vital capacity

**Definition and calculation** - VC = IRV + TV + ERV, or equivalently TLC − RV. - Forced vital capacity (FVC) is VC measured with a maximal-effort, rapid exhalation. It is the standard spirometry measurement. - Typical adult value is about 4–5 L (about 60 mL/kg). It is larger in taller people and men, and smaller in older people and women.

**Components** - Tidal volume (TV): about 500 mL. - Inspiratory reserve volume (IRV): about 3000 mL. - Expiratory reserve volume (ERV): about 1200 mL. - Residual volume (RV): about 1200 mL. RV is not part of VC and cannot be measured by spirometry. Measure it with helium dilution, nitrogen washout, or body plethysmography. The same applies to FRC and TLC, which both include RV.

**Pathophysiology and clinical significance** - Restrictive disease: VC, FVC, and TLC are decreased. FEV1/FVC is normal or increased (>0.8) because both fall proportionally, or FEV1 falls less than FVC. - Intrinsic (parenchymal) causes: idiopathic pulmonary fibrosis, sarcoidosis, pneumoconioses, and hypersensitivity pneumonitis. Compliance is decreased. - Extrinsic causes: obesity, pleural effusion, kyphoscoliosis, and ankylosing spondylitis. - Neuromuscular causes: Guillain-Barré syndrome, myasthenia gravis, ALS, and diaphragmatic paralysis. Compliance is normal, and PEmax and PImax are decreased. - Obstructive disease (COPD, asthma): FEV1 falls more than FVC, so FEV1/FVC is <0.7. Air trapping raises RV and TLC, which can reduce VC and FVC as disease progresses. - Pregnancy: VC is essentially unchanged. Increased tidal volume comes at the expense of ERV and RV, so FRC and RV fall. - Aging: RV and FRC increase, and VC decreases.

**Clinical use** - Serial bedside FVC (or negative inspiratory force) monitors respiratory muscle strength in Guillain-Barré syndrome and myasthenic crisis. - Intubation or mechanical ventilation is typically considered if FVC is <20 mL/kg (or <15–20 mL/kg), if NIF is weaker than −30 cmH2O, or if MIP is falling quickly. The "20/30/40" rule uses VC <20 mL/kg, MIP weaker than −30 cmH2O, and MEP <40 cmH2O. - Pulmonary function tests are also used to follow ILD progression, with FVC decline as a trigger to start or change antifibrotic therapy.

**High-yield pearls** - VC cannot give RV, FRC, or TLC. Use helium dilution or body plethysmography for those. - FRC is the volume at which the elastic recoil of the lung (inward) equals that of the chest wall (outward). In emphysema FRC increases, and in fibrosis it decreases. - Inspiratory capacity (IC) = TV + IRV. FRC = ERV + RV. TLC = VC + RV. - Dead space is not measured by spirometry. Use the Bohr equation or Fowler's method.