inspiratory capacity

**Definition and calculation** - IC = TV + IRV = TLC − FRC. - Typical values (70 kg adult male): TV ~500 mL, IRV ~3000 mL, so IC ~3500 mL. Values are somewhat lower in women. - It is one of the four lung capacities. Each capacity is the sum of two or more volumes: IC, FRC (ERV + RV), vital capacity (IRV + TV + ERV), and TLC (all four volumes). - IC is measured by spirometry. Spirometry cannot measure RV, FRC, or TLC. These need helium dilution, nitrogen washout, or body plethysmography.

**Physiology** - IC reflects how far the lung can expand above resting end-expiration (FRC). FRC is the point where the inward elastic recoil of the lung balances the outward recoil of the chest wall. - It depends on respiratory muscle strength (diaphragm and external intercostals), lung and chest wall compliance, and airway patency.

**Clinical significance** - **Restrictive lung disease** (e.g., pulmonary fibrosis, sarcoidosis, neuromuscular weakness, obesity, kyphoscoliosis): IC is decreased because compliance is reduced or muscles are weak. TLC, VC, and FRC also fall, and FEV1/FVC is normal or increased. - **Obstructive lung disease** (COPD/emphysema): air trapping and hyperinflation raise FRC and RV, which leaves less room to inspire. IC falls, and the **IC/TLC ratio** falls. This is a marker of dynamic hyperinflation, and a low IC/TLC (≤0.25) predicts worse outcomes and mortality in COPD. During exercise, IC decreases further. Bronchodilators raise IC by reducing hyperinflation, which relieves dyspnea. - **Pregnancy:** the diaphragm rises, so ERV, RV, and FRC fall by ~20%. IC increases slightly because tidal volume rises, and TLC is mostly unchanged or slightly reduced. - **Position:** FRC falls when supine, so IC rises slightly.

**High-yield board points** - IC = TLC − FRC, which is a common calculation question. - IC starts at FRC, whereas VC starts at RV. - RV, FRC, and TLC cannot be measured by spirometry alone. - Hyperinflation (COPD) raises FRC, which lowers IC. - Fibrosis lowers all volumes and capacities. Compliance falls and elastic recoil rises, so FRC falls.