14.4 Exercise, Oximetry & Special Test Calculations

Special-test math still lives in Domain III.A

Beyond spirometry, volumes, DLCO, and blood gases, the DCO lists calculation/reference tasks for exercise and field walking, oximetry-related indices, airways resistance, respiratory muscle pressures, bronchoprovocation, and related special measurements (grouped here as III.A.6–14, III.A.17 literacy). The theme is unchanged: derive the right number, attach the right reference, and do not over-call noise as change.

Six-minute walk test (6MWT): distance and %predicted

Primary reported values

Predicted equations for 6MWD use demographics (age, sex, height, sometimes weight) and differ by population—select the lab’s configured equation and enter correct height/age/sex, same discipline as spirometry references.

Worked mini-example — distance %predicted

Measured 6MWD = 420 m. Predicted = 560 m.

• %predicted = (420/560) × 100 = 75%.

If the patient stopped twice and used 2 L/min O₂, report 420 m (75% pred) on 2 L/min O₂ with two stops—never a naked distance that hides conditions.

SpO2 nadir concept

• Record continuous or frequent SpO2; nadir is the lowest valid reading (reject artifact from poor perfusion/motion when you can identify it).
• Example: resting SpO2 96%, readings during walk 93%, 91%, 88%, 90% → nadir 88% if pulse quality was adequate.
• Desaturation definitions (e.g., ≥3–4% fall or drop below 88–90%) are protocol/interpretation rules; technologist calculation duty is accurate nadir and ΔSpO2 = nadir − rest when requested.

Worked mini-example — desaturation delta

Rest SpO2 95%, nadir 87% → Δ = −8 points (absolute percentage points, not “8% of 95”).

Monitored exercise / oximetry calculation notes (high level)

For labs performing more than field walks (e.g., titrated O₂ evaluation, simplified exercise oximetry):

• Report workload or speed/grade when known, SpO2/HR at stages, and O₂ flow if titrated.
• Time to desaturation and recovery SpO2 may be calculated from annotated timelines.
• Do not compute fancy VO2 max claims from a hallway walk without metabolic cart data—report what was measured.

Airways resistance and specific conductance (Raw / sGaw)

Body plethysmography yields airways resistance (Raw) from the relationship between alveolar pressure changes (box pressure calibrated to P alv) and flow during gentle panting, conceptually:

$$Raw \propto \frac{\Delta P}{\dot{V}}$$

Specific conductance (sGaw) relates conductance (1/Raw) to lung volume (often FRC/TGV):

$$sGaw = \frac{1/Raw}{V_{TG}} \quad (\text{conceptual})$$

Technologist calculation literacy

• Software computes Raw and sGaw from acceptable pant loops; you select median/mean of acceptable efforts per SOP.
• High Raw / low sGaw track obstruction patterns; volumes must be valid—garbage TGV makes sGaw garbage.
• Report units as configured (e.g., cmH2O/L/s for Raw).

Worked mini-example — conductance

If Raw = 2.0 cmH2O/L/s, Gaw = 1/2.0 = 0.5 L/s/cmH2O. If TGV = 4.0 L, sGaw = 0.5/4.0 = 0.125 (unit labeling per system).

You will not hand-derive every calibration constant on the exam; you must know Raw rises when pressure swing is large for a given flow and that sGaw indexes conductance to volume.

MIP / MEP %predicted

MIP (PImax) — maximum inspiratory pressure; MEP (PEmax) — maximum expiratory pressure. Reported in cmH2O (magnitude; inspiratory often as negative gauge, reported as absolute strength).

$$\%predicted = \frac{measured}{predicted} \times 100$$

Predicted sets use age, sex, and sometimes other demographics—enter correctly.

Worked mini-example

MIP measured −60 cmH2O (report magnitude 60 cmH2O inspiratory strength as per lab convention), predicted 100 cmH2O → 60% predicted.
MEP 80 cmH2O, predicted 120 → 67% predicted.

