By Will Boggs MD

(Reuters Health) – Blood pressure measured at the wrist is commonly higher than pressure measured at the upper arm, which has implications for the accuracy of devices that measure blood pressure, researchers report.

Current hypertension guidelines are based on blood pressure measured with a brachial cuff, the kind typically used in doctor’s offices, and applied on the upper arm. But many devices used by patients at home, including the increasingly available wearable monitors, measure blood pressure at the wrist and other locations.

Dr. James E. Sharman of the University of Tasmania, in Hobart, Australia, and his team measured blood pressure consecutively at the upper arm and wrist in 180 middle aged and older individuals undergoing coronary angiography.

Systolic blood pressure – the top number in a blood pressure reading that reflects pressure within the arteries when the heart beats – averaged 5.5 mmHg higher at the wrist than at the upper arm, the researchers report in the journal Hypertension.

“We expected to see a lot of variation in how systolic blood pressure changed from the upper arm to the wrist, but were interested to see the amount of people (14 percent) with what may be considered as very large differences; on average 20 mmHg (or more),” Sharman said in an email. “So the magnitude of variation between individuals was substantial.”

Fewer than half of the patients had wrist systolic pressure readings within 5 mmHg of their upper arm reading; for 46 percent the readings differed by 5mmHg or more, including 27 percent whose readings differed by 10mmHg or more.

One in nine participants (11 percent) had wrist systolic pressure readings that were lower by 5 mmHg or more than their upper arm reading.

On average, wrist systolic blood pressure was 12.9 mmHg higher than systolic pressure measured in the aorta, the main artery delivering blood from the heart to the rest of the body. More than half of this difference was accounted for by the pressure difference between aorta and upper arm measurements, and the rest was the difference between upper arm and wrist measurements, researchers found.

“If you do an online search, you will see there are hundreds of devices available for sale, including many that measure blood pressure with a cuff or a band at the wrist,” Sharman said. “Our research adds further weight to the evidence that the vast majority of these wrist devices will not record blood pressure values that are comparable with the clinical standard of upper arm cuff blood pressure.”

Sharman suggested that doctors and patients can use an “easy-to-use” and comprehensive guide to measuring blood pressure at home, published (here: in 2016 by the journal Australian Family Physician. “The guide provides instructions on the standardized method, as well as a diary to record blood pressure values.”

He added, “The study findings feed into a bigger story on the need to improve the accuracy of blood pressure devices in general.”

Blood pressure guidelines are based on external upper-arm cuff measurements, noted Dr. Giacomo Pucci of the University of Perugia and Terni University Hospital in Italy, who wasn’t involved in the study.

“Unfortunately,” Pucci told Reuters Health, there is distortion of pressure readings taken externally, compared to blood pressure measured inside an artery. And, “we are not able, at present, to predict the degree of such distortion in each individual in a non-invasive way. Therefore, we should keep relying on non-invasive brachial BP” as a surrogate for internal measurements.

“There is increasing availability on the market of devices proposing the non-invasive assessment of central BP,” he said. “Unfortunately, many of these devices are lacking in scientific background. Further research is needed in this field, and rigorous quality assessment of novel devices is obviously mandatory.”

SOURCE: Hypertension, March 25, 2019.