Residual Volume: Definition, Normal Values, and Clinical Significance

Residual Volume (RV) is the volume of air remaining in the lungs after a maximal forced exhalation. It cannot be measured by spirometry (which only captures volumes that can be exhaled) and requires body plethysmography, helium dilution, or nitrogen washout.

Normal Values

• Adult male: ~1,200 mL (1.2 L).
• Adult female: ~1,100 mL (1.1 L).
• Varies with age, height, body composition, and lung disease.

Physiological Role

• Prevents alveolar collapse (atelectasis) at end-expiration by maintaining positive transpulmonary pressure.
• Allows continuous gas exchange during the respiratory cycle (oxygen and CO₂ diffusion never cease).
• Buffers against large fluctuations in alveolar gas concentrations.

Clinical Significance

• Obstructive Lung Disease (COPD, asthma): RV is increased due to air trapping (premature airway closure, loss of elastic recoil). Elevated RV/TLC (total lung capacity) ratio is a hallmark.
• Restrictive Lung Disease (fibrosis, neuromuscular): RV may be normal or reduced proportionally to TLC.
• Pulmonary Function Testing: RV is essential for calculating TLC = VC + RV, and for assessing hyperinflation.
• Mechanical Ventilation: Auto-PEEP (intrinsic PEEP) reflects dynamic hyperinflation; understanding RV helps set PEEP and avoid barotrauma.

RV is a critical parameter in comprehensive pulmonary function testing and ICU management.