According to the American Heart Association, MMHG, commonly written as mm HG, stands for "millimeters of mercury." This unit is used in the measurement of blood pressure.
When medical personnel take a blood pressure reading, such as "115/70 mm HG," it is verbalized as "115 over 70 millimeters of mercury." The top number, the systolic reading, measures arterial pressure during the heart's contraction. The bottom diastolic number assesses arterial pressure when the heart is relaxing between beats, refilling itself with blood. In general, a healthy blood pressure reading for adults over the age 20 is a top number less than 120 and a bottom number less than 80. A higher reading indicates a potential problem with blood pressure.