RPFT Lung Volume Measurement & Diffusing Capacity — 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.
Question 8: A patient's TLC is 65% of predicted and RV is 60% of predicted, with proportional reductions in all lung volumes. This pattern is most consistent with:
- Obstructive lung disease
- Restrictive lung disease (Correct answer)
- Mixed obstructive-restrictive disease
- Normal variation
Correct answer: Restrictive lung disease
A proportional reduction in all lung volumes (TLC < 80% predicted) without disproportionate RV elevation defines a restrictive pattern, indicating reduced lung or chest wall capacity.
Question 9: In a patient with severe emphysema, which combination of lung volume findings is most expected?
- Low TLC, low RV, normal RV/TLC ratio
- High TLC, high RV, elevated RV/TLC ratio (Correct answer)
- Normal TLC, low FRC, low RV
- Low TLC, high RV, normal FRC
Correct answer: High TLC, high RV, elevated RV/TLC ratio
Emphysema causes hyperinflation with increased TLC and severe air trapping with disproportionately elevated RV, resulting in a high RV/TLC ratio.
Question 10: A DLCO reported as 55% of predicted is classified as:
- Mildly reduced
- Moderately reduced (Correct answer)
- Severely reduced
- Normal
Correct answer: Moderately reduced
Per ATS/ERS guidelines, DLCO 40–60% predicted is classified as moderately reduced; below 40% is severely reduced and above 70% is normal.
Question 11: A patient has an obstructive pattern on spirometry with a markedly elevated DLCO. Which diagnosis should be considered?
- COPD with emphysema
- Asthma or pulmonary hemorrhage (Correct answer)
- Idiopathic pulmonary fibrosis
- Pulmonary embolism
Correct answer: Asthma or pulmonary hemorrhage
Polycythemia, pulmonary hemorrhage, or active bleeding into the lungs provides additional hemoglobin for CO binding, elevating DLCO; asthma alone may also show mildly elevated or normal DLCO.
Question 12: Which of the following best explains why helium dilution FRC may be falsely LOW in a patient with a large pulmonary bulla?
- Helium is absorbed by bullous tissue
- The bulla does not communicate with ventilated airways, so helium never reaches it (Correct answer)
- Bullae increase airway resistance and reduce helium mixing
- The patient cannot achieve a valid FRC starting point
Correct answer: The bulla does not communicate with ventilated airways, so helium never reaches it
Bullae are air-filled spaces isolated from the ventilated airway tree; helium cannot enter them during the closed-circuit maneuver, causing an underestimate of true FRC.
Question 13: When interpreting lung volumes, which of the following isolated findings would suggest neuromuscular disease rather than parenchymal restriction?
- Low TLC with disproportionately low RV
- Low TLC with disproportionately elevated RV and low maximal inspiratory pressure (MIP) (Correct answer)
- Normal TLC with elevated RV
- Elevated TLC with low DLCO
Correct answer: Low TLC with disproportionately elevated RV and low maximal inspiratory pressure (MIP)
Neuromuscular disease impairs both inspiration (reducing TLC) and expiration (reducing ability to fully exhale, raising RV); reduced MIP confirms muscle weakness as the etiology.
Question 14: After completing a lung volume measurement by plethysmography, the technologist notices the FRC value is much higher than the VC-derived measurements suggest. The most appropriate next step is:
- Report the plethysmography result without further action
- Check for mouth leaks, panting frequency errors, or shutter timing artifacts and repeat if needed (Correct answer)
- Switch to helium dilution and average both results
- Add 500 mL to account for equipment dead space
Correct answer: Check for mouth leaks, panting frequency errors, or shutter timing artifacts and repeat if needed
Artifacts such as mouth leaks, panting at incorrect frequency (>2 Hz), or shutter timing errors are common sources of falsely elevated plethysmographic FRC and must be investigated before reporting.
