Blood pressure readings taken on arms parallel to the body are up to 10 percent higher than readings taken when the elbow is at a right angle to the body with elbow flexed at heart level.
Researchers measured blood pressure in 100 emergency room patients, ages 18 to 88, who were being seen for a symptom that wasn't associated with cardiovascular instability. The patients' blood pressures were measured six times using perpendicular and parallel arm positions while laying, sitting and standing. They found that in the number of seated patients classified with high blood pressure, 22 percent had the arm perpendicular and 41 percent had the arm parallel to the body.
Researchers found that blood pressure measured with the arm perpendicular to the body was much lower than with the arm in a parallel position. This was an important finding because blood pressure readings determine certain treatment options for patients. The researchers pointed out that arm position should be consistent throughout the whole test when measuring blood pressure.
Annals of Internal Medicine January 6, 2004;140(1):74-5