Pressure is a fundamental physical measurement quantified using various units across science and engineering. The millimeter of mercury, abbreviated as mmHg, is a specific and historically significant measure of pressure. Although it is not part of the modern International System of Units (SI), the mmHg unit maintains a strong presence in particular fields. It is especially recognized in clinical medicine, where it serves as the standard for measuring and reporting common patient readings.
Defining Millimeters of Mercury
The millimeter of mercury is a manometric unit of pressure derived from the historical use of mercury barometers and manometers. It is based on the physical principle that a given pressure can support a column of liquid to a certain height against the force of gravity. Specifically, one mmHg is defined as the pressure exerted by a column of mercury precisely one millimeter high under standard gravitational conditions.
Scientists historically favored mercury because its high density allowed for much shorter and more manageable column heights compared to using water. Standard atmospheric pressure at sea level, for example, is equal to 760 mmHg. The unit is closely related to the Torr, named after Evangelista Torricelli.
While the SI unit for pressure is the Pascal (Pa), the mmHg unit remains in use due to its long-standing tradition and practicality in certain fields. For conversion, one mmHg is precisely equivalent to 133.322 Pascals.
Primary Application: Measuring Blood Pressure
The most common application of this unit is the measurement of arterial blood pressure. Blood pressure is routinely measured using a device called a sphygmomanometer, which historically employed a mercury column to directly display the pressure reading. Although modern devices often use aneroid or digital technology, the results are still universally reported in mmHg.
A blood pressure reading consists of two numbers: systolic and diastolic pressures. Systolic pressure is the higher number, representing the maximum pressure in the arteries when the heart ventricles contract and push blood out into the body. A normal systolic reading is generally considered to be less than 120 mmHg.
Diastolic pressure is the lower number, representing the minimum pressure in the arteries when the heart is at rest and refilling with blood between beats. An example of a healthy adult reading is 120/80 mmHg, where 80 mmHg is the diastolic value.
Other Medical and Scientific Uses
Beyond blood pressure, the mmHg unit is also commonly used to measure other physiological pressures within the body and in specialized scientific settings.
- Intraocular pressure (IOP), quantified by ophthalmologists during eye exams, often for screening glaucoma.
- Cerebrospinal fluid pressure.
- Intracranial pressure.
- Pressures in vacuum systems, where very low pressures are observed.
- Respiratory pressures in medical or research contexts.