Which of the following lung volumes cannot be measured by simple spirometry?

Explanation

## Spirometry Limitations **Key Point:** Spirometry measures only the volumes that can be breathed in and out. Residual volume (RV) — the air remaining in the lungs after maximal expiration — cannot be measured by spirometry because it cannot be expelled from the lungs. ## Lung Volumes Measurable vs. Non-Measurable by Spirometry | Volume | Definition | Measurable by Spirometry? | |--------|-----------|---------------------------| | Tidal Volume (TV) | Air inspired/expired during quiet breathing | **Yes** | | Inspiratory Reserve Volume (IRV) | Maximum air that can be inspired after normal inspiration | **Yes** | | Expiratory Reserve Volume (ERV) | Maximum air that can be expired after normal expiration | **Yes** | | Residual Volume (RV) | Air remaining in lungs after maximal expiration | **No** | ## Methods to Measure Residual Volume Residual volume requires indirect measurement techniques: 1. **Helium dilution** — patient rebreathes helium-enriched air; equilibration allows RV calculation 2. **Nitrogen washout** — patient breathes 100% O₂; nitrogen washout is measured 3. **Body plethysmography** — gold standard; uses Boyle's law to calculate RV from pressure changes **High-Yield:** Functional residual capacity (FRC) = ERV + RV. Since RV cannot be measured by spirometry, FRC also cannot be measured by spirometry alone. **Clinical Pearl:** Total lung capacity (TLC) = VC + RV. Because RV is unmeasurable by spirometry, TLC must be calculated using one of the indirect methods above.