Millimeters of mercury, abbreviated as mmHg, is a common unit for measuring pressure. It is frequently encountered in various fields, particularly within healthcare, where precise pressure measurements are performed. Understanding mmHg helps in interpreting medical readings and appreciating its historical and scientific foundations.
What is Millimeters of Mercury (mmHg)?
Millimeters of mercury (mmHg) is a unit of pressure derived from the height of a mercury column. “mm” represents millimeters, a unit of length, and “Hg” is the chemical symbol for mercury. This unit originated from Evangelista Torricelli’s invention of the mercury barometer in the 1640s. Torricelli’s experiments showed a mercury column remained suspended due to atmospheric pressure.
Mercury was chosen for pressure-measuring devices due to its unique properties. Its high density allows for a more compact column height compared to other liquids like water, which would require a significantly taller apparatus. Mercury also maintains a liquid state at room temperature and has a very low vapor pressure, ensuring accurate readings. This combination of density, liquid state, and low vapor pressure made mercury a suitable substance for early precise pressure measurements.
mmHg and Blood Pressure Readings
The most common encounter with mmHg is in blood pressure measurement. Blood pressure readings are presented as two numbers. The top number is systolic pressure, reflecting the force of blood against artery walls when the heart beats. The bottom number is diastolic pressure, representing the pressure in the arteries when the heart rests between beats.
For instance, a reading of “120/80 mmHg” signifies a systolic pressure of 120 mmHg and a diastolic pressure of 80 mmHg. These measurements provide insight into blood circulation efficiency and cardiovascular health. Regular blood pressure monitoring helps identify conditions like high or low blood pressure. Readings outside the normal range (typically 90/60 mmHg to 120/80 mmHg) can indicate health concerns such as hypertension, which increases the risk of heart disease and stroke.
Beyond Blood Pressure: Other Uses of mmHg
While blood pressure measurement is its most widespread application, mmHg is also utilized in other medical and scientific contexts. In ophthalmology, for example, intraocular pressure (IOP), the fluid pressure inside the eye, is measured in mmHg. Normal intraocular pressure typically ranges from 10 to 21 mmHg, and values outside this range can indicate conditions like glaucoma.
Historically, mmHg was a standard unit for measuring atmospheric pressure, particularly with mercury barometers. Although Pascals are now the SI standard for pressure, mmHg persists in specialized fields due to historical continuity and practical convenience. It is also used in vacuum science and older meteorological equipment.