Select the best (most extreme) acceptable efforts after validity rules (Chapter 12 concepts), then apply %pred—do not average a leak trial with a good trial.

Bronchoprovocation: PC20 interpolation concept

PC20 (provocative concentration causing a 20% fall in FEV1 from baseline/post-diluent) is often not exactly one of the scheduled doses. When FEV1 fall crosses 20% between two concentrations, interpolate (commonly log-concentration interpolation in software).

Conceptual linear interpolation (exam-level)

If at concentration C1 the %fall is F1 < 20%, and at next concentration C2 the %fall is F2 > 20%, PC20 lies between C1 and C2. Software typically uses logarithmic spacing of concentrations; a simplified story for literacy:

You identify the bracket and accept the instrument’s PC20 output when challenges were valid—not a hand-waved “about C2.”

Worked mini-example — percent fall and bracket

Baseline FEV1 = 3.00 L.

• After 1 mg/mL: FEV1 2.70 L → %fall = (3.00−2.70)/3.00 × 100 = 10%.
• After 2 mg/mL: FEV1 2.25 L → %fall = 0.75/3.00 × 100 = 25%.

PC20 is between 1 and 2 mg/mL (software interpolates; result might be ~1.7 mg/mL class example—not a universal constant). If %fall never reaches 20% at the highest dose, report PC20 greater than highest concentration (or non-responsive per protocol), not a fake interpolated value below the final dose.

PD20 (provocative dose) uses cumulative dose rather than concentration—know which your protocol reports.

Serial PFT change: variability vs real change (technologist level)

Patients repeat PFTs over months. Domain III expects you to understand that measurement noise + biologic within-subject variation exists; not every 50 mL shift is “improvement.”

Concepts (not a license to invent cutoffs)

Worked mini-example — don’t over-call

Visit A best FEV1 2.00 L; Visit B best FEV1 2.06 L after a mild cold, both quality A.

• Absolute change +0.06 L (+3%).
• This may lie within ordinary variability; report both values accurately and avoid technologist language claiming “definite bronchodilator-range improvement” without context.

Visit A DLCO 22; Visit B DLCO 15 with Hb drop from 14 to 9 g/dL and recent smoking.

• Large numeric fall may be partly correction/condition artifact—apply Hb/COHb pathways before declaring pure membrane decline.

Practical selection rules

1. Compare like with like (same posture, similar BD state, same equation set when possible).
2. Prefer best acceptable efforts each visit.
3. Annotate quality grade changes (A→D) that explain numeric shifts.
4. For DLCO serials, align Hb-corrected values when policy uses them.
5. Know that significant change thresholds differ by parameter (FEV1 vs FVC vs DLCO) and by guideline era—exam items often test variability awareness more than a single magic percent.

Other special calculations (III.A.6–14 / 17 survey)

Depending on lab menu, Domain III.A also touches selecting/calculating:

• Oximetry trending during procedures (resting vs exercise SpO2).
• Shunt estimates or other blood-gas derived indices if performed (only with proper samples/FIO2 data).
• Pre/post intervention %change for any ordered special maneuver using (post−pre)/pre × 100.
• Reference LLN/%pred for Raw, sGaw, MIP/MEP when equations exist in software.

Always: correct demographics → correct predicted → correct measured selection → transparent derived index.

Integrated RPFT vignettes

Vignette A — 6MWT: 350 m, predicted 500 m → 70% pred; SpO2 rest 94%, nadir 86% on room air. Report distance, %pred, nadir, and that room air was used.

Vignette B — PC20: Falls of 8%, 12%, then 22% at successive concentrations—interpolate PC20 between the last two doses; do not pick the first dose.

Vignette C — Raw/sGaw: Excellent Raw efforts but TGV failed validity—do not report sGaw as if volume were solid.

Vignette D — serial: FEV1 down 200 mL with cough and grade D efforts vs prior grade A—flag quality before “progression.”

Closing Domain III.A map

Master the arithmetic, the reference selection, and the honesty about conditions—that is how high-cut Data Management items are scored.