Blood Pressure

One of the first excursions away from urine in the search for markers of function and disease came in 1555 with the publication of a book called Sphygmicae artis iam mille ducentos annos perditae & desideratae Libri V by a physician from Poznan in Poland named Jozef Strut (better known by his Latinized name, Iosephus Struthius). In this 366-page work, Struthius described placing increasing weights on the skin over an artery until the pulse was no longer able to lift the load. The weight needed to achieve this gave a crude measure of what he called "the strength of the pulse" or, as we would call it today, blood pressure.

Early attempts at quantitative measurement of blood pressure had to be conducted in animals rather than human subjects because of the invasiveness of the technique. The first recorded success with these techniques dates from 1733, when the Reverend Stephen Hales, a British veterinary surgeon, inserted a brass pipe into a horse's artery and connected the pipe to a glass tube. Hales observed the blood rising in the tube and concluded not only that the rise was due to the pressure of the blood in the artery but also that the height of the rise was a measure of that pressure.

By 1847, experimental technique had progressed to the point where it was feasible to measure blood pressure in humans, albeit still invasively. Carl Ludwig inserted brass cannulas directly into an artery and connected them via further brass pipework to a U-shaped manometer. An ivory float on the water in the manometer was arranged to move a quill against a rotating drum, and the instrument was known as a kymograph ("wave-writer" in Greek).

Meanwhile, in 1834, Jules Hérisson had described his sphygmomètre, which consisted of a steel cup containing mercury, covered by a thin membrane, with a calibrated glass tube projecting from it. The membrane was placed over the skin covering an artery and the pressure in the artery could be gauged from the movements of the mercury into the glass tube.

Although minor improvements were suggested by a number of authors over the next few years, credit for the invention of the true sphygmomanom-eter goes to Samuel Siegfried Karl Ritter von Basch, whose original 1881 model used water in both the cuff and the manometer tube. Five years later, Scipione Riva-Rocci introduced an improved version in which an inflatable bag in the cuff was connected to a mercury manometer, but neither of these early machines attracted widespread interest. Only in 1901, when the famous American surgeon Harvey Cushing brought back one of Riva-Rocci's machines on his return from a trip to Italy, did noninvasive blood pressure measurement really take off.

Sphygmomanometers of the late nineteenth century relied on palpation of the pulse and so could only be used to determine systolic blood pressure. Measurement of diastolic pressure only became possible when Nikolai Korotkoff observed in 1905 that characteristic sounds were made by the constriction of the artery at certain points in the inflation and deflation of the cuff. The greater accuracy allowed by auscultation of these Korotkoff sounds opened the way for the massive expansion in blood pressure research that characterized the twentieth century.

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