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.

Nikolai Korotkoff, who observed and described the sounds made by the heart pumping the blood beneath the cuff as it was deflated, made the final real advance in 1905. This required the use of a stethoscope to listen but was the first method to allow the diastolic pressure to be measured as well.

In addition, the measurement of both systolic and diastolic pressures was more accurate and reliable than previous methods.

Doctor's Practical Guide:

It's difficult to realize but we only began to take blood pressures about one hundred years ago.

Thus the blood pressure unit of measurement today is still millimeters of mercury (mm Hg).

The sounds we observe when taking a blood pressure are still called the Korotkoff sounds. This only requires the operator to deflate the cuff and observe at what pressure the Korotkoff sounds start and at what pressure they stop. Voila! These are the systolic and the diastolic pressures and are written for example as 120/80 or 120 over 80.

I believe that everyone who can should learn to take his or her own blood pressure. It is easy to learn for a willing person, our modern units practically do it for you! See this page for how and why: How To Take Blood Pressure