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FRIDAY, Jan. 16 (HealthDayNews) -- Blood pressure readings taken on an arm that is slack can be up to 10 percent higher than those taken on an arm that is at a right angle to the body, new research shows.

The surprising study warns that this disparity can throw off treatment choices. Unfortunately, the researchers could not say which kind of reading was more accurate, although the American Heart Association (AHA) recommends bending your elbow at a 45-degree angle during a blood pressure reading.

Regardless of whether patients sat, stood or lay down, blood pressure numbers rose when their arms were slack and dipped when their lower arms were at right angles to their upper bodies. In the first position, 41 percent of patients appeared to have high blood pressure levels; in the second position, only two in every 10 did.

"Arm position is important, and it should be consistent from measurement to measurement," says study co-author Dr. David Guss, director of emergency medicine at the University of California at San Diego Medical Center.

Doctors consider blood pressure to be one of the best gauges of a person's cardiovascular health. The top and bottom numbers of a blood pressure reading -- such as 115 over 75 -- measure how hard the heart has to work to push oxygenated blood into the circulation system and pull it back for recycling.

Higher numbers show that the heart is working harder because it's fighting greater resistance, typically because arteries have lost their flexibility, explains Dr. Dennis Goodman, chief of cardiology at Scripps Memorial Hospital-La Jolla in San Diego. "It's as if you put fluid in a pipe that can't stretch."

In the worst cases, high blood pressure can lead to heart attack or stroke by contributing to ruptured or blocked blood vessels.

In recent years, heart experts have revised their estimates of ideal blood pressure levels. While the cutoff used to be 140/90, cardiologists now consider rates higher than 115/75 to be unhealthy. Someone who has a blood pressure reading of 135/85 will double his or her risk of heart attack compared to someone at the 115/75 level, Goodman says.

Guss says he first became interested in the issue of arm position and blood pressure levels while measuring his own pressure as a medical student. He noticed the readings went up and down depending on where he put his arm.

In the study, Guss and two medical students checked the blood pressure levels of 100 patients, from age 18 to 88, at the medical center's emergency room. All of the patients were visiting the hospital for reasons not directly related to heart problems, such as colds and headaches. The findings appear in a recent issue of the Annals of Internal Medicine.

The researchers measured each patient's blood pressure a total of six times, testing two arm positions (with the lower arm jutting out at a right angle to the body, as if they were pointing forward, or simply parallel to the upper body) while they were sitting, standing and lying down.

Among seated patients with their arms at right angles, the blood pressure readings suggested that 22 percent suffered from hypertension. But nearly twice as many patients got that diagnosis when their levels were measured while their arms were slack, parallel to their upper bodies.

So which measurement is the correct one? The study authors didn't explore that issue, and Guss says he isn't certain why the difference exists. However, he notes the AHA recommends placing your arm in a 45-degree position, in the middle of the two positions explored in the study.

In more bad news for patients, other research has suggested medical professionals don't even follow the AHA guidelines rigorously.

Goodman says the study findings are surprising and important. Patients should pay attention when their blood pressure is measured and make sure their arms are in the recommended position, he says. If they're not, he says, patients shouldn't be afraid to speak up.

More information

You can learn about hypertension and what to do about it from the American Heart Association or the National Heart, Lung, and Blood Institute.

(SOURCES: Dennis Goodman, M.D., chief, cardiology, Scripps Memorial Hospital-La Jolla, San Diego; David Guss, M.D., director, emergency medicine, University of California at San Diego Medical Center; Jan. 6, 2004, Annals of Internal Medicine)

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