mmHg (millimetres of mercury) is the unit used to measure blood pressure and other pressures within the cardiovascular system. It describes the height in millimetres to which the pressure would raise a column of liquid mercury in a manometer: a type of pressure measurement device historically used before modern electronic transducers became standard.
At normal atmospheric conditions, a column of mercury 1 mm high corresponds to approximately 133 pascals of pressure. Blood pressure is expressed as two values in mmHg: systolic pressure (the peak pressure during ventricular contraction, typically around 120 mmHg in a healthy adult) over diastolic pressure (the pressure during relaxation between beats, typically around 80 mmHg). A normal reading of 120/80 mmHg is a widely recognised reference point, though normal ranges vary with age, fitness, and other factors.
The mmHg unit is also used for other cardiovascular pressure measurements: central venous pressure (typically 4 to 12 mmHg), pulmonary arterial pressure (typically 25/10 mmHg), and arterial blood gas partial pressures in older conventions (modern UK practice uses kilopascals, kPa). Pressure gradients across heart valves are also expressed in mmHg during echocardiography.
Mercury manometers are no longer used clinically (mercury is a toxic heavy metal), but the unit mmHg has been retained because of its deep entrenchment in medical practice. Electronic sphygmomanometers and pressure transducers generate readings calibrated to mmHg equivalents.
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