Blood pressure/wrong cuff size

[mwaso]

so, when measuring blood pressure, if you use a cuff that's too large for the patient, you get an incorrectly low reading. This doesn't make sense to me. why?

 

[hypatia]

A large cuff doesn't apply enough pressure to compress the artery, with smaller arm size.

It also works the other way. Some obese people are incorrectly diagnosed with high blood pressure when in fact, their blood pressure is actually normal! This false diagnosis is based on improper use of blood pressure equipment, namely using a cuff that's to small.

 

[mgb_phys]

Add to that "white coat hypertension" = I'm in a doctors office, he's going to tell me off for being fat , I'm going to die... puts most people into the high blood pressure category.

 

[Moonbear]

A large cuff doesn't apply enough pressure to compress the artery, with smaller arm size.

Actually, it's the opposite. The large cuff doesn't need to be inflated as much to compress the artery on a smaller arm, so gives an erroneously low reading. Likewise, if a cuff is too small, it will have to be overinflated to get it to compress the artery adequately, so will give an erroneously high reading.

 

[berkeman]

Actually, it's the opposite. The large cuff doesn't need to be inflated as much to compress the artery on a smaller arm, so gives an erroneously low reading. Likewise, if a cuff is too small, it will have to be overinflated to get it to compress the artery adequately, so will give an erroneously high reading.

As Moonbear says, too small a cuff will give an artificially high reading:

http://www.nature.com/jhh/journal/v11/n7/abs/1000470a.html

With the smallest bladder (13 ´ 23 cm) the highest systolic and diastolic BP was measured (mean SBP 127.2 mean DBP 77.0 mm Hg), followed by the bladder of 13 ´ 36 cm (125.1 resp. 75.4 mm Hg). The lowest BP was measured with the bladder of 16 ´ 23 cm (123.7 resp. 74.4 mm Hg).

An interesting point made by some studies is that hypertension may be misdiagnosed in a number of obese patients for this reason. The variation with cuff size seems to be more pronounced with obese patients as well.

 

[Moonbear]

An interesting point made by some studies is that hypertension may be misdiagnosed in a number of obese patients for this reason. The variation with cuff size seems to be more pronounced with obese patients as well.

I wonder how much variation or error is introduced by the obesity by itself. In other words, regardless of the size of cuff, would the artery be properly occluded with a consistent inflation pressure when there is so much fat is between the cuff and artery? Fat has such a different texture than muscle, would having to compress a lot of fat to compress the artery give an inaccurate reading all by itself?

The only comparison I can think of to explain my reasoning is when I collect blood samples from farm animals. It takes relatively little effort to compress the jugular vein of a sheep enough to "puff" it up to get a blood sample...just a light pressure with my thumb. In contrast, to get a blood sample from a cow, light pressure with my thumb isn't going to cut it. With all the fat, you need to lean your whole fist into their neck to adequately compress the jugular vein to get a blood sample. It takes more pressure to occlude the vein of a cow with a fat neck than a sheep with a skinnier neck.

Has anyone tested if any blood pressure cuff gives an accurate reading in obese patients? How does the use of a sphygmomanometer compare with bp measurements using an intra-arterial catheter?