The hanging arm effect

Blood pressure readings differ wildly depending on arm position

By Carrie O'Hara

The position of a patient's arm during blood pressure testing can have a dramatic effect on test results, according to research in the January 6 issue of Annals of Internal Medicine. Readings taken from an arm hanging limply at the patient's side are liable to be 10% higher than readings taken when the arm is held parallel to the heart with the elbow flexed horizontally, the position dictated by most clinical guidelines.

The study's investigators from the University of California San Diego and the Medical College of Wisconsin measured blood pressure in 100 emergency room patients, aged 18-88. All were seen for a chief symptom unlikely to be associated with cardiovascular instability. The patients' blood pressure was measured six times -- in perpendicular (right angle, flexed at elbow) and parallel arm positions while laying, sitting and standing. The proportion of seated patients classified as hypertensive was 22% with the arm perpendicular, but rose to 41% when the arm was parallel .

"In every body position, the systolic and diastolic blood pressure measured with the arm perpendicular to the body was significantly lower than with the arm in a parallel position".

The researchers note that since a 10% change in measured blood pressure could lead to a significantly different prognosis and treatment, physicians should make sure their testing is standardized. They point out that a previously published study found that 73% of healthcare workers failed to use proper arm positions and blood pressure cuff positions, as defined by the American Heart Association.

The research mirrors results reported last June in the Journal of Human Hypertension. That study also found an approximate 10% increase in pressure readings taken from a hanging arm. It noted that readings taken in the correct horizontal position remained steady whether the extended arm was supported or not. A notable result of that study was the finding that the "hanging arm effect" on measured blood pressure is most marked in patients whose underlying blood pressure is high.