Blood Pressure & Units

The Powers That Be have now decided that high blood pressure is defined as 130/80 or above. One has to wonder what percentage of the population can come in under this hurdle?

130/80? The medical profession measures blood pressure in millimeters of mercury – the height (under standard gravity) of the column of liquid mercury that would be supported by the pressure of the blood within certain arteries. But that unit is not Systeme International!

If the medical profession were to knuckle under to Political Correctness and adopt French units, the high blood pressure threshold would be 17,332/10,666 Pascal. For medical professionals dealing with urgent life-and-death situations, there does seem to be a definite advantage in ignoring Systeme International units in favor of units which are more appropriate in scale.

This might be seen as analogous to the frequent practice of measuring tire pressures in pounds per square inch – 32 psi is more practical than 220,632 Pascal. In the same way, astronomers measure distances in (non-Systeme International) units such as Astronomical Units, parsecs, or light-years; it would be futile to talk about such distances in SI meters.

While SI units have a place, there is much to be said for measuring items of interest in practical unit systems that provide numerical values with a reasonably appropriate scale – especially with today’s technology where conversion (when required) of practical units to academic SI units is a trivial task.


In using SI units, one is free to use any of the scale prefixes to scale the fundamental unit to one appropriate for the quantity being measured. An SI unit with a scale prefix is still an SI unit. For example, when describing normal adult resting blood pressure in SI, it is usually cited 16 kPa (kilopascals) systolic and 11 kPa diastolic, equivalent 120/80 mmHg as doctors write it.

Similarly, tire pressure in Europe is usually cited in either kPa or sometimes bar (atmospheres), where 1 bar is 100 kPa. It is much easier to think of atmospheric pressure as 100 kPa than 14.7 psi.


SI users typically use kPa easily. 17/10 seems easy to remember, with the same number of significant figures as mmHg.


So, you get five units of measure between systolic and diastolic? Not very discriminating. There’s a reason many shun SI in favor of a system that the lords and ladies can use.


Reality is that every set of units is arbitrary. Witness, for example, Systeme International’s decision to make the base unit of length be the meter while the base unit of mass is 1,000 grams. It seems that the most useful way to approach measurement is to adopt units which are appropriate to the task at hand. For some reason, the medical profession seems to display various examples of how to do it – and how not to do it.

Measurement of blood sugar is an interesting example. Doctors played around and found that if they adopted the rather arbitrary unit of milligrams per deciLiter, then the normal human blood sugar level was approximately 100 – a very easy score to remember & communicate. This also provided a scale for blood sugar with an appropriate sensitivity.

But this was not “SI” enough for the Euros. They decided that it would be better (more precise, whatever) to use millimole per deciLiter as the unit for blood sugar – yielding a normal blood sugar reading of an arbitrary-sounding 5.6 and a rather coarse scale.

The Systeme International has its place in the academic world, but it seems sensible always to be flexible about units and tailor them to whatever real world measurement is under study.


All you say, @Gavin is true.

But there’s more. Take BP. Today it is extremely rare to find a manometer using mercury, yet that is the ONLY accurate measurement. The current analog methods are common but have no way to standardize or adjust inaccuracies out of them. THAT makes them FAR LESS USEFUL. You do NOT KNOW if your BP really IS greater than 130/80 or it’s just a machine error. We “rely” on “technology” way too much, often not even comprehending what we are relying upon.


So true! I asked my medical professional how often the manual blood pressure cuffs they use are calibrated? Answer was – we don’t do that; these manual blood pressure measurement devices are reliable. However, he went on to say they do not use automatic blood pressure measuring devices because they are unreliable and they don’t know when they go out of calibration.


It’s not just the automatic machines. That doctor’s apparatus is not standardizable. IT may use human ears to hear the sounds but the numbers at which they can be heard cannot be adjusted nor checked, nor calibrated in any way.