cmH2O

cmH2O, short for centimeter of water, is a non-SI unit of pressure defined by the hydrostatic pressure exerted by a 1-centimeter-high column of water at standard gravity. Using P = ρgh with ρ ≈ 1000 kg/m3 and g ≈ 9.80665 m/s2, 1 cmH2O equals about 98.0665 pascals (Pa), or 0.0980665 kilopascals (kPa). It is commonly used in medical settings to express small pressure differences in respiratory care.

In clinical practice, cmH2O is used to quantify airway pressures in devices such as CPAP machines and mechanical ventilators. It appears in measurements like positive end-expiratory pressure (PEEP), peak inspiratory pressure, and mean airway pressure. Typical noninvasive respiratory support settings range from roughly 5 to 15 cmH2O, though higher or lower values may be used based on patient needs and lung mechanics.

Measurement is performed with pressure transducers or manometers connected to the airway circuit. Values are often displayed directly in cmH2O on medical equipment. While cmH2O is widely used in healthcare, the SI unit of pressure is the pascal; cmH2O is accepted for use with SI in certain contexts.

Related notes include that 1 cmH2O equals 10 millimeters of water (mmH2O), a smaller commonly used subunit. The unit’s practicality stems from its direct relation to hydrostatic pressure in small water columns, a familiar physical basis for clinicians monitoring airway pressures.