How can a normotensive blood pressure reading be converted from millimetres of mercury (mm Hg) to centimetres of water (cm H₂O)?

Converting Blood Pressure from mm Hg to cm H₂O

To convert 120/80 mm Hg to cm H₂O, multiply each value by 1.36: systolic becomes 163 cm H₂O and diastolic becomes 109 cm H₂O.

Conversion Formula

The standard conversion between millimeters of mercury (mm Hg) and centimeters of water (cm H₂O) is based on the relative densities of mercury and water:

• 1 mm Hg = 1.36 cm H₂O 1
• Conversely, 1 cm H₂O = 0.735 mm Hg 1

Applying the Conversion to 120/80 mm Hg

For a normotensive blood pressure reading of 120/80 mm Hg:

• Systolic pressure: 120 mm Hg × 1.36 = 163.2 cm H₂O 1
• Diastolic pressure: 80 mm Hg × 1.36 = 108.8 cm H₂O 1

Clinical Context

While mm Hg remains the international standard unit for blood pressure measurement in clinical practice, cm H₂O is commonly used for measuring other physiological pressures 1:

• Respiratory pressures (esophageal, gastric, transdiaphragmatic pressures) are typically measured in cm H₂O, with ranges from a few cm H₂O up to 200 cm H₂O 1
• Central venous pressure and other low-pressure vascular measurements may use cm H₂O 1

Important Caveats

• The mm Hg unit is the gold standard for blood pressure measurement and should be used for all clinical blood pressure documentation and decision-making 1, 2
• Mercury sphygmomanometers, when properly maintained, remain the reference standard for blood pressure measurement accuracy 1
• Converting blood pressure to cm H₂O is primarily an academic exercise, as clinical guidelines, treatment thresholds, and diagnostic criteria are all established in mm Hg 1, 2