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Comment by lordnacho | Hacker News Reader

I don't get how the traditional cuff method actually works. It's not clear to me how having the thing on your arm tells you what the pressure is inside your blood vessels. Isn't your arm made of things that can stress, therefore the pressure is not the same on either side of it? Same as how how do you measure the pressure inside a tyre or other pressure vessel? The point of the vessel is to make the pressure not the same as outside.

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To put it simply, it measures how much pressure is required to stop your pulse being detected. The higher your blood pressure, the more pressure required to stop the pulse in the arm

It's apprently not very accurate but I suppose good enough.

https://www.acc.org/latest-in-cardiology/articles/2017/07/24...

So I can actually feel my pulse in my upper arm presumably when the device is in it's measurement range so I've always presumed that it's sensing that with a pressure transducer and a simpleish algorithm, but I never thought to ask about how it actually works, and now I wonder if other people can feel them working.

Wont the stiffness and amount of tissue around the vessels change that ? And thus won't it be different from person to person not because the blood has a different pressure but because of the way we are measuring it.

It doesn't just stop the pulse from being detected, it stops the blood from passing into the limb where the cuff is applied.

That's going to mostly depend on how much pressure is behind the blood.

I never knew that, thanks. So when the machine ramps up the pressure, then ramps up some more, then some more, you know you're in trouble?

I've always asked myself the question how this works, but actually never looked it up. Thank you for this explanation!

Brilliant and concise, thanks!

You measure the pressure you need to apply, to stop the blood going through the cuff.

It’s probably not possible to measure blood pressure perfectly. +- 5mmHg seems to be a completely normal error, probably even more. But it seems to be good enough.

The gold standard for BP measurement is direct measurement via an arterial catheter - an invasive and relatively risky procedure.

For a device to be approved as a Class 1 medical device for blood pressure measurement, the FDA wants +/- 3mmHg to an established BP measuring method.

Wow, I didn’t know that.

I still think though, that in a common setup the +-3mm is impossible to reach. People don’t sit perfectly still, may flex their muscles during the measurement, etc

It is a standard way to measure it. So, you might just say that it measures "blood pressure on the upper arm" and that it is taken as a measure. Like temperature with a thermometer.

Like, is the air pressure/temperature/contamination everywhere the same around my city? Well, obviously not, but we make do with what we have.

Medical sensors are fascinating, the one the really blew my mind in simplicity is a light based blood oxygen meter.

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