9.2 Body Plethysmography Lung Volumes

Why Body-Box Volumes Are High-Yield for RPFT

Under the PFT DCO, body-plethysmographic lung volumes are a distinct skill path from gas dilution: II.A.8.b (select) and II.B.8.b (perform). Equipment setup and calibration of the cabin were Domain I topics; this section is the patient procedure that produces FRC (as TGV), TLC, and RV.

Thoracic gas volume (TGV or VTG) is the compressible gas volume in the thorax at the moment of shutter closure. When closure is timed at relaxed end-expiration, TGV ≈ FRC_box (often called plethysmographic FRC).

Boyle’s Law Applied to TGV

In a constant-volume (variable-pressure) body box, the patient sits in a sealed cabin. At end-expiration the mouthpiece shutter closes, stopping airflow at the mouth. The patient makes small panting efforts. With no flow past the shutter:

• Mouth pressure (Pmouth) approximates alveolar pressure changes.
• Chest expansion/compression changes cabin free-gas volume, producing box pressure (Pbox) swings.

Under near-isothermal conditions:

P₁V₁ = P₂V₂

For small changes, thoracic gas volume is recovered from the relationship between ΔPmouth and ΔPbox and the calibrated cabin volume (minus the subject’s body displacement). Software displays a Pmouth–Pbox loop; the slope of the open loop during occlusion is proportional to TGV.

Worked conceptual example (Boyle’s law intuition)

Suppose alveolar pressure at shutter closure is atmospheric (≈ 1000 cm H₂O absolute for easy arithmetic—real labs use actual barometric pressure in consistent units). A gentle pant raises alveolar pressure by 10 cm H₂O (about +1%). If thoracic gas compresses by ΔV, Boyle’s law says the product PV stays constant:

If V is TGV and pressure rises from P to P + ΔP, then approximately ΔV / V ≈ −ΔP / P (for small ΔP).

If measured ΔV corresponding to that compression (inferred from cabin ΔPbox and calibration) is −0.040 L when ΔP / P = 0.010, then:

V ≈ 0.040 / 0.010 = 4.0 L

So a 40 mL volume change with a 1% pressure change implies TGV ≈ 4.0 L. Real systems use simultaneous ΔPmouth and ΔPbox with precise calibration factors—the exam cares that you know larger TGV → larger chest volume change for a given ΔPmouth, and that the law is P₁V₁ = P₂V₂ under near-isothermal panting.

Panting technique

Coach:

• Gentle, small-volume pants (not maximal inspiratory/expiratory efforts)
• Rate often near 0.5–1 Hz (about 30–60/min)—follow lab standard; avoid frantic panting
• Hands on cheeks if trained to reduce compliant upper-airway volume artifact (lab-dependent coaching)
• Maintain tight mouth seal; nose clip on
• Support the cheeks/mouthpiece so the shutter occlusion is true

Why gentle? Rapid, large pants drive adiabatic (non-isothermal) compression: heat does not exchange fully with tissues/walls, pressure swings exaggerate, and TGV can be wrong. Gentle pants keep conditions closer to the isothermal Boyle’s law assumption used in many algorithms.

Why the Body Box Captures Trapped Gas Better Than Dilution

Gas dilution and N₂ washout require tracer communication with the mouth over minutes. Gas behind closed airways or in extremely slow units may not participate → underestimated FRC.

Plethysmography does not require gas to leave the lung. Any compressible thoracic gas that changes volume when alveolar pressure changes contributes to the Boyle’s law signal—including trapped gas. Therefore in obstruction:

FRC_box (TGV) ≥ FRC_dilution (often strictly greater when trapping is present)

Exam pitfall: a higher box FRC is not automatically “wrong” relative to dilution—it may be the more complete physiologic answer in COPD. Validity still requires good pants, no leak, and thermal stability (Section 9.3).

