Decreased lung compliance (CL) results in what change to the lung mechanics time constant?

• A. Short Time Constants
• B. Long Time Constants
• C. Short Inspiratory Time
• D. Fast Time Constants

Answer: (D)

The key idea is how the lung’s time constant is defined and what happens when compliance changes. The time constant for the respiratory system is τ = Resistance × Compliance. If compliance decreases, the compliance term gets smaller, so τ becomes smaller as well. A smaller time constant means the system reaches a given fraction of its volume more quickly in response to a driving pressure, i.e., the response is faster. Therefore, decreased CL leads to fast (shorter) time constants.