The study, which was a reanalysis of data from 24 international studies that included nearly 54,000 patients, found that a difference of 10 mmHg or higher between the two arms increased the 10-year risk of cardiovascular events and death. For each degree above 10 mmHg, the risk of over the next decade of angina, heart attack and stroke rose by 1%, according to the report published in Hypertension.
“Blood pressure should be measured in both arms during cardiovascular assessment, not only to identify the higher reading arm, but also to identify additional risk conferred by an inter-arm difference,” said the study’s lead author, Dr. Chris Clark, a clinical senior lecturer in general practice at the University of Exeter College of Medicine and Health in England.
Based on the new findings, Clark and his colleagues say that a difference of more than 10 mmHg between the two arms should be a cause for concern.
“We think the difference we are measuring is caused by changes leading to stiffening of the arteries,” Clark said in an email. “Stiff arteries are associated with cardiovascular events and death.”
While it might seem that a lower blood pressure in one arm might be good news, the opposite is true.
“It is likely that meaningful differences in blood pressure between the left and right arms indicates a narrowing of major arteries due to atherosclerosis (a build up of fats, cholesterol and more on the artery walls),” said study co-author Dr. Mary McDermott, a professor at the Northwestern University Feinberg School of Medicine in Chicago.
“Specifically, the arm with the lower blood pressure likely has blockages in the arterial flow that cause the lower blood pressure,” McDermott said in an email.
To take a closer look at the importance of different readings between arms, the researchers scoured the medical literature for studies that included blood pressure measurements from both arms and long-term health outcomes.
They ultimately focused on 24 studies which included data from 53,827 patients.
The new study is an “important paper,” said Dr. Matthew Muldoon, a professor of medicine at the University of Pittsburgh School of Medicine and director of the UPMC Heart and Vascular Institute Hypertension Center. “It is the most important paper to address the question of blood pressure arm difference,” said Muldoon, who was not affiliated with the new research.
One limitation of the study is that it didn’t address the variability of blood pressure from moment to moment even in one arm, Muldoon said. “When you measure blood pressure repeatedly, it’s not stable,” he added.
Nevertheless, the study shows that differences in blood pressure between the arms is correlated with a higher risk of cardiovascular events. And that higher risk might be enough to help doctors convince patients that it’s necessary to work at lowering blood pressure.
While the authors suggest that doctors routinely measure blood pressure in both arms, that’s unlikely to happen since there is already so much to cover in each visit.
A solution could be for patients to measure their blood pressure in both arms on their own.
“I am a fan of engaging patients in their hypertension management,” Muldoon said. “Automated cuffs are not expensive. In some respects it’s more practical to expect patients to do this.”
The American Heart Association offers some tips on how to get the most accurate home blood pressure readings:
- Clear your schedule and relax: Don’t smoke, drink caffeinated beverages or exercise 30 minutes prior to measuring your blood pressure and empty your bladder to make sure you’ll have at least five minutes of quiet rest prior to the measurement.
- Your body position is important: Sit with your back straight and supported (a dining room chair would be better than a sofa, for example). Place your feet flat on the floor and do not cross your legs. Place your arm on a flat surface for support with the upper arm at heart level. Make sure the cuff sits directly above the bend in your elbow.
- The cuff should be directly on your skin: Don’t take the measurements over your clothes.
- Take measurements at the same time every day: It’s best to take readings around the same time each day, such as morning or evening.
- Make multiple measurements and record all the results: Each time you take your blood pressure make two or three readings a minute apart. If your monitor has built-in memory to store the readings take it with you to your doctors’ appointments.
If your numbers are concerning, talk to your doctor. It's helpful to have a record of past readings for an informed discussion about your risk and next steps.