Which unit is used to express airway or intrapulmonary pressures in respiratory mechanics (often seen as cmH2O)?

• A. mmHg
• B. kPa
• C. psi
• D. cmH2O

Answer: (D)

Small airway and intrapulmonary pressures are small and change around atmospheric pressure, so the most natural way to express them is as a height of a column of water: centimeters of water (cmH2O). This unit directly maps pressure to a tangible, intuitive measure, since 1 cmH2O corresponds to about 98 Pa (0.098 kPa). In clinical ventilator settings, numbers are typically in the range of several to a few dozen cmH2O (for example, a PEEP of 5 cmH2O or peak pressures around 15 cmH2O), which keeps values easy to read and compare. Other units like mmHg, kPa, or psi exist, but they describe the same pressures in scales that are less convenient for the small pressures encountered in respiratory mechanics; for instance, 15 cmH2O is about 0.98 kPa or roughly 0.21 psi. So cmH2O is the standard unit used to express airway or intrapulmonary pressures.