Key Takeaways

• If your arm isn’t correctly positioned for a blood pressure reading, you can get a number that is falsely high.

• To get the most accurate reading, your arm should be supported on a flat surface and at heart level.

• An inaccurate reading due to poor arm position could lead to misdiagnosis of high blood pressure or unnecessary treatment.

As many as 1 in 6 American adults may have inaccurate information about their blood pressure because their arm was in the wrong position during readings.

A study published this week in JAMA Internal Medicine found that two nonstandard arm positions commonly used during blood pressure measurements can overestimate results.

Based on data from the National Health and Nutrition Examination Survey, researchers estimated that improper arm position would result in 16 percent of U.S. adults (40 million individuals) being misclassified as having high blood pressure.

“Because home measurements are an important part of the diagnosis, it is essential to properly educate patients on blood pressure measurement technique, as inaccurate readings may contribute to a misdiagnosis or inappropriate choice in therapy,” says senior study author Tammy Brady, MD, PhD, vice chair for clinical research in the department of pediatrics at the Johns Hopkins University School of Medicine and medical director of the pediatric hypertension program at Johns Hopkins Children’s Center in Baltimore.

Which Arm Positions Raise Blood Pressure Readings?

The American Heart Association advises people to support their arm on a flat surface and at heart level while having their blood pressure checked in order to get the most accurate reading.

Despite these guidelines, healthcare providers still take measurements in nonstandard positions — with the arm resting on a patient’s lap or hanging at their side, according to study authors.

To evaluate the effects of these nonstandard positions, researchers recruited 133 adults for this study. Almost 8 out of 10 were Black, just over half were female, and they ranged in age from age 18 to 80.

Each person was fitted with an upper-arm blood pressure cuff selected and sized based on their upper-arm size. They had three sets of triplicate measurements taken 30 seconds apart with a digital blood pressure device.

The results confirmed previous evidence indicating that arm positions might make a big difference when having blood pressure taken.

Researchers found that lap and side position readings were significantly higher for both systolic pressure (the top number of a reading, measuring the force of blood flow when the heart is pumping) and diastolic pressure (the bottom number, showing the force of blood flow when the heart is relaxing).

Blood pressure is measured in millimeters of mercury (mmHg). On average, lap support overestimated systolic pressure by nearly 3.9 mmHg and diastolic by 4 mmHg.

An arm hanging at the side overestimated systolic pressure by 6.5 mmHg and diastolic by 4.4 mmHg.

“Even though those numbers sound low, they can make a real difference in putting someone on medicine or not putting someone on medicine, or increasing the dose of a medicine versus keeping it the same,” says Gregory Katz, MD, a cardiologist with NYU Langone Health and assistant professor of medicine at the NYU Grossman School of Medicine in New York City, who was not involved in the study

Why Do Incorrect Arm Positions Lead to Higher Blood Pressure Measurements?

Dr. Brady explains that readings are higher when the arm is either in 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 these blood vessels.

“It also is harder for the blood to return to the heart when the arm is lower than the heart; so to compensate for this and improve blood return, the blood vessels in the arm constrict,” she says. “In addition, when the arm is unsupported, the muscle isn’t fully at rest and contracts. All of these body responses lead to a higher BP [blood pressure] reading.”

Other Factors Matter for a Proper Pressure Check

In addition to keeping your cuffed arm on a flat surface and at heart level, the American Heart Association advises sitting upright with back straight and feet flat on the floor to get an accurate reading.
The cuff also needs to be the right size and positioned in the right place. Previous research has shown that using a cuff that is one or two sizes too small can overestimate systolic blood pressure by almost 10 and 20 mmHg, respectively.

Findings May Be a ‘Call to Action’

Brady suggests that the findings may serve as a call to action for healthcare providers to pay better attention to best practice guidelines, and for patients to “advocate for themselves in the clinical setting and when measuring their BP at home.”

Blood pressure might be the most important modifiable risk factor for heart disease, according to Dr. Katz.

“But the only way to control blood pressure well is to know what the actual value is,” he says.

Credit: Everyday Health