Which condition is associated with long time constants due to increased resistance?

Which condition is associated with long time constants due to increased resistance?

• A. ARDS
• B. Normal lungs
• C. COPD
• D. Pulmonary edema

Answer: (C)

Long time constants reflect how slowly the lungs empty or fill, and they depend on the product of airway resistance and lung compliance (tau = R × C). When resistance rises, the time constant increases. In COPD, airway resistance is markedly elevated because of chronic inflammation, mucus plugging, and airway remodeling. This high resistance makes expiration slower and lengthens the time constant, producing air trapping and dynamic hyperinflation. In contrast, conditions like ARDS or pulmonary edema mainly reduce lung compliance (the lungs become stiffer), which changes mechanics but does not produce long time constants through increased resistance. Normal lungs have low resistance and normal time constants, so they wouldn’t exhibit the prolonged expiratory time constant seen with COPD.

Long time constants reflect how slowly the lungs empty or fill, and they depend on the product of airway resistance and lung compliance (tau = R × C). When resistance rises, the time constant increases. In COPD, airway resistance is markedly elevated because of chronic inflammation, mucus plugging, and airway remodeling. This high resistance makes expiration slower and lengthens the time constant, producing air trapping and dynamic hyperinflation.

In contrast, conditions like ARDS or pulmonary edema mainly reduce lung compliance (the lungs become stiffer), which changes mechanics but does not produce long time constants through increased resistance. Normal lungs have low resistance and normal time constants, so they wouldn’t exhibit the prolonged expiratory time constant seen with COPD.