Sunday, July 12, 2009

Wrist Blood Pressure Monitors

There are three kinds of blood-pressure monitors available to the public: Upper-arm monitors, wrist monitors, and finger monitors. Upper-arm monitors are quite similar to the automatic monitors now used in most doctor's offices, and employed in most research; these are the standard monitors recommended for home blood-pressure monitoring.

Blood pressure readings can in principle be taken anywhere that the arteries can be compressed to the point where the flow is suppressed. But the vascular resistance to flow changes as you progress down an artery. If you think about it, this makes sense: if blood didn't flow more easily toward the extremities, it would be hard to ensure that blood reached the fingertips. The net effect is that, all else held equal, blood pressure will tend to be higher measured at the wrist than at the upper arm, and higher in the fingertip than at the wrist.

In addition, the height at which the wrist or finger is held relative to the heart also affects the blood pressure. This also stands to reason: it is easier to cut off blood flow when it is pushing "uphill" than when it is flowing "downhill."

In principle, these factors can all be compensated for so that a measurement made at the wrist or finger can be translated back to an equivalent in upper-arm terms. In practice, this long proved difficult in the case of wrist monitoring, and has thus far proved impossible in finger monitors.

Recently, however, a few manufacturers have overcome the problems with wrist monitors, and a number of models have been validated by the two key certification agencies, the Association for Advancement of Medical Instuments (AAMI) and the British Hypertension Society. The better versions of these monitors will not take readings unless the wrist is supported at exactly the level of the heart--the position that ensures accurate results.

It is still possible to screw up a reading, even with the automatic monitoring of wrist position. If the cuff inflation begins and the user then moves their arm to another position, the result will be inaccurate. For this reason, no agency recommends wrist monitors over upper arm monitors. (Although you can also produce incorrect readings by moving your arm with an upper arm monitor, it is less easily done by accident--and you can't move your upper arm as far above or below your heart as you can move your wrist.) In other words, the powers that be don't want you using wrist monitors because they don't trust you to follow the directions.

Despite all that, I use a wrist monitor (though I started out on an upper-arm monitor.) Why? I think wrist monitors have a number of advantages:

1) Wrist monitors are easy to transport. You can slip them into a purse, briefcase, or the glove compartment of your car. Though they are much larger than a wristwatch, I've seen women wearing clunky bracelets that were about the same size as my wrist monitor.

2) Wrist monitors can be used almost anywhere. Well, anywhere you can sit down. Conventional upper-arm monitors require a table nearby to station the monitor and rest your arm, and the monitor is connected to the cuff by an inconvenient tube. I've used my wrist monitor to take my blood pressure in the locker room after exercise, in airport transit lounges while traveling, and sitting on the freeway stuck in traffic. Try doing any of those with an upper-arm monitor.

3) Wrist monitors don't make you feel as if you're being strangled. Upper-arm cuffs are huge. Most people find the massive crushing sensation on their upper arm to be unpleasant, and a substantial number of people respond to it with surges in blood pressure--whcih rather defeats the whole purpose. The squeeze from a wrist monitor isn't like a hug from your grandmother, but it's a whole lot easier to ignore than the full-arm crush of a standard monitor.

4) The better wrist monitors don't "overpressure." To measure blood pressure, the cuff needs to be inflated to the point where the sounds of blood flow are completely cut off, and then gradually deflated as the sounds of the pulse are restored. Most automatic upper-arm units inflate to some high value; pause; listen...and then if sounds are still heard, inflate to an even higher pressure, and repeat the process. This is slow (giving the panic-prone time to shoot their blood pressure through the roof), and well-respected research shows that an overinflated cuff in itself results in higher blood pressure readings [1]. Most AAMI/BHS-validated wrist monitors are smarter, and use techniques (such as Omron's "Intellisense") to minimize overinflation. (Anti-overpressuring techniques are now also built in to some upper-arm monitors as well.)

I'm not in the business of recommending specific products, but, for the record, I use an Omron HEM-650 Wrist Monitor, and I'm very happy with it. It takes a little time to learn the proper technique, but after a few sessions the supporting and postioning of the arm becomes instinctive (and it also beeps to signal whether your wrist is too low, too high, or just right). It's good to calibrate/validate the readings against readings in a doctor's office or against an upper-arm monitor, but this sort of calibration is advised for all home-monitoring units.

Although most users find this device to be excellent (see the 200+ reviews at Amazon), a few reported discrepancies in validation or calibration. On the other hand, it's worth noting that many of the aneroid blood pressure monitors in clincal settings are out of whack: in the 1990s, one study found that 30-40% of all automatic monitors in use by physicians were out of calibration by 4 mm or more, and 10% were out of calibration by 10 mm or more. Since the majority of physicians and nurses don't follow the AHA guidelines for taking blood pressure, the combination of inaccurate monitors plus incorrect readings mean that a very large percentage of patients are probably misdiagnosed in clinical settings[2]. It's disturbing that the medical industry considers the calibration of home monitors to be a major problem while ignoring the fact that a huge percentage--perhaps the majority--of measurements taken in clinics are also wrong. Apparently mistakes by professionals are somehow less worrisome than mistake by patients.

The truth is, blood pressure measurement isn't as exact as most people in the medical profession pretend. Take a few repeated measurements--at home, or in your doctor's office--and you'll see what I mean. Both systolic and diastolic numbers fluctuate considerably over the period of only a few minutes.

For that reason, I never put much faith in any single measurement. I record my blood pressure at least twice a day (in the morning, and 1-2 hours after exercise), but the number I record is the average of three measurments--one after five minutes of sitting quietly according to American Heart Association guidelines, and then two more, each after another wait of three minutes. (Some research protocols claim that a two-minute pause between measurements is sufficient.)

Doing it right--waiting five minutes before measuring and then taking two more measurements spaced apart by three minutes--ends up requiring about fifteen minutes of your time. But sitting quietly and relaxing for fifteen minutes twice a day isn't such a bad idea even for people who aren't taking their blood pressure.

[1] Sprafka, JM et al. The Effect of Cuff Size on Blood Pressure Measurements in Adults. Epidemiology. May 1991; 2(3):214-7.

[2] Campbell, Norman R.C., and McKay, Donald W., Accurate Blood Pressure Measurement: Why Does it Matter?, CMAJ. August 1999; 161 (3) 277

7 comments:

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