The symbol mmHg represents the millimeter of mercury, a unit of pressure that remains the universal standard for clinical measurements in medicine. This measurement is encountered most frequently when assessing human blood pressure, a fundamental indicator of cardiovascular health. The mmHg value describes the force exerted by the blood against the walls of the arteries.
Defining the Millimeter of Mercury
The term mmHg is a composite unit that stands for millimeters of mercury, where “mm” denotes millimeters and “Hg” is the chemical symbol for the element mercury. This unit is rooted in the physics of pressure measurement, specifically the use of a barometer developed in the 17th century. Early pressure measurements were determined by the height of a column of liquid that a specific pressure could support against gravity.
Mercury was the fluid of choice for these instruments because of its high density. This dense nature meant the column required to measure pressure was relatively short and manageable, unlike a water column which would need to be over 33 feet tall. While modern technology has largely replaced liquid mercury devices with digital sensors, the unit of measurement remains standardized in the medical field.
The Standard Measurement for Blood Pressure
Blood pressure readings are comprised of two distinct numbers, both of which are expressed in millimeters of mercury. The measurement is taken using a device called a sphygmomanometer, which applies external pressure to temporarily stop blood flow in an arm artery. As the pressure is slowly released, the device records the force at two specific points in the cardiac cycle.
The first and higher number is the systolic pressure, which reflects the maximum force exerted on the artery walls when the heart muscle contracts and pushes blood out. The second and lower number is the diastolic pressure, which measures the lowest pressure in the arteries when the heart is at rest, filling with blood between beats. Although many blood pressure cuffs now use oscillometric or aneroid technology without actual mercury, the medical community reports the results in mmHg to maintain consistency with established clinical practice.
Interpreting Blood Pressure Categories
The numerical values expressed in mmHg are categorized by health organizations to determine a person’s risk level for cardiovascular complications. A reading in the Normal category is defined as a systolic pressure of less than 120 mmHg and a diastolic pressure of less than 80 mmHg. Maintaining readings within this range indicates a healthy workload for the heart and blood vessels.
Readings that fall into the Elevated category show a systolic pressure between 120 and 129 mmHg, while the diastolic pressure remains less than 80 mmHg. This stage signals an increased risk and suggests lifestyle changes are necessary.
If the systolic reading is between 130 and 139 mmHg or the diastolic reading is between 80 and 89 mmHg, the patient is classified as having Stage 1 Hypertension.
The next level, Stage 2 Hypertension, is defined by a systolic reading of 140 mmHg or higher or a diastolic reading of 90 mmHg or higher. At this stage, a healthcare provider recommends both lifestyle modifications and medication to reduce pressure and prevent organ damage.
The highest category is the Hypertensive Crisis, defined as a reading higher than 180 mmHg systolic and/or higher than 120 mmHg diastolic. This level requires immediate medical attention due to the risk of stroke or other serious events.