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New device may underestimate blood pressure

Automated blood pressure measuring devices are replacing old-fashioned mercury manometers in doctor's offices and clinics around the world. But newer isn't necessarily better.
/ Source: Reuters

Automated blood pressure measuring devices are replacing old-fashioned mercury manometers in doctor's offices and clinics around the world. But a new head-to-head comparison of the two techniques suggests that the newer version isn't necessarily better — and could even be missing some people with high blood pressure who are in need of blood pressure-lowering treatment.

Readings were about two points lower, on average, with the automated device compared to the mercury-based technique, Drs. Robert A. Koner, Stanley H. Wishner, and Johanna Landgraf of Good Samaritan Hospital in Los Angeles found. What's more, discrepancies were larger in people 65 and older.

The older "auscultatory" method (from the Latin word for listening) uses a stethoscope and a mercury manometer. The stethoscope placed over an artery in the patient's arm allows a doctor or nurse to listen for when pulse sounds return (systolic blood pressure), and then disappear again (diastolic blood pressure), as a blood pressure cuff deflates and blood flows back into the artery. The mercury manometer indicates the pressure at each of these time points in millimeters of mercury (mm Hg).

The newer automated oscillometric devices calculate systolic and diastolic blood pressure based on the average pressure in the artery. Oscillometric measurement requires less skill than the older technique, and may be suitable for use by untrained staff and for automated patient home monitoring of blood pressure.

Researchers testing blood pressure medications in large clinical trials have generally used the older auscultatory technique, Kloner noted in an interview, but the mercury manometers are "sadly" being phased out in many clinics and doctor's offices.

There's been controversy over whether oscillometric devices are as accurate as the older technique, he added, and studies comparing the two approaches have had mixed results.

Kloner and his team compared auscultatory and oscillometric measurement by linking both devices to a single blood pressure cuff, making it possible to compare measurements for the same patient, for the same heartbeat. They looked at 337 consecutive patients who were visiting their heart doctor.

Average systolic blood pressure (the top number) as measured with the automatic device was 131, compared to 133 with the mercury manometer. Diastolic pressures averaged 70 versus 72, respectively.

While 2 points may not seem like much, Kloner noted, there's strong evidence that even a 2-point reduction in blood pressure can reduce the risk of heart attack and stroke.

What's more, he added, the differences were larger for a significant number of patients — and "huge" for a few of them. For example, 7 percent of patients showed a discrepancy of 10 to 15 mm Hg and 4 percent had a 15 to 20 mm Hg difference; 2 percent had discrepancies of 20 mm Hg or higher.

And while about 10 percent of patients younger than 65 showed a discrepancy in blood pressure readings between the two techniques, nearly 30 percent of those 65 and older did.

This may have been because older people have stiffer, less resilient arteries, making the newer oscillatory technique less accurate, Kloner said; with the mercury manometer technique, pulse sounds remain audible despite arterial stiffness.

Kloner recommends that physicians using automatic oscillometric devices in their offices test them against a mercury manometer. Patients should also discuss different approaches to blood pressure measurement with their doctor, he added.

The most important thing, Kloner said, is for people to get their blood pressures checked, to know if they do have high blood pressure, and to get treatment if they need it.