What are the units used for a respiratory time constant?

What are the units used for a respiratory time constant?

• A. Seconds
• B. Liters
• C. Millimeters of mercury
• D. Minutes

Answer: (A)

The main idea is that the respiratory time constant is a time-based measure of how quickly the lungs respond to a pressure change. It’s defined as the product of airway resistance (Raw) and lung compliance (Crs). Because Raw has units of cmH2O·s/L and Crs has units of L/cmH2O, their product yields seconds, making the time constant a unit of time. In practical terms, one time constant is the time it takes for the lung to reach about 63% of the way from its starting volume to the new volume after a pressure change. If the time constant is small, filling happens quickly; if it’s large, filling takes more time—information that helps tailor inspiratory time during mechanical ventilation.

The main idea is that the respiratory time constant is a time-based measure of how quickly the lungs respond to a pressure change. It’s defined as the product of airway resistance (Raw) and lung compliance (Crs). Because Raw has units of cmH2O·s/L and Crs has units of L/cmH2O, their product yields seconds, making the time constant a unit of time. In practical terms, one time constant is the time it takes for the lung to reach about 63% of the way from its starting volume to the new volume after a pressure change. If the time constant is small, filling happens quickly; if it’s large, filling takes more time—information that helps tailor inspiratory time during mechanical ventilation.