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History Of The Measurement Of Blood Pressure
The Role Of Mercury 200Hg
Advances in anatomy, histology and physiology were directly related to the technology of the day. Thus relaxation of religious constraints helped the progress of anatomy and macroscopic pathology.
The increase of knowledge in pathology and physiology came only with the advent of microscopy and advances in physics. The measurement of blood pressure is directly related to Evangelista Torricelli, who was the first to measure atmospheric pressure.
The barometer (from the Greek baros "weight", and metron via the French mètre, "meter") is generally considered to have been created by the Itallian scientist and mathematician Evangelista Torricelli around 1643. He was trying to discover why vacuum pumps could not raise water more than 10 metres. Knowing that mercury was approximately 13 times denser than water, he was able to scale the experiment down to a more manageable size. He filled an approximately 1 metre long tube, sealed at one end, with mercury, and then placed it open side down in a bowl or mercury. The height of the mercury in the tube then dropped to about 760mm above the mercury in the bowl, and varied over time, and was later found to depend on altitude. This lead to the idea of air having mass, and hence of atmospheric pressure.
The force of the mercury in the column must be balanced by the force of the mercury in the bowl pushing into the column, which depends on the atmospheric pressure. The atmospheric pressure could then be determined from two simple equations: F=ma (force = mass x acceleration) and F=PA (force = pressure x area).
substituting mass = volume * density, and volume of a cylinder = cross-sectional area * height, and g = acceleration due to gravity, and ρ is the density of mercury:
Since we are balancing the forces inside the tube, Aair = AHg = A = cross-section area of the tube.
The density of mercury is 13,545 kg/m3 at 20°C (or 13,595 at 0°C), so we can now calculate that the air pressure was approximately 101 kPa.
Because the weight of the mercury column is dependant on both the density of mercury (which in turn depends on the temperature), and the local gravity (which varies with altitude, etc), the barometer needs to be calibrated for maximum accuracy if is not used at sea level and near room temperature.
The low vapour pressure (essentially, the volatility) of mercury allowed for a good vacuum to form, and its low viscosity and surface tension were also beneficial, especially in the small tubes of later meters. As mercury has a very linear relationship between temperature and density (eg: see mercury's physical properties at The Engineering Toolbox), it was also well suited to use in thermometers.
In recognition of Torricelli's work, the pressure in a vacuum is measured in torr (1 Torr = 1/760 standard atmospheres, and extremely close to 1 mmHg). The pressure above the mercury column in Torricelli's experiment is known as a Torricellian vacuum, and is approximately 10-3 Torr (0.13 Pa), comprised of mercury vapour. In 1959, the USSR honoured Torricelli by placing his image on a stamp.
A later improvement to the barometer came in 1844 from the French scientist Lucien Vidi. He created the aneroid barometer (meaning "without fluid"), which uses a flexible sealed metal box, called an aneroid cell. Often several of these cells are stacked together to increase the amount of movement. These cells are then linked via a series of levers, further increasing the movement, to a dial displaying the pressure.
Another common type of aneroid sensor was patented in 1849 by Eugene Bourdon, and became known as a Bourdon gauge. He realised that a curved tube will try to straighten when it is pressurised. One end of the tube is attached to the input, and immobilised. The other end is free to move, and is linked to the display by a series of levers and gears, magnifying the distortion.
Blood Pressure Measurement
The estimation of blood pressure was due to the discovery of blood circulation by William Harvey in 1628. He published a treatise entitled "Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus" ("An Anatomical Exercise on the Motion of the Heart and Blood in Living Beings") in which described the heart as a pump, and arteries as efferent and veins as afferent pipes (vessels).
One of the first to measure blood pressure was Stephen Hales in 1727. Among his other achievements (eg: measuring the volume of the heart), Reverent Hales is noted for siting a cannula in the leg artery of a 14 year old mare, connecting it to a long glass tube and measuring the height to which the blood rose. After some time the blood rose to 8 feet and 3 inches above the ventricles, and oscillated 3-4 inches with each contraction (Hales, S. "Haemastaticks", 3rd Edition, 1738).
In 1855, Karl von Vierordt asserted that blood pressure could be measured by non-invasive means by compressing the arteries with weights, and measure the pressure required to stop the blood from flowing.
Vierordt's design was difficult to use, and largely unsuccessful. It was significantly improved by Étienne Jules Marey in 1860. Marey by replacing the counterweight with a water filled chamber, in which the pressure could be increased, and used a sphygmograph to record the pulse, and a manometer to record the blood pressure. Marey's design improved the accuracy of measurement and recording, but was still very complicated to operate, and wasn't used often.
