Millimeter of mercury
A millimeter of mercury is a manometric unit of pressure, formerly defined as the extra pressure generated by a column of mercury one millimetre high. It is now defined precisely as 13.5951 × 9.80665 = 133.322387415 pascals. It is denoted by the symbol "mmHg".
One millimeter of mercury is approximately 1 Torr, or 1/760 of standard atmospheric pressure. The two units are not exactly equal; however, the difference (less than 0.000015%) is negligible for most practical uses.
History and definition
The mercury manometer was the first accurate pressure gauge; they are still used today in many scientific and technical fields due to mercury's sensitivity to temperature and atmospheric and acoustic pressures. They displayed the pressure difference between two fluids as a vertical difference between the mercury levels in two connected reservoirs. For this reason it became customary to measure pressure in "millimeters (or inches) of mercury."
Actual pressure is read as the height difference of the two columns, times the density of mercury, times the local gravitational acceleration. Because the density of mercury depends on temperature and speed of gravity, both of which vary with location and altitude, specific standard values for these two parameters were adopted. This resulted in defining a "millimeter of mercury" as the pressure exerted at the base of a column of mercury 1 millimeter high with a precise density of 13.5951 g/cm3 when the acceleration of gravity is exactly 9.80665 m/s2.
The density 13.5951 g/cm3 chosen for this definition is the approximate density of mercury at 0 °C (32 °F), and 9.80665 m/s2 is the standard gravity.
In practice, measurements are made using local values, which vary little at the Earth's surface. These assumptions limit both the validity and the precision of the mmHg as a unit of pressure.
Relation to the pascal
According to the UK’s National Physical Laboratory (NPL), the assumption of fixed and exact (but ultimately incorrect) values of density and gravity will inherently limit knowledge of the relationship between the millimetre of mercury and the pascal. By contrast, the magnitude of pressure values expressed in the SI pressure unit, the pascal, can flex (albeit not by much) to take account of technological improvements in the underlying definitions of mass, length and time—the SI base quantities from which pressure is derived.
Relation to the torr
The precision of modern transducers is sufficient to show the difference between the torr and the millimetre of mercury. However, instrument readings purported to be in "mmHg" may include large errors due to different definitions, different values of gravity, or varying assumptions about the density and temperature. Misunderstandings about temperature assumptions alone can lead to errors of several tenths of a percent. 
Use in medicine and physiology
In medicine, pressure is still generally measured in millimeters of mercury. These measurements are in general given relative to the current atmospheric pressure: for example, a blood pressure of 120 mmHg, when the current atmospheric pressure is 760 mmHg, means 880 mmHg relative to perfect vacuum.
Routine pressure measurements in medicine include:
- Blood pressure, measured with a sphygmomanometer
- Intraocular pressure, with a tonometer
- Cerebrospinal fluid pressure
- Intracranial pressure
- Intramuscular pressure (compartment syndrome)
- Central venous pressure
- Pulmonary artery catheterization
- Mechanical ventilation