Centimetre of water

Centimetre of Water

A centimetre of water (cmH₂O) is a unit of pressure that is commonly used in medicine and respiratory physiology. It is defined as the pressure exerted by a column of water 1 centimetre high at 4 degrees Celsius at the standard acceleration of gravity. This unit is particularly useful in measuring pressures in the human body, such as blood pressure, intracranial pressure, and pressures in mechanical ventilation systems.

Definition

The centimetre of water is defined as:

1 cmH₂O = 98.0665 pascals (Pa)

This definition is based on the density of water at 4°C, which is 1000 kg/m³, and the standard acceleration due to gravity, which is 9.80665 m/s².

Applications in Medicine

In the medical field, the centimetre of water is often used to measure:

• Blood pressure: Although blood pressure is typically measured in millimetres of mercury (mmHg), cmH₂O can be used in certain clinical settings.
• Intracranial pressure (ICP): Monitoring ICP is crucial in patients with head injuries or conditions like hydrocephalus.
• Mechanical ventilation: In respiratory care, pressures such as positive end-expiratory pressure (PEEP) and continuous positive airway pressure (CPAP) are often measured in cmH₂O.

Conversion to Other Units

The centimetre of water can be converted to other units of pressure:

• 1 cmH₂O = 0.980665 mbar
• 1 cmH₂O = 0.735559 mmHg
• 1 cmH₂O = 0.014223 psi

Historical Context

The use of water columns to measure pressure dates back to early scientific experiments. The simplicity of using water, a readily available and non-toxic fluid, made it a practical choice for early pressure measurements. The centimetre of water remains in use today due to its convenience and relevance in specific medical applications.

Also see

• Millimetre of mercury
• Pascal (unit)
• Pressure measurement
• Respiratory physiology