Define vital capacity. What is its significance?

Vital capacity (VC) is defined as the maximum volume of air that can be exhaled after taking the deepest possible inhalation

• It equals the sum of tidal volume (TV) + inspiratory reserve volume (IRV) + expiratory reserve volume (ERV)

Typical values in healthy adults range from 3 to 5 L, averaging around 4.8 L for men and 3.1 L for women

Significance of vital capacity:

• Serves as a fundamental spirometric measure of lung function and respiratory health

• Helps diagnose and differentiate lung diseases:

  • Restrictive disorders (e.g., pulmonary fibrosis) reduce VC due to limited lung expansion

  • Obstructive disorders (e.g., COPD, asthma) may maintain near-normal VC, but airflow rates (e.g., FEV₁) are impaired

• Assesses respiratory muscle strength and neuromuscular involvement (e.g., in Guillain–Barré, myasthenia gravis)Monitors disease progression and treatment response over time

Simple calculation and clinical use:

• VC = TV + IRV + ERV, or alternately VC = TLC – RV, where TLC = total lung capacity and RV = residual volumeOften measured as slow vital capacity (SVC) or forced vital capacity (FVC) depending on flow requirements

In summary, vital capacity is a critical indicator of lung expansion ability, muscle strength, and overall pulmonary health, informing diagnosis, management, and prognosis of respiratory conditions.