Linked Spirometry → TLC and RV

TGV alone is not TLC. After acceptable TGV at FRC:

1. Open shutter; patient returns to quiet breathing if needed.
2. From the FRC level, inspire fully → measure IC.
3. TLC = TGV + IC (when TGV was at FRC).
4. From FRC, expire fully → ERV; RV = TGV − ERV.
5. Optionally verify VC and consistency: TLC ≈ RV + VC.

Numeric link example

Acceptable mean TGV (FRC_box) = 4.20 L, IC = 2.10 L, ERV = 0.90 L, slow VC = 3.40 L:

• TLC = 4.20 + 2.10 = 6.30 L
• RV = 4.20 − 0.90 = 3.30 L
• RV/TLC = 3.30 / 6.30 ≈ 52% (elevated—supports trapping/hyperinflation pattern when quality is good)
• Check: RV + VC = 3.30 + 3.40 = 6.70 L vs TLC 6.30 L → investigate spirometric linkage, IC/ERV quality, or TGV variability before releasing (may need repeat linked maneuvers).

Linked spirometry must occur without losing the FRC reference—same session, proper coaching, and software linkage. Measuring IC hours later on a different device without a new FRC breaks the chain.

Selecting Body Plethysmography (II.A.8.b)

Choose body-box volumes when:

• Obstruction, hyperinflation, or air trapping is in the clinical question
• Prior dilution FRC seems too low for the spirometric pattern
• Serial lung volumes for advanced disease management and the patient can pant
• Airways resistance / specific conductance will also be measured in the same cabin visit

Prefer dilution or document limitations when:

• Severe claustrophobia or inability to seal the door safely
• Patient cannot perform acceptable panting (cognitive, neuromuscular, pediatric limits—lab-specific)
• Continuous equipment incompatible with cabin policy
• Acute pain, recent surgery, or instability making shutter occlusion inappropriate

Selection is a clinical + technical judgment: the “best” method is the one that answers the order with valid data the patient can produce.

Performing the Procedure (II.B.8.b)

Sequence

1. Equipment ready — calibrations current; door gasket intact; thermal baseline stable after prior door openings (Domain I carry-forward).
2. Patient education — explain door closure, communication method, panting against a closed shutter, and that the shutter will open again quickly.
3. Position — seated upright, mouthpiece height comfortable, nose clip, hands positioned per lab coaching.
4. Door closed; equilibrate — wait for stable Pbox baseline before critical trials.
5. Tidal breathing — establish relaxed FRC; avoid forced end-expiratory pushes that lower volume below FRC.
6. Shutter closure at end-expiration — for FRC-level TGV.
7. Pant — gentle, rhythmic; capture several loops meeting acceptability (next section).
8. Shutter opens — patient resumes free breathing; avoid panic breaths that wreck linkage.
9. Linked IC / ERV / VC as protocol requires.
10. Repeat TGV trials for reproducibility; report per standards (often mean of acceptable trials).

Shutter timing—detail that decides pass/fail

Some protocols measure TGV at volumes other than FRC for research or special cases; clinical static lung volumes for TLC/RV almost always anchor TGV at FRC. If the stem says shutter closed at TLC, the computed TGV is TLC-level thoracic gas—not FRC.

Simultaneous Raw option

Many visits combine TGV with airways resistance (Raw) panting (open shutter, slightly different coaching). Do not confuse loops: occlusion loops for TGV vs open-shutter flow–Pbox relationships for Raw. Selection (II.A) may include both; performance (II.B) requires the correct shutter state for each metric.

Clinical Scenario

Order: “Lung volumes; evaluate hyperinflation” in a patient with severe emphysema who failed helium equilibration after prolonged rebreathing (He still drifting). RPFT selection (II.A.8.b): proceed to body box if the patient can pant and tolerate the cabin. Performance (II.B.8.b): equilibrate cabin, close shutter at end-expiration, gentle pants, linked IC/ERV, multiple acceptable TGV trials. Expect TGV and TLC higher than the incomplete dilution attempt suggested—and document why dilution was abandoned.

Link to Practice

/practice/rpftPractice questions with detailed explanations