Lung Volume Measurement & Diffusing Capacity Flashcards

Which technique measures total lung capacity (TLC) by having the patient breathe a known concentration of helium in a closed circuit until equilibration?

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.

During body plethysmography for FRC measurement, the patient performs panting maneuvers against a closed shutter. This technique measures:

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.

In the nitrogen washout technique for measuring FRC, the patient breathes 100% oxygen, and FRC is calculated from:

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.

Which lung volume compartment represents the volume of air remaining in the lungs after a maximal forced exhalation?

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.

An elevated RV/TLC ratio in a patient's pulmonary function results most commonly indicates:

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.

When body plethysmography overestimates lung volumes compared to helium dilution in the same patient, this discrepancy most likely indicates:

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.

During a helium dilution FRC maneuver, the test should be terminated when end-tidal helium concentration changes by less than:

Answer: 0.02% over 30 seconds

ATS/ERS guidelines define equilibration as < 0.02% change in He concentration over 30 seconds, ensuring accurate measurement.

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:

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.

In a patient with severe emphysema, which combination of lung volume findings is most expected?

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.

A DLCO reported as 55% of predicted is classified as:

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.

A patient has an obstructive pattern on spirometry with a markedly elevated DLCO. Which diagnosis should be considered?

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.

Which of the following best explains why helium dilution FRC may be falsely LOW in a patient with a large pulmonary bulla?

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.

When interpreting lung volumes, which of the following isolated findings would suggest neuromuscular disease rather than parenchymal restriction?

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.

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:

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.