17 October, 2014    #20

mmHg BP Units

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Blood Pressure (BP) was officially first discovered in 1733 by Stephen Hales, an English researcher.


Hales inserted a brass tube into the artery of a tethered and supine horse while observing pulsating blood rising to a height of 8 feet 3 inches (251 cm) in an attached 10 foot* vertical glass tube open to the atmosphere.  In other words, the horse’s Systolic Blood Pressure, the pressure required to push blood to that height, was measured at 96 inches or 253 cm of blood.


In 1828 Poiseuille, a French physician-physicist, won the gold medal of the Royal Academy of Medicine for his doctoral thesis on the use of a Mercury (Hg) Manometer for the measurement of arterial blood pressure. By connecting his U-shaped glass manometer tube partially filled with mercury to an artery, he effectively reduced Hales 10 foot BP device to approximately 9 inches.


Mercury is around 13.6 times the relative density or weight of blood. Thus, we can calculate Hales horse BP results in corresponding units of mercury (Hg), the units we use today.


251cm of blood / 13.6 = 18.4 cmHg >>

18.4 cmHg x (10mm / 1cm) = 184 mmHg


Since a horse’s optimal Systolic BP is 115 mmHg, similar to humans, you might say that Hales horse was a little upset!


In 1881, Samuel Siegfried Karl Ritter von Basch of Prague created an apparatus that measured BP indirectly on the wrist without severing an artery, and therefore, greatly reducing the risk of infection.


Von Basch’s method of indirect BP measurement not only lowered the risk of infection, but paved the way for the amazing evolution of non-invasive BP monitors we use today.


Mercury is an element with the symbol Hg.  Commonly known as quicksilver and formerly as hydrargyrum: “hydra” meaning watery and “argyrum” silver, or “Liquid Silver” from the Latinized Greek lexicon.


*NOTE: I had to estimate the height of Hales device to separate the horse’s BP from the height of his device.


Read more about the history of Blood Pressure at the link below:




A Short History of BP Measurement by Jeremy Booth.



Thank you Dr. Booth.

Mike Kohut, President, DDMS




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