How accurate are home blood pressure monitors?

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Given that some 50 million Americans have high blood pressure, it's no wonder sales of devices that enable people to check their own blood pressure reached $126 million last year. Medical experts approve. Numerous groups, including the American College of Physicians and the Joint National Committee on Detection, Evaluation, and Treatment of High Blood Pressure, have found that when done properly, self-monitoring can be helpful in high blood pressure diagnosis and treatment.

One reason is that diagnosing high blood pressure in the doctor's office may not always be straightforward. About 1 in 5 people who shows elevated blood pressure when readings are taken by physicians (and to a lesser extent by nurses or other medical personnel) actually has normal blood pressure that rises in the examining room due to nervousness. Self-monitoring of blood pressure at home can help identify the condition, known as white coat hypertension.

Self-monitoring is also beneficial because people using their own equipment can take far more readings than could reasonably be taken in a doctor's office. High blood pressure, or hypertension, generally cannot be diagnosed on the basis of one measurement, and several readings at home can be more telling than, say, measurements taken on just a couple of visits to the doctor. When medication or lifestyle strategies are being used to treat hypertension, self-monitoring is also valuable in determining appropriate drug dosages and the efficacy of habit changes such as losing excess weight and cutting back on salt.

Of course, self-monitoring should always be conducted in conjunction with, and not as a replacement for, the services of a physician. Your equipment also needs to be used correctly. Bringing your monitor to your doctor's office serves a dual purpose: It allows you to check your technique as well as check your monitor against readings obtained by your physician to make sure your device shows reasonably similar results.

Choosing a monitor
Home monitors come in two basic varieties: electronic (battery operated) models, which are semi- to fully automatic depending on the model chosen; and what are called aneroid models, which are operated manually.

To find out about the ups and downs of home monitors, we recently purchased seven units from drug-stores and discount home appliance stores in the Boston area.[*] The models represented major brands in a wide range of prices. We tried them out under the guidance of Madelyn Hackett, RN, head nurse of the Metabolic Research Unit at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University. Four of the units we bought were electronic models. Three were aneroid, or manual, devices.

To estimate their accuracy, Ms. Hackett first measured the blood pressure of Tuffs Letter staffers with a mercury sphygmomanometer, the reference standard for blood pressure measurement used in doctors' offices. This reading was then compared to readings obtained with the home equipment.

All varied by no more than a few points--a reasonable range of accuracy, especially considering that blood pressure can fluctuate a little even in the few minutes it takes to switch from one monitor to another. The machines were not similar in how they operated, however.

Aneroid models
We tested three aneroid machines: the Lumiscope 100-019 ($15.99), the Omron HEM-18 ($19.99), and the Sunbeam 7683 ($21.99). Whatever brand you buy, the aneroid blood pressure monitors require the most effort. The user first wraps a cuff around the upper arm (see photo on the top of page 5). The cuff contains a stethoscope head (a round disc) that must be positioned over the brachial artery (just above the crook of the arm). The stethoscope head connects to long, tube-like pieces that are placed in the ears and acts like a microphone to amplify the sound of the heart.

Next, the cuff is inflated by rapidly squeezing a flexible rubber bulb attached to it, cutting off the blood supply to the artery. The home tester then turns a knob by the bulb, slowly letting out air and allowing blood to begin flowing through the arm again.

Readings from a gauge resembling a compass are taken twice: when the heartbeat can first be heard after the blood starts flowing (that's systolic pressure, the upper number) and when the beat disappears as the heart goes into its resting phase (that's diastolic pressure, the lower number).

While this procedure sounds complicated, the equipment is fairly easy to use once you get the hang of it. The big plus is that aneroid monitors are considerably less expensive than the electronic models--around $20 as opposed to $50 or more. The caveat: to use an aneroid monitor, you need a fair amount of manual dexterity as well as reasonably good hearing and vision (to read the com-pass-like dial).

Electronic models
Electronic monitors do away with the need for a stethoscope. Furthermore, while some of them require manual inflation of the cuff with a rubber bulb, deflation is always automatic. Thus, you don't have to turn any knobs and listen for heartbeats. And results appear on an easy-to-read digital display (see photo at right).

By far, the easiest electronic models to use automatically inflate as well as deflate the cuff--no bulb squeezing. Once the cuff has been positioned on the arm, the touch of a button is all that is needed to obtain a reading. Automatic inflation, however, has a price--the two models with this feature that we tested (the Omron HEM 704-C and the Walgreens 91) were also the most expensive, costing $99.99 and $64.99, respectively. However, if you are expecting to use your machine on a regular basis and you can spring for it, an automatic inflate-deflate model is what we recommend. It's as close to a no-fuss procedure as you can get.

Electronic models using a manually inflated cuff are the next best thing for ease of use--and cost somewhat less. We looked at the Marshall 82 ($49.99) and the Sunbeam 7622 ($49.99), and both presented no problems. As with the aneroid models, however, users must be able to rapidly squeeze a bulb to inflate the cuff--a drawback for anyone with impaired grip strength.

The hardest aspect of using any of the models we tested--both electronic and aneroid--was putting on the cuff. It must be positioned correctly and tightened around the upper arm using only one hand.

Some devices that use a wrist cuff rather than an upper-arm cuff help address this difficulty. However, when these devices first appeared, they were not very accurate. Thus, professional organizations such as the American Heart Association are still not recommending them--at least not until they have a chance to reevaluate the newer wrist-cuff equipment. The same goes for devices that attach to the finger. These models also have not been deemed reliable.

* The units we tested do not represent the entire array of blood pressure machines available. They do, however, represent the types of machines available.

Tufts University Diet & Nutrition Letter (ISSN 0747-4105) is published monthly by Tufts University Diet & Nutrition Letter, 53 Park Place, New York, NY 10007, for $20 per year. Second class postage paid at New York, NY, and additional mailing offices. Postmaster: Send address changes to Tufts University Diet & Nutrition Letter, PO. Box 57857, Boulder, CO 80322-7587.

TAKING BLOOD PRESSURE MANUALLY
The man pictured here is slowly letting air out of the blood pressure cuff by turning a knob (covered by his right thumb) in a counterclockwise direction. The number that appears on the gauge in his left hand as soon as he starts to hear his heartbeat is his systolic pressure. Normal is below 140 (meaning the heart is pumping hard enough to drive a column of mercury up a tube to a height of 140 millimeters). The number on the gauge just before the heartbeat disappears (when the heart rests between beats) is his diastolic pressure. Normal is below 90.

PHOTO (BLACK & WHITE): A man taking his blood pressure manually

TAKING BLOOD PRESSURE ELECTRONICALLY
It's easiest to take a blood pressure reading with an electronic machine that has an automatically inflating cuff. After slipping on the cuff, the user simply presses a button. The machine then inflates the cuff, deflates it, and flashes a digital reading, all within about 25 seconds.

PHOTO (BLACK & WHITE): Everybody now can take their blood pressure electronically

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