Measurement of Blood Pressure
Once the circular motion of blood in a closed system was understood, investigators became interested in studying the properties of this system. The pulse was correlated with heart rate; the total volume of blood could be determined by observation. The measurement of blood pressure was more difficult. The first recorded measurement was not by a physician, but by a clergyman, Stephen Hales, who was interested in science and physiology. In 1733, he inserted long hollow tubing into an artery in the neck of a horse. He was astounded to see the blood rise in the tubing to a height of nine feet! This crude pressure measuring device, a manometer, was later made more practical by allowing the blood pressure to displace a column of mercury.

Mercury is about 13.6 times denser, or heavier, than blood or water. Even modern blood pressure measuring devices, sphygmomanometers, which use an air pressure system (aneroid manometers) still record the pressure as "millimeters of mercury". This refers to the height of a mercury column that could be balanced by the pressure. "Millimeters of mercury" is often abbreviated mm. Hg., since "Hg" is the chemical symbol for mercury. The nine-foot column of blood observed by Stephen Hales is equivalent to 2,743.2 millimeters (304.8, the number of millimeters in a foot, times 9). Since mercury is approximately 13.6 times heavier than blood or water, the blood pressure of the horse, in modern terminology, was about 202 mm. Hg. (2,743.2 divided by 13.6).Stephen Hales did not pursue the measurement of blood pressure. He became interested the movement of gases in the lungs. It was he who first demonstrated "artificial ventilation" or "artificial respiration" in animals. The modern importance of this discovery is well known, as it applies to mechanical ventilation during general anesthesia and in resuscitation of persons near death.

Stephen Hales 1677-1761

In spite of Rev. Hales' experiment with the horse, it was to be 143 years before an inventor, named Ritter von Basch, produced a measuring device, a sphygmomanometer, which was practical and accurate, and did not require insertion into an artery! This apparatus was introduced in 1876. A "blood pressure cuff" similar to modern instruments, was introduced in 1896 by Scipione Riva-Rocci. The early sphygmomanometers used an inflatable cuff that was placed on the upper arm. It was inflated to a pressure sufficient to compress the artery and obliterate the pulse. The actual pressure was measured by balancing the pressure in the cuff with a reservoir of mercury. The mercury was displaced upward into a calibrated glass tube from which air had been evacuated. The systolic pressure was measured by observing the pulse as pressure was released from the upper arm. The systolic pressure is produced in the circulatory system during contraction, or systole, of the main pumping chambers of the heart, the ventricles. This is the higher pressure recorded. A lower pressure, called the diastolic pressure, is the pressure in the system when the heart is relaxed, or in diastole.

Dr. Rene Theophile Hyacinthe Laennec

The systolic pressure could then fairly well be determined by feeling for the pulse, and noting the pressure when the pulse first returned during deflation of the cuff. To measure more accurately required the use of a listening instrument, the stethoscope. The first stethoscope was simply a tube made of stiff rolled paper, later devised as a wooden cylinder with an ear-piece. It was invented in 1816 by René Théophile Hyacinthe Laënnec, a French physician. Before the invention of the stethoscope, the physician placed his ear against the bare chest to examine the sounds of the lungs and heart. While the stethoscope improved the quality of the sounds heard, the real reason for its invention was to allow the physician to examine the chests of his female patients more discreetly. It also made it easier to examine those patients who were very obese, and to put some distance between the physician and patients with particularly loathsome or malodorous diseases. The Laënnec stethoscope evolved into an instrument with flexible tubing, using both ears, by 1826. A stethoscope very similar to those now used was available by 1855.

In 1905, N. S. Korotkoff, using the stethoscope, described the sounds heard over the artery (pulse) as the pressure in the cuff was released. He was able to show that the disappearance of the sounds corresponded to the pressure as the heart relaxed, called diastolic pressure. The sounds heard when measuring blood pressure are still referred to as the Korotkoff sounds. Blood pressure is therefore now recorded with two numbers; for example, 120/80 mm, Hg. In this example the pulse sound is first heard at a systolic pressure of 120, and disappears at a diastolic pressure of 80.

Twentieth Century Medicine
With the blood pressure cuff and stethoscope in hand, physicians entered the 20th Century. In the 19th Century rapid advances had begun in all fields of science. Important to the practice of medicine was the technology of measurements, statistics, and record keeping, along with growing knowledge of chemistry and physiology. Records were kept more carefully, and case reports were subjected to scientific analysis. The invention and rapid growth of communications technology allowed clinicians and researchers to share their discoveries more rapidly.