Everybody knows that blood pressure is measured in millimeters of mercury, written as mmHg. But not everyone knows why. Let’s take a closer look at the history of mercury and blood pressure measurement.
What are mmHg?
One mmHg is how much pressure is needed to move a column of mercury up a column by one millimeter. One mmHg is equal to 133.322387415 pascals, and approximately equal to one Torr, a unit of standard atmospheric pressure.
MmHg is a very common way to measure pressure in a wide range of devices. Microns (micrometers of mercury), Torrs, and inHg (inches of mercury) are also used to measure and express pressure in vacuum systems.
Why is mmHg Used to Measure Blood Pressure?
To understand that, we have to go all the way back to 1643, when Evangelista Torricelli invented the mercury manometer. It was a vertical tube filled with mercury, and the first hydrostatic gauge. It led to the first accurate barometer in history.
A few years later, in 1661, Christiaan Huygens invented the U-tube manometer, which quickly displayed pressure differences.
As more and more pressure-measuring devices were created, mercury continued to be the fluid of choice.
Mercury has a high density, over 13 times denser than water, which makes it a compact fluid to use in instruments (in other words, a glass tube would need to be 13.6 times longer if it was using water as the working fluid).
Mercury is highly cohesive, so it does not cling to the inside of a glass tube, leading to errors in measurement that may occur when oils are used. Mercury has low vapor pressure, so it does not expand as easily as other fluids.
So mercury was the natural choice of working fluid when Samuel Siegfried Karl Ritter von Basch invented the sphygmomanometer in 1881.
By the early 1900s, mercury sphygmomanometers were standard equipment in medical practices around the world, often known by the brand name Baumanometer.
How do Mercury Sphygmomanometers Work?
Mercury sphygmomanometers work in combination with the ears of the clinician to give accurate blood pressure readings.
A cuff is placed around the patient’s arm (or any extremity with a superficial artery). The cuff is inflated with air, applying increasing pressure until the artery collapses.
The clinician, listening with a stethoscope, can hear that the blood flow has halted.
Pressure in the cuff is slowly released, and the clinician can hear the turbulent flow and capture the moment in which the arterial pressure falls below the pressure of the cuff. The reading on the mercury manometer at that moment is the systolic blood pressure.
The clinician continues to listen and release pressure from the cuff. When laminar blood flow resumes, it can no longer be heard, and the reading at that moment is the diastolic blood pressure.
Why do We Still Use Mercury to Measure Blood Pressure?
Of course, we now know that mercury is a highly toxic material, and it is being phased out of medical devices around the world.
Unfortunately, mercury sphygmomanometers remain the most accurate way to measure blood pressure. They are also incredibly easy to evaluate to see if they need calibration; the unused, inactive meter should read “0,” and if it doesn’t, it needs servicing.
Other sphygmomanometers are much more complex to evaluate for accuracy and to calibrate. In fact, aneroid sphygmomanometers are calibrated by measuring them against a mercury sphygmomanometer to determine if they are inaccurate.
Poorly maintained mercury sphygmomanometers may leak and pose health hazards, and studies show that not all of these devices are maintained or serviced as often as they should be.
But modern mercury devices are much less prone to leaks or damage, and other blood pressure measuring devices to date remain far less accurate.
Conclusion
We measure blood pressure in mmHg not just because our blood pressure tools are based on ancient technologies that required mercury.
It’s also because those tools led to a standard of measurement that is in use in a wide range of devices that measure pressure of all kinds, so even when we no longer use mercury-based devices, we will continue to use mmHg as a measure.
It’s a lot like using a “foot” as a measure of distance — at one time it was literally any person’s foot, then it was standardized to be only a single person’s foot, then it was standardized to be equal to 12 inches, and is still used as a unit of measure today, even though the original relationship has been lost in time.