During routine spirometry in a healthy 28-year-old female, the most common lung volume that is measured directly by the spirometer is which of the following?

Explanation

## Lung Volumes: Direct vs. Indirect Measurement **Key Point:** The spirometer measures only the air that can be breathed in and out. Volumes that include residual volume (RV) cannot be measured directly by spirometry and require additional techniques (helium dilution, nitrogen washout, or body plethysmography). ### Classification of Lung Volumes | Volume | Definition | Measured by Spirometry? | Method if Not | |--------|-----------|------------------------|----------------| | **Tidal Volume (TV)** | Air breathed in/out at rest (~500 mL) | ✓ Yes | — | | **Inspiratory Reserve Volume (IRV)** | Max air inspired after normal inspiration (~3000 mL) | ✓ Yes | — | | **Expiratory Reserve Volume (ERV)** | Max air expired after normal expiration (~1100 mL) | ✓ Yes | — | | **Residual Volume (RV)** | Air remaining after max expiration (~1200 mL) | ✗ No | He dilution, N₂ washout, plethysmography | | **Vital Capacity (VC)** | TV + IRV + ERV (~4600 mL) | ✓ Yes | — | | **Functional Residual Capacity (FRC)** | ERV + RV (~2300 mL) | ✗ No | He dilution, N₂ washout, plethysmography | | **Total Lung Capacity (TLC)** | VC + RV (~5800 mL) | ✗ No | He dilution, N₂ washout, plethysmography | **High-Yield:** Remember: **Spirometry measures only volumes that do NOT contain residual volume.** Any volume that includes RV (FRC, TLC) or IS RV itself cannot be measured directly. **Mnemonic:** **STRIVE** = Spirometry measures **S**mall volumes (TV, IRV, ERV) and their sum **V**ital Capacity. Everything with **R**esidual volume needs other methods. **Clinical Pearl:** Vital capacity is the single most useful spirometric measurement in clinical practice—it reflects overall respiratory muscle strength and lung compliance. A reduced VC suggests either restrictive disease or respiratory muscle weakness.