# Physics of bp cuffs

1. Oct 1, 2008

### mwaso

this really is a physics question! (I think)

so, when you measure blood pressure, you wrap a cuff around the subject's arm and fill it with air past the point where the artery is occluded. Then, you listen with a stethoscope as you slowly release the air...you'll be able to hear beats for a range of pressures. These beats signify that the blood flow is obstructed and the top and bottom numbers read give the systolic and diastolic blood pressures (pressure during compression and relaxation of ventricles, respectively). Make sense?

Ok, so the cuffs come in different sizes and are calibrated accordingly. If you use the wrong size cuff on a person's arm, you get an inaccurate reading. For a cuff that's too large for the arm, the bp comes out lower than the true pressure. And here's where my conundrum is, bc that doesn't make sense! To me, it makes sense that the cuff, being too large, might possibly give an abnormally high reading because it's harder to make snug on the arm and therefore would require more pressure to occlude the artery...but I can't think of any reason why the cuff would give a low reading.

does anyone understand the physics behind this?

2. Oct 1, 2008

### atyy

I'm guessing that a too large cuff is easily over-wrapped around the arm, so that it becomes tight with only a small pressure.

3. Oct 1, 2008

### Andy Resnick

I wonder if it has to do with balancing the compressibility of a person's arm against the compressibility of the cuff.

The claim is that too large a cuff gives a low reading, meaning that less pressure is required to stop blood flow. Is the converse true for a too-small cuff?

4. Oct 4, 2008

### schroder

Hmmmm. It does seem somewhat counter-intuitive that the smaller cuff gives the higher reading.One might be tempted to think that the smaller cuff would compress the artery more and with a given inflation pressure give a lower reading. But I believe that when the smaller cuff is stretched around a large arm it will be more difficult to inflate the bladder because the stretching pulls the membranes closer together with an elastic force. So it will require more inflation pressure to expand the cuff and compress the artery, resulting in a higher reading. The converse would be true with the larger cuff; it is more easy to inflate and compress the artery so it will give a lower reading. In my experience, most nurses are not properly trained to take BP, and it should be taken in BOTH arms as a comparison! (But that is another story)

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