SN2
SHORT NOTE
5M
★ High-yield
Define lung volumes and capacities. Give their normal values and state which of them cannot be measured by spirometry.
Definitions
Lung volume = a primary, non-overlapping subdivision of lung gas.Lung capacity = sum of two or more lung volumes.
Lung Volumes
| Volume | Definition | Value |
|---|---|---|
| Tidal volume (TV) | Air inspired or expired in one quiet breath | 500 mL |
| Inspiratory reserve (IRV) | Extra air inspired after normal inspiration | 3000 mL |
| Expiratory reserve (ERV) | Extra air expired after normal expiration | 1100 mL |
| Residual volume (RV) | Air left after maximal expiration | 1200 mL |
Lung Capacities
| Capacity | Formula | Value |
|---|---|---|
| Inspiratory capacity (IC) | TV + IRV | 3500 mL |
| Functional residual capacity (FRC) | ERV + RV | 2300 mL |
| Vital capacity (VC) | IRV + TV + ERV | 4600 mL |
| Total lung capacity (TLC) | VC + RV | 5800 mL |
Important Points
- RV cannot be measured by spirometry — it never leaves the lung
- Hence FRC and TLC also cannot be measured by spirometry (both contain RV)
- Measured instead by helium dilution, nitrogen washout, or body plethysmography
- FRC acts as a buffer → prevents wide swings in alveolar gas between breaths
- Values 20–25% lower in females; ↓ with age and in the supine position
Dynamic Lung Volumes
| Test | Definition | Normal value |
|---|---|---|
| FVC | Forced vital capacity — VC expired as forcefully and rapidly as possible | 4800 mL |
| FEV1 | Volume expired in the first second of FVC | 4000 mL |
| FEV1/FVC | Ratio — the key discriminator | > 80% |
| PEFR | Peak expiratory flow rate | 400–600 L/min |
| MVV | Maximum voluntary ventilation | 125–170 L/min |
- Timed vital capacity — 83% in 1 s, 94% in 2 s, 97% in 3 s
Applied Aspects
| Obstructive (asthma, COPD) | Restrictive (fibrosis, kyphoscoliosis) | |
|---|---|---|
| RV, FRC, TLC | ↑ (air trapping) | ↓ |
| VC | ↓ | ↓ |
| FEV1/FVC | < 70% | Normal or ↑ |
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