Unit of measurement for blood pressure

Blood Pressure Monitor and Home Blood Pressure Monitors

Blood Pressure Unit

Some hospitals have a special discrete area dedicated to the treatment of high blood pressure. There are also special clinics dedicated to researching blood pressure. Much of what we know about blood pressure comes from these units.

Blood Pressure Unit of Measurement

In the early 1700's a British veterinarian demonstrated that blood was under pressure by inserting a tube into a horse’s artery and connecting it to a glass tube. He observed the blood rising in the vertical tube and concluded that it had pressure.

It was not until 1847 that a human blood pressure was demonstrated but again by a catheter inserted directly into an artery. The blood would rise in the tube until the weight of the column of blood was equal to the pressure of the blood. Unfortunately, this required a tube 5 or 6 feet tall and, to be able to demonstrate hypertension, even 12 or 13 feet.

Neither the invasive technique nor the huge column was practical. In 1881 Ritter von Basch developed a device to encircle the arm with pressure sufficient to obliterate the pulse in an artery beyond the cuff. Connected to a manometer (a pressure measuring device) one could read how much pressure was required to shut off the pulse. Intra-arterial measurement confirmed the accuracy. This method read only the systolic pressure.

In 1896 an Italian, Riva-Rocci, developed the prototype of the mercury sphygmomanometer used to this day. He reasoned that the very high column could be greatly shortened if a heavy liquid could be used. Fortunately, mercury (Hg) was available.

A silvery liquid that is 13.6 times as heavy as water, mercury could shorten the column to less than a foot. Thus he connected the cuff wrapped around the arm to a glass column of mercury that showed the pressure in the cuff. The observer could then read how many millimeters of mercury were required to shut off the pulse below the cuff.

The use of mercury is still the gold standard today and the millimeters of mercury still the units of pressure measurement (mm Hg) regardless of the type of apparatus used. A column of mercury of a specific height is a certain pressure no matter how you look at it.

This design was brought to the United States by a neurosurgeon, Harvey Cushing, who was traveling through Italy at the time.