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There are a number of sources that confirm blood pressure cuff size can have a significant impact upon the measured reading. (See 1, 2, 3, 4) The most succinct description I have seen is this:

The error that is introduced by too small a blood pressure cuff is a reading that is too high and the error can be 50 mm and this is not rare. ...
Too large a cuff introduces the opposite error, an artificially low reading but not nearly the magnitude of the too small cuff error.

Disclaimer, I haven't reviewed the material too closely on the site I quoted, so it's likely they're trying to sell something. But their summary is in alignment with other non-commercial sites I've found.

What I'd like to understand is why the blood pressure cuff size affects the measured reading.

From my simplistic understanding, a blood pressure cuff squeezes against the brachial artery.

brachial artery

The compression of the brachial artery temporarily stops blood flow and allows the practitioner to measure when flow commences again.

Assuming the cuff is large enough to stop blood flow, I would like to understand the physiological principles that are affecting the read measurement. And I think this is a valid engineering question as I'm interested in the interaction between the measuring device (blood pressure cuff) and the system being measured (blood pressure via the brachial artery). In other (much bigger) words, what biomedical engineering principle is at work to cause the measured blood pressure to differ?

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The first clue is in your reference to the succinct description:

You may need a large cuff if your arm size is 11 or 12 inches. By 13 inches you do need a large cuff.

The brachial artery is protected by the muscles and fat in the upper arm. The larger the girth of the upper arm the thicker the muscles and fat protecting the artery.

Any pressure applied to the upper arm to squeeze the brachial artery must be applied through the thickness of muscle and fat (soft tissues) between the artery of the surface of the arm.

Given that the aim of the blood pressure cuff is to squeeze the brachial artery to temporarily prevent blood flow, if a small cuff is used on a large-girth arm, with thick muscles and/or lots of fat, a higher pressure will need to be applied than when a larger cuff is used. The opposite occurs when a large cuff is used on a small-girth arm, with small muscles and very little fat: less pressure is needed to squeeze the brachial artery shut because of the lower resistance offered by the arm muscles and fat. Hence to apply the correct pressure to squeeze the brachial artery, the correct size of cuff needs to be used.

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  • $\begingroup$ Can you elaborate on the "resistance offered by the arm muscles and fat" to pressure? It's not clear to me whether you're suggesting this effect is due purely to the geometry of the arm, or relates to compression of the tissue between the cuff and artery (for example). $\endgroup$ – Air Apr 23 '15 at 23:31
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I would guess that there is more than one reason to that (However, I am speculating, since I could not find any technical research on it). But in general the question is what effects can lead to a change in “pressure-reading”:

In order to compress the artery in a way, that the blood flow is stopped the arm needs to be compressed as well. For simplicity one could think of it as reducing the diameter/cross-area of the arm. When inflating the cuff the inner side of the cuff moves inward (and thereby reduces the area, and thereby compressing the artery). Applying the same pressure to two cuffs (one small one large). The larger one will extend more and thereby reduce the area to a larger extend (Since assuming the cuffs have the same thickness but differ in length, there is more fabric for the larger one).

Since one needs some way of “calibrating” the cuff-pressure-reading with the blood-pressure there will be a standard-cuff which allows for a calibration/correlation of both pressures. However, I think one will also need a standard-arm to provide the calibration. Therefore, I would argue, that the cuff size might play a role in the pressure reading, it is however not the only influence. For example I would guess that also the size/area-ratio between cuff and arm-cross-area will play a role in the correlation, so that measuring blood-pressure on children might require a smaller cuff.

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