Do you have high blood pressure? Check the tonometer cuff size and arm position

About one in six adult patients has inaccurate information about their blood pressure because their arm is in the wrong position during measurement or the tonometer cuff size is too small.

A study published this week in JAMA Internal Medicine found that two non-standard hand positions commonly used during BP measurement may overestimate blood pressure measurements.

Based on the study data, researchers estimate that improper hand positioning causes 16 percent of American adults (that’s 40 million people) to mistakenly categorize themselves as having high blood pressure.

“Because home blood pressure measurement is an important part of diagnosis, it is essential to educate patients on proper blood pressure measurement techniques, as inaccurate readings can contribute to misdiagnosis and inappropriate choice of therapy,” said senior study author Tammy Brady, MD, PhD, deputy chair of the Department of Clinical Research in the Department of Pediatrics at Johns Hopkins University.

According to the study authors, even though the American Heart Association advises people to rest on a flat surface and keep their arm at the level of the heart when measuring blood pressure, health care providers still take measurements in non-standard positions – with the arm resting on the patient’s lap or hanging down at the side.

To assess the impact of these non-standard positions, the researchers recruited 133 adult participants into the study. Each study participant was fitted with a blood pressure cuff sized to match the size of their upper arm. A digital device was used to take three series of triplicate measurements at 30-second intervals.

The results confirmed earlier findings that hand position can have a large effect on blood pressure readings.

The researchers found that pressure readings in the hand position at the knees and at the side were significantly higher for both systolic pressure (the top measure that characterizes the force of blood flow when the heart is working) and diastolic pressure (the bottom measure that characterizes the force of blood flow when the heart is relaxed).

Blood pressure is measured in millimeters of mercury (mmHg). Knee support elevated systolic pressure by an average of 3.9 mmHg and diastolic pressure by 4 mmHg. The arm hanging at the side overestimated systolic pressure by 6.5 mmHg and diastolic pressure by 4.4 mmHg.

“While these figures seem insignificant, they can make a real difference in whether or not to prescribe a medication to someone, increase the dose of a medication or leave it the same,” says Gregory Katz, a cardiologist at NYU who was not involved in conducting the study.

Dr. Brady explains that measurement results increase when the arm is resting on the lap or at the side because the blood vessels in the arm are at a greater vertical distance from the heart, and gravity increases the pressure in those vessels. Also, when the arm is below the heart, blood has a harder time returning to the heart, so to compensate for this and improve blood return, the blood vessels in the arm narrow. In addition, when the arm is unsupported, the muscles are not at complete rest and contract. All these reactions of the body lead to an increase in BP.

Cardiologists advise that for the most accurate readings, not only should the arm with the cuff be held flat and level with the heart, but also that you should sit upright, with a straight back and feet flat on the floor. The cuff should also be the correct size and positioned in the correct place.

Previous studies have shown that using a cuff that is one or two sizes smaller can overestimate systolic blood pressure by almost 10 and 20 mmHg, respectively.

The only way to reliably control blood pressure is to know its real value.