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What is the difference between [cmH2O], [kPaand], and [mmHg]?

圧力換算 血圧 mmHg cmH2O kPa


The following article is about the units of pressure used during the perfusion process: mmHg, cmH2O and kPa.

But how do you convert them?

And what are the differences between them in the first place?

-Read on, to find out my own answers to these questions.

Table of contents

  1. Conclusion

  2. Why do we use mmHg for blood pressure?

  3. History of sphygmomanometer

  4. Indirect and direct expressions

  5. Summary


First, I’ll start with the conclusion (for those who would like to see some quick answers)!




Remember these formulae and you won’t be afraid while converting kPa, mmHg, and cmH2O.

Why do we use mmHg for blood pressure?

As an engineer, kPa is more familiar to me.

There is an SI system in the unit, which is already firmly established as the unit of choice used in the international standards system.

The pascal (PA) is the SI derived unit of pressure used as in international standards. The Kilopascal (kPa) which represents 1000 times this amount is also used.

So if that's the case, why is mmHg used in blood pressure etc,. in the ‘medical field’?

After a little research, I was able to find out…

"Revision of lightweight unit order pertaining to units of measure of pressure in vivo" Chairman of Japanese Medicine

Somehow, I found myself reading such amazing material. It was taken on behalf of the Subcommittee of the Japanese Medical Association from Dr. Takahisa, Chairman of The Japanese Medical Association, on October 2, 2013.

According to the text, there seems to have been a request from the Director of the General Affairs Division of the Medical Affairs Bureau of the Ministry of Health, Labour and Welfare.

Because it is a quite and a lengthy sentence and difficult to summarize (especially in English!):


Based on the Measurement Law, the unit of mmHg was scheduled to be abolished, but a cabinet order to amend it came out, and you may use mercury column meters "only when used for weighing pressure in vivo".

This seems to be accurate.

It was firmly stipulated by the Cabinet Order that mmHg was due to be removed at the time it was written.

So, as far as I can tell, it seems that this Cabinet Order continued after that point. As of June 9, 2020, mmHg was still being with the chest (only when being done so for weighing pressure in vivo).

Since the pressure gauges that I typically see as an engineer are mainly used for industrial purposes and not for "weighing pressure in vivo", the kPa of the SI unit system is the one I was familiar with. I could see why now. It was nice to be able to understand the difference.

But what if you want to use a pressure gauge for industrial applications to measure pressure in vivo?

The decision is hard to make, and is ultimately subjective, but you can probably use mmHg.

In other words, it’s a better option for researchers who are measuring blood pressure.

History of sphygmomanometer

The world's first sphygmomanometer was conceived in 1896, by Italian doctor. It consists of a pressure gauge part, that reads pressure via a mercury column, and a cuff part, that pressurizes by the gripping of a rubber ball. It is the standard type of sphygmomanometer used for blood pressure measurement by nurses and doctors in hospitals, with the support of stethoscopes. (Kubota, History of Blood Pressure Measurement, Medical Engineering Vol.80, No.6 (2010)).

In fact, the discovery of the Korotkoff sound occurred through this process. The first thing that often happens when a stethoscope is used inside the cuff is to hear this Korotkoff sound.

I also participated in a medical education program for working adults and heard my own Korotkoff sound, but I remember hearing one that surprised me greatly.

Indirect and direct expressions

There are indirect formulas and direct formulas in the method of blood pressure measurement: the indirect formula is basically a method of measuring blood pressure indirectly, by winding the cuff around the arm; the direct formula is a method of measuring blood ornamentally, by directly pointing the tube into the blood vessel.

The observation of blood using a sphygmomanometer was first performed by American medical scientist Earl Wood and others in 1947 and is a method that can measure blood pressure directly and continuously (Kubota, History of blood pressure measurement, Medical And Machine Science Vol. 80, No. 6 (2010)).

Perfusion pressure measurement in perfusion culture requires this type of direct formula. IT makes it possible to continuously measure the pressure of the flow path (≈ artery) in the perfusion of organs and tissues.

With our constant pressure and pulsating constant pressure pumps, it is easy to perform direct flow path pressure measurement, as well as a constant pressure supply that automatically controls the pressure by feeding back to the pump.

The unit shown in the constant-pressure liquid delivery unit is not in vivo, but mmHg is adopted from the viewpoint of measuring the same conditions in the body.

In order to convert kPa to mmHg, it is necessary to check inside the device. If there is an opportunity to introduce one into the range, I will do so further down the line its development story.


To summarize this article above:

  • The relationship between mmHg, kPa and cmH2O is: 1mmHg⇒0.1333kPa⇒1.340cmH2O 1kPa⇒7.5mmHg⇒10.05cmH2O 1cmH2O⇒0.736mmHg⇒0.0981kPa

  • MmHg is frequently used in blood pressure measurement because the world's first sphygmomanometer used mercury.

  • There are indirect and direct formulas for blood pressure measurement; the direct formulas are used for perfusion pressure measurement.

I hope you have gained a deeper understanding of the units used in medical research fields, such as mmHg, kPa, and cmH2O!

Thank you for reading to the end.


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