Blood pressure may read falsely high if the arm isn’t positioned properly

When the arm is on the lap or the side, a blood pressure reading can be erroneously high. But when the arm is supported and at heart height, a blood pressure reading is more likely to be right.

In a clinical trial, researchers investigated the effect that different arm positions had on blood pressure readings, which consist of two numbers. The first, the systolic, represents the blood’s pressure against the artery walls when the heart beats. The second, the diastolic, is the pressure between beats, when the heart rests.

For trial participants with their arm in their lap, systolic and diastolic readings skewed about 4 millimeters of mercury higher, on average, compared with the readings of participants with the recommended arm position: at heart level and supported by a desk or table. For participants whose arm hung at their side, the systolic reading was close to 7 millimeters of mercury higher and the diastolic reading was 4 millimeters of mercury higher, researchers from Johns Hopkins University report online October 7 in JAMA Internal Medicine.

An estimated 120 million adults in the United States have high blood pressure — defined as a reading equal to or greater than 130/80 millimeters of mercury — or take blood pressure lowering drugs. High blood pressure increases the risk of cardiovascular disease and stroke (SN: 5/29/18).

To get the most accurate blood pressure reading, medical guidelines recommend people have their legs uncrossed and feet flat on the floor, their back supported, their arm positioned correctly and a properly fitting blood pressure cuff. But how closely medical offices follow these guidelines with their patients varies, the study’s authors note.

That could mean inaccurate readings for some people, which might lead to unwarranted diagnoses of high blood pressure. With data from a national survey on health and nutrition from 2017–2018, the Johns Hopkins researchers calculated that, at a cut off of 130 millimeters of mercury, 54 million adults might be misclassified as having high blood pressure if the arm isn’t positioned properly.