What effect does fat and muscle have on bp readings?
May 28, 2015 5:08 AM   Subscribe

If you have a substantial amount of fat or muscle between a blood pressure cuff and the artery it's compressing, what effect does that have on the recorded blood pressure?

It occurred to me that a layer of compressible fat (or relatively incompressible muscle) should have an effect on the pressure applied to the artery beneath that layer. In either case I *assume* that the pressure applied on the artery would be reduced, effectively increasing the recorded BP.

Is there any logic to this? Is there any research on it, or consensus on how much of an effect it has?

[ Every google search I can think of gets polluted with results for the metabolic body fat to BP correlation instead of a mechanical arm fat/muscle to BP correlation ]
posted by sodium lights the horizon to Health & Fitness (8 answers total) 2 users marked this as a favorite
Has no effect, as long as the cuff fits properly (that's why there are different cuff sizes).
posted by tinkletown at 5:29 AM on May 28, 2015 [4 favorites]

Response by poster: So (with the right sized cuff) I could stick a hunk of pork belly between my arm and the pressure cuff and get the same reading? That sounds counterintuitive.
posted by sodium lights the horizon at 7:02 AM on May 28, 2015

Best answer: The different mechanical properties (compressibility) of the tissue surrounding the artery being compressed can introduce small amounts of error into the reading. This study suggests ~ 5% over estimation in some elderly or those with lower compressibility in arm soft tissue, and 5% under estimation in children or those with high compressibility.

Unfortunately, they didn't do the experiment with pork belly.

posted by JeanDupont at 7:29 AM on May 28, 2015 [2 favorites]

Related, using the wrong cuff size (common in large-armed people because the medical assistant or nurse or whatnot may not have a large cuff nearby), results in inaccurate readings, where a too small cuff leads to a falsely high reading.
posted by latkes at 7:33 AM on May 28, 2015 [1 favorite]

The way one measures blood pressure is to apply enough pressure to collapse the artery and stop blood flowing. Then you slowly release the pressure until the blood just barely gets flowing again - that's your systolic pressure, the pressure at which the blood can just barely "win" against the cuff. This causes characteristic sounds/vibrations which either a trained person or machine can detect. Then, you reduce the amount of pressure further, until the characteristic sounds/vibrations completely stop, which happens when the artery is totally open - that's your diastolic pressure. Apologies if you already knew all this.

I imagine there IS some degree of variability in blood pressure measurements based on the size/composition of the arm, but human bodies are probably not as compressible as you're imagining - squishy yes, but not so compressible (like water)*. I doubt the potential impact is very large.

*This old paper lists the compressibility of human soft tissue around 0.4-0.5 m^2/GN, almost exactly the same as water http://www.sciencedirect.com/science/article/pii/0021929078900143
posted by Cygnet at 7:35 AM on May 28, 2015 [6 favorites]

You may want to add "intra-arterial" to your searches as that would be the gold standard to compare non-invasive BP measures to.
posted by latkes at 7:36 AM on May 28, 2015

Best answer: In practice, the error introduced by misfitting the cuff (especially easy to do in an emergency setting, in my experience), patient movement, environmental noise, etc. vastly exceed any variations based on fat and muscle. You can take (and I have taken) a BP on somebody's thigh using an ordinary long cuff; you get pretty much the same result as you get with the same equipment on the arm.
posted by fifthrider at 8:48 AM on May 28, 2015 [3 favorites]

Anecdotally, arterial line direct monitoring of BP is almost always different from cuff readings when I've had both to refer to. There are a lot of variables with cuffs -- size, positioning, patient movement, the amount of intervening tissue (which I'm sure has some impact) -- that go away with direct monitoring. In addition to up-to-the-moment pressures without waiting for a cuff to cycle, this is another one of the reasons an art line is preferable in a patient you're concerned about, even though it means an arterial stick and can randomly be godawfully difficult to place.
posted by killdevil at 12:05 PM on May 28, 2015

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