Which statement about CRS units is true?

Which statement about CRS units is true?

• A. They are expressed as mL/cm H2O or L/cm H2O
• B. They are expressed as mmHg
• C. They are dimensionless
• D. They are expressed as L/s

Answer: (A)

Respiratory system compliance describes how much volume changes for a given change in pressure. It’s defined as ΔV/ΔP, so the units are volume per pressure. In practice, those units are mL per cm H2O (or L per cm H2O). This reflects how easily the lungs and chest wall expand with pressure during inspiration. It’s not mmHg, which is just a pressure unit without relating to how much volume changes per pressure, and it’s not dimensionless since there’s a volume per pressure dimension. It isn’t a flow rate either, since L/s would be volume per unit time, not per unit pressure. For example, a Crs of 50 mL/cm H2O means that each centimeter of water in driving pressure yields about 50 mL of tidal volume.

Respiratory system compliance describes how much volume changes for a given change in pressure. It’s defined as ΔV/ΔP, so the units are volume per pressure. In practice, those units are mL per cm H2O (or L per cm H2O). This reflects how easily the lungs and chest wall expand with pressure during inspiration. It’s not mmHg, which is just a pressure unit without relating to how much volume changes per pressure, and it’s not dimensionless since there’s a volume per pressure dimension. It isn’t a flow rate either, since L/s would be volume per unit time, not per unit pressure. For example, a Crs of 50 mL/cm H2O means that each centimeter of water in driving pressure yields about 50 mL of tidal volume.