Size matters in blood pressure measurement

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The wrong cuff size can turn normal readings into hypertension and vice versa, a study has found.

Using the wrong cuff size when measuring blood pressure can lead to huge inaccuracies in readings and diagnosis, a new study reveals.

Preliminary research presented at the American Heart Association’s Epidemiology, Prevention, Lifesytle & Cardiometabolic Health Conference 2022 in Chicago, revealed patients who had their reading taken with a cuff that was too small for them had significantly higher readings than when an appropriate sized cuff was used, with almost two in five being misclassified as hypertensive. Subjects who were given a larger cuff than they needed returned notably lower readings, leading to a significant number of hypertensive people being mistakenly identified as normotensive.

Study author Dr Tammy Brady, of Johns Hopkins University in Baltimore, said while clinical practice guidelines highlighted the importance of individualised cuff selection for measuring blood pressure, most of the research looking at cuff size and measurement accuracy had so far been done using mercury sphygmomanometers. 

“We looked at how cuff size impacts blood pressure readings with today’s widely used automated blood pressure monitors,” she said.

“On average, individuals who required an extra-large cuff based on their measured mid-arm circumference had a blood pressure of 143.9/86.5mmHg when a regular adult cuff was used, which is a measurement in the range for a diagnosis of stage two hypertension.

“When using the appropriately sized cuff for the same participants, blood pressure readings were an average of 124.2/79.1mmHg, which is within the normal range. In our study, the overestimation of blood pressure due to using a cuff that is too small misclassified 39% of participants as being hypertensive, while the underestimation of blood pressure due to using a cuff that is too large missed 22% of participants with hypertension.”

Natalie Raffoul, manager of the Australian Heart Foundation’s healthcare programs, told The Medical Republic the differences in readings were “quite astounding”. 

She said she believed GPs understood the need to use the right cuff size when measuring blood pressure, but perhaps not the level of importance. She said this was the first time she had seen it so “clearly quantified” why.

“I think [GPs] will be quite shocked – that’s a really clinically significant difference,” she said.

“This is more data and more evidence that the technique we use to measure blood pressure is very important. We need to increase the awareness that you have to use the right size cuffs – it’s not just a one size fits all.”

In the study, researchers analysed blood pressure readings of 165 adults with an average age of 55 years. During a single setting, researchers compared the average of three blood pressure readings using a regular, adult-sized cuff versus the average of three blood pressure readings obtained using a cuff appropriately sized for the individual.

Among the participants who required a small adult cuff, a regular-sized adult cuff returned lower readings: an average of 3.8mmHg lower for systolic and 1.5mmHg lower for diastolic pressure.

Worryingly, among those who required a large or extra-large adult cuff, a regular-sized cuff was associated with significantly higher readings, including an average 4.8mmHg higher diastolic and 19.7mmHg higher systolic reading.

Based on these findings, individuals who require the large adult and extra-large adult cuff sizes may be most at risk for mismeasurement, misclassification and potential over-treatment in the clinical setting, Dr Brady said.

“It’s important for health care professionals and the public to recognise the importance of cuff size for the clinician’s office, kiosk and home blood pressure measurement,” she said.

“For example, cuff size may be even more important in communities with high prevalence of obesity, since large or extra-large cuff sizes may provide more accurate blood pressure readings in those individuals.”

Ms Raffoul said the study findings provided a “great platform” to talk about the use of automated blood pressure machines and pointed out that the issues were not only related to cuff size.

Only 7% of blood pressure machines available in Australia have been validated for accuracy according to international standards, and there are no Australian standards that regulate the sale of blood pressure machines in this country, she said.

Citing a research article that appeared in the journal Hypertension in 2020, she said non-validated devices, including upper arm cuff, wrist cuff and wearable wristbrand devices dominated the online marketplace in Australia.

“You can easily jump online and buy a blood pressure machine, and [if it’s not validated] it doesn’t mean it’s not correct, it just means nobody’s checked,” Ms Raffoul said.

The study revealed validated cuff devices were more expensive than non-validated devices, creating a “major barrier to accurate home BP monitoring and cardiovascular risk management”, the researchers wrote.

Ms Raffoul said people could check whether a blood pressure machine had been validated or tested for accuracy at this University of Tasmania website. The Heart Foundation also has resources for patients on measuring blood pressure at home, clinical guidelines and a 2021 clinical webinar for GPs on hypertension.

Hypertension 2020, online 10 April

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