9.3 Evaluating Lung Volume Validity

Validity Is Domain II.C—Not Optional Paperwork

After selecting and performing static lung-volume methods, the NBRC outline expects you to evaluate validity: II.C.8.a (gas dilution) and II.C.8.b (body plethysmography). A clean-looking printout with wrong FRC still harms clinical decisions (surgery risk, hyperinflation assessment, restriction vs obstruction).

Validity has three layers:

1. Technical signal quality — analyzers, seals, shutter, thermal baseline, end-points
2. Maneuver quality — pant loops, leak-free seal, correct starting volume, patient cooperation within ability
3. Internal consistency — reproducibility across trials; TLC/RV linkage; comparison between methods when both are done

Acceptability of Pant Maneuvers (Body Box)

What a good TGV pant looks like

• Shutter fully closed at the intended lung volume (usually FRC)
• Closed loops relating Pmouth and Pbox with a stable, readable slope
• Gentle frequency and amplitude without wild pressure spikes
• No progressive baseline drift of Pbox during the occlusion from thermal or leak effects
• Several acceptable trials with TGV reproducibility within lab/ATS-aligned limits (know the concept of repeatability; exact mm limits follow current standards and software)

Reject patterns (high-yield)

Effort vs validity: Maximal straining is not the goal. A frail patient with gentle, reproducible loops may produce more valid TGV than a strong patient thrashing against the shutter.

Leak Detection

Body box

Leaks occur at the cabin (door gasket, ports) or patient interface (mouthpiece, nose, shutter). Clues:

• Failure of manufacturer leak check
• Pbox cannot hold a stable baseline with door closed
• TGV nonsensically low or unstable without physiologic explanation
• Patient or technologist hears/feels air escape during occlusion

Gas dilution / N₂ washout

• He dilution: unexpected fall in He not explained by lung uptake → room-air in-leak → FRC overestimated
• N₂ washout: secondary N₂ rise, failure to reach end-tidal threshold, or excess N₂ recovery → room-air leak → FRC overestimated
• Mouthpiece leak with lost volume on the spirometer side corrupts linked IC/ERV even if FRC math looks complete

Validity rule: if a leak is plausible, do not “correct by eye.” Fix the seal and repeat the measurement.

Thermal Drift

A closed cabin with a warm patient is a heat source. Until temperature stabilizes, Pbox baseline drifts. Measuring ΔPbox for Boyle’s law on a moving baseline injects systematic error into TGV.

Validity actions:

• Require software/stability criteria before accepting trials
• After prolonged door-open cleaning, re-equilibrate each patient
• If drift appears mid-series, pause, stabilize, repeat—do not average drifted trials with stable ones without review

Thermal drift is an equipment–environment validity issue, not proof of restriction or obstruction.

Comparing Box FRC vs Dilution FRC

When both methods are acceptable:

Clinically meaningful difference is interpreted with disease context, absolute liters, and quality grades—not a single universal cutoff memorized in isolation. Exam stems often use a large gap in COPD to point to trapping when both tests are described as technically good.

Do not average box and dilution FRC into a blended number that hides method disagreement. Report method-specific values and the implication of discordance.

Gas-Dilution Validity Specifics (II.C.8.a)

Check before release:

1. Start at FRC — end-expiratory switch-in documented/coached
2. End-point met — He stability criterion or N₂ washout threshold actually reached
3. No leak signatures in tracer tracings
4. Time to equilibration / washout plausible; extremely long times with never-stable tracer → incomplete communication; consider box or report limitation
5. Linked IC/ERV/VC acceptable and internally consistent (TLC vs RV+VC)
6. Reproducibility — obtain at least two technically satisfactory trials; the reported FRC is the mean of acceptable results that agree within 10% (ERS/ATS lung volumes, 2023 update). Wait at least twice the washout/dilution time between maneuvers, longer in severe obstruction or bullous disease
7. Inspired O₂ / CO₂ control adequate during He rebreathing (patient distress or volume drift flags problems)

Reject examples:

• Washout stopped at end-tidal N₂ still 8% “because the patient was tired,” without a valid early-termination protocol note
• He still falling steeply when the test auto-stopped on time limit without equilibration
• VC from linkage 1.0 L lower than sitting spirometry VC without explanation

Body-Box Validity Specifics (II.C.8.b)

Check before release:

1. Door sealed; thermal baseline stable
2. Shutter timing at intended volume
3. Acceptable pant loops; reject artifact trials from the average
4. TGV repeatability — aim for at least three acceptable FRCpleth values agreeing within 5%, calculated as (largest FRC − smallest FRC) ÷ mean FRC, and report the average (ERS/ATS lung volumes, 2023 update). Sets exceeding 5% may still be usable, but say so in the quality comment rather than reporting them silently
5. Linked spirometry yields coherent TLC/RV
6. Comparison with spirometry pattern: e.g., reported TLC 2.5 L with FVC 4.0 L is physiologically inconsistent—something is invalid
7. If Raw measured, ensure occlusion vs open-shutter states were not mis-assigned in software selection

Common artifacts and reject/repeat decisions

Integrated Validity Scenario

Same-day results:

• N₂ washout FRC 2.9 L, reached the 1/40th-of-starting (~2% N₂) end-point over three consecutive breaths, two trials within 5% of the mean
• Body box TGV trials: 4.5, 4.6, 3.1 L — the 3.1 L trial shows open loops and door seal concern noted by technologist
• After reseating door gasket and re-equilibration, TGV 4.5 and 4.55 L
• Linked IC/ERV coherent with TLC and elevated RV/TLC

Valid report: dilution FRC ≈ 2.9 L; box FRC ≈ 4.5 L; interpret difference as trapping; exclude the 3.1 L artifact trial. Releasing a mean TGV that includes 3.1 L would falsely lower box FRC and shrink the trapping signal—exactly the II.C.8.b failure mode.

Reporting Discipline

Valid lung-volume reports state:

• Method(s) used (He, N₂, body box)
• FRC/TGV, TLC, RV, RV/TLC (and reference comparisons when provided)
• Quality comments (equilibration time, pant quality, leaks corrected, trials rejected)
• Method discordance when clinically relevant

Invalid data withheld or clearly flagged beats a tidy table of wrong liters. That judgment is core RPFT practice under II.C.8.a/b.

Link to Practice

/practice/rpftPractice questions with detailed explanations