The first recorded direct measurement of human blood pressure was made by the French surgeon Faivre in 1856. He connected the brachial and femoral arteries of a patient to a mercury manometer, and recorded their pressures.
The first modern sphygmomanometer (from the Greek "sphygmos", pulse; and French "manomètre", pressure meter) was developed in 1881 by the Austrian physician Samuel Siegfried Karl Ritter von Basch. Basch was a graduate of the Karl University in Prague and the physician to Maximillian, the Emperor of Mexico. Basch used a water filled bag surrounding a rubber bulb which was connected to a column of mercury. This was placed on an artery in the upper arm. Water was pumped into the bag until the pulsation stopped. The height of the mercury column in millimetres was then recorded as the systolic blood pressure. Von Basch's device was experimentally confirmed to be more accurate than earlier devices.
In 1889, Pierre-Carl Potain improved on Basch's design, by replacing the water with air, and using an aneroid meter instead of a mercury manometer.
The Basch design was later improved by Scipione Riva-Rocci in 1896, who made it easier to use. He created a non-expanding cuff that encircled the whole arm, and had an inflatable bag on the inside. This bag was then connected to both a rubber bulb, which inflated it, and to a mercury manometer to measure its pressure. The patient's arm was placed through this ring, and then the bag was inflated until the pulse stopped. The pressure was slowly decreased until the pulse could be felt again, which was then recorded as the systolic pressure.
In a report to the Imperial Military Medical Academy in 1905, the Russian surgeon Nikolai Korotkov described a non-invasive way to measure blood pressure. He wrote that by inflating a cuff wrapped around the upper arm until circulation was stopped, and then slowly reducing the pressure, one could measure both the systolic and diastolic blood pressure. A stethoscope placed under the cuff was used to listen to the blood flow.
As normal blood flow is non-turbulent, it does not make any sound. The sounds that can heard are caused by the turbulent flow of blood as it passes through the intermittently opened artery.
The first tapping sounds occur when the cuff pressure is the same as the systolic (maximum) blood pressure, and the blood is able to pass through. Eventually, as the pressure in the cuff is decreased, there are no more sounds produced. This indicates that the blood is flowing freely, which gives the diastolic (minimum) blood pressure.
These sounds have become known as Korotkov sounds and are still widely used.
After the arrival of the microprocessor, it has been possible to measure the pulse digitally. These machines often use a piezoelectric pressure sensor to measure the oscillations caused by the pulse, and then calculate the systolic and diastolic pressures. This enables them to be used in areas too noisy to use the analogue auscultation method, and also removes the need for the hazardous mercury. Unfortunately, they may not be as accurate (depending on the size of the cuff, placement, etc), and there are a number of cases where the use of these digital machines is inadvisable, such as arteriosclerosis, arrhythmia, pulsus paradoxus, etc; and in these cases a trained operator using an analogue sphygmomanometer is more accurate.
For a more detailed history, you may wish to read "A Short History of Blood Pressure Measurement" by Jeremy Booth, in the "Proceedings of the Royal Society of Medicine", vol 70, Nov 1977.
Blood Pressure Meters
A mercury manometer kindly loaned to us by professor Donald Simpson. It belonged to his aunt, Dr. Hubbe. She studied arts and medicine at the University of Adelaide, taught anatomy in Professor Watson's department, and had a small private practice.
The gauge measures to over 220 millimetres of mercury. The rubber bulb is used to inflate the cuff and the meter is contained in the stainless steel container (the dial). The knurled screw on the inflating bulb is used to control the flow of air into and out of the meter.
Three blood pressure cuffs. Those attached to the aneroid and mercury manometers are connected to two tubes. One tube connects to the inflating rubber bulb and the other to the manometer. The top centre one has only one tube, and is connected to a digital manometer.
Above is a digital manometer purchased from the local chemist for 90 dollars. The cuff is inflated by a battery operated pump, and the systolic and diastolic pressures are recorded digitally during the deflation and stored. They reported as reliable and accurate, but some physicians still use mercury or aneroid types.
A professional blood pressure meter, which comes with a range of cuff widths for more accurate readings over a wider range of arm diameters, allowing for more accurate readings.
Some modern blood pressure monitors are integrated with other devices, and are able to record pulse, blood pressure, oxygen saturation, temperature and an ECG. These data can be printed or stored for later analysis and reference.