogen exhaled until end-tidal N2 reaches <2%
Nitrogen washout collects all exhaled N2 over successive breaths until < 2% N2 remains; FRC is back-calculated from the known initial alveolar N2 (~80%) and total N2 exhaled.
Question 4: Which lung volume compartment represents the volume of air remaining in the lungs after a maximal forced exhalation?
- Expiratory reserve volume (ERV)
- Residual volume (RV) (Correct answer)
- Functional residual capacity (FRC)
- Inspiratory reserve volume (IRV)
Correct answer: Residual volume (RV)
Residual volume (RV) is the volume remaining after maximal exhalation and cannot be measured by simple spirometry; it requires gas dilution or plethysmography.
Question 5: An elevated RV/TLC ratio in a patient's pulmonary function results most commonly indicates:
- Restrictive lung disease
- Air trapping consistent with obstructive lung disease (Correct answer)
- Respiratory muscle weakness
- Pulmonary vascular disease
Correct answer: Air trapping consistent with obstructive lung disease
A high RV/TLC ratio (>35–40%) indicates that a disproportionate fraction of TLC is trapped gas, a hallmark of obstructive conditions such as COPD or emphysema.
Question 6: When body plethysmography overestimates lung volumes compared to helium dilution in the same patient, this discrepancy most likely indicates:
- Instrument calibration error in the plethysmograph
- The presence of poorly ventilated or non-communicating lung regions (Correct answer)
- Patient hyperventilation during helium washout
- An error in the BTPS correction factor
Correct answer: The presence of poorly ventilated or non-communicating lung regions
Plethysmography measures all thoracic gas including bullae and trapped regions, while helium dilution only reaches ventilated areas; a large discrepancy signals significant trapped gas or bullous disease.
Question 7: During a helium dilution FRC maneuver, the test should be terminated when end-tidal helium concentration changes by less than:
- 5% over 30 seconds
- 0.02% over 30 seconds (Correct answer)
- 1% over 1 minute
- 10% over 2 minutes
Correct answer: 0.02% over 30 seconds
ATS/ERS guidelines define equilibration as < 0.02% change in He concentration over 30 seconds, ensuring accurate measurement.
RPFT Lung Volume Measurement & Diffusing Capacity 1
Which technique measures total lung capacity (TLC) by having the patient breathe a known concentration of helium in a closed circuit until equilibration?