12.2 Airways Resistance & Conductance by Plethysmography

Airways Resistance on the RPFT Blueprint

Domain II lists airways resistance and conductance as procedure content: II.A.16 (select), II.B.16 (perform), and II.C.16 (evaluate validity). These skills build on body-box equipment knowledge from Domain I and lung-volume TGV technique from static volumes—but Raw is a flow–pressure relationship, not a Boyle’s-law volume alone.

Definitions

Why “specific” indices matter: absolute Raw falls as lung volume rises (airways tether open). Measuring at different volumes confounds comparison. sGaw (and sRaw) normalize for the lung volume at which resistance was assessed—critical when FRC differs between visits or after bronchodilator.

Numeric intuition

If Raw = 2.0 cm H₂O·L⁻¹·s, then Gaw = 1/2.0 = 0.50 L·s⁻¹·cm H₂O⁻¹. If TGV at the measurement is 4.0 L, sGaw = 0.50 / 4.0 = 0.125 L·s⁻¹·cm H₂O⁻¹·L⁻¹. Lower sGaw indicates reduced volume-specific conductance (more obstruction in a volume-adjusted sense).

Body-Box Measurement Concept

In a constant-volume (variable-pressure) plethysmograph:

1. Patient sits in a sealed cabin with nose clip and mouthpiece.
2. Quiet breathing establishes FRC.
3. For Raw, the patient pants gently with the shutter open so airflow occurs at the mouth while box pressure (Pbox) swings reflect alveolar pressure changes (after calibration and thermal considerations).
4. The system relates flow (pneumotach) to the pressure driving flow (derived from Pbox and calibration) to compute Raw (and display loops).
5. TGV for specific indices typically uses closed-shutter panting (Boyle’s law) as in lung-volume measurement; many protocols obtain TGV and Raw in the same visit with the correct shutter state for each metric.

Open vs closed shutter—do not confuse

Exam trap: reporting Raw from closed-shutter TGV loops alone, or TGV from open-shutter Raw pants alone, without the correct signal set.

Panting at FRC

Coach:

• Small-volume, rhythmic pants near FRC (not maximal IC efforts), tidal volume about 50–150 mL
• Two different pant rates apply, and mixing them up is an exam trap: the ERS/ATS lung volumes standard (2023 update) specifies 1.5–2 pants per second (90–120/min) for the open-shutter Raw loop, but only 0.5–1.0 Hz (30–60/min) for the closed-shutter TGV used to normalize sRaw and sGaw
• Because the optimal frequencies differ, the standard states that Raw must not be measured during the same maneuver used to measure lung volumes—longer mouthpiece time also invites leak
• Support cheeks if trained; maintain tight seal
• Avoid glottic closure, talking, or swallowing mid-loop
• Multiple acceptable loops; report mean or median per lab standard

Hands on cheeks reduce upper-airway compliance artifact in some coaching traditions—follow the written procedure for your system.

Clinical Use (II.A.16)

Select airways resistance/conductance testing when:

Bronchodilator response by resistance: a meaningful improvement may appear as decreased Raw and increased sGaw after bronchodilator. Percentage change criteria are standard- or lab-defined; know the direction of change expected with relief of obstruction and that volume shifts after BD can affect interpretation—hence specific conductance is often preferred.

Spirometry remains the primary obstruction metric for most orders. Raw is selected when it answers the clinical question or when forced spirometry cannot be obtained validly—not as an automatic replacement for every FEV1/FVC.

Performing the Procedure (II.B.16)

1. Verify body-box calibration, leak check, and thermal readiness (Domain I carry-forward).
2. Explain door closure, panting, and that the shutter may close briefly for TGV if linked.
3. Seat patient upright; adjust mouthpiece height; apply nose clip.
4. Close door; allow Pbox baseline to stabilize.
5. Establish relaxed tidal breathing at true FRC.
6. Perform open-shutter panting for Raw loops meeting acceptability.
7. Perform closed-shutter TGV if specific indices or volumes are ordered (shutter at end-expiration for FRC-level TGV).
8. Repeat for reproducibility; avoid prolonged sealed-cabin sessions if the patient is distressed.
9. If pre/post bronchodilator Raw is ordered, keep technique identical before and after drug and wait the prescribed post-BD interval.

Document BTPS/conditions as the system requires; ensure the correct patient height/weight if software uses them for any derived displays (body size primarily for volumes/references, but complete demographics still matter for the visit).

Validity (II.C.16)

Reject or repeat when quality fails—even if a number prints:

Acceptable Raw pants show consistent, closed, readable flow–pressure (or system-equivalent) relationships without leak spikes. Compare with clinical context: an extremely high Raw in a young healthy athlete with perfect spirometry should trigger technical review before clinical alarm.

Pre/post comparison validity

For bronchodilator assessment via resistance:

• Same device, same cabin, same coaching
• Comparable FRC/TGV if using specific indices—large volume shifts without noting them mislead
• Acceptable loops both pre and post
• Document drug, dose, and wait time

Clinical Scenario

A postoperative patient cannot perform reproducible forced spirometry because of pain but can sit in the body box and pant. Order: evaluate obstruction and BD response. Selection (II.A.16): Raw/sGaw with pre/post bronchodilator. Performance (II.B.16): stable cabin, open-shutter pants at FRC, closed-shutter TGV for sGaw, albuterol, repeat identical technique. Validity (II.C.16): first post-BD set shows irregular panting and a cheek leak—reject, re-coach hands-on-cheeks and tight seal, then report only acceptable pairs. sGaw rises substantially with lower Raw; you report resistance-based BD response with a note that FVC/FEV1 were not achievable.

Link to Practice

/practice/rpftPractice questions with detailed explanations