Aneroid Manometers

Are they accurate? Do they last forever? What maintenance is necessary? Excerpts from AAMI, AHA, NIH, plus commentary.
Copyright, 6/1/03.

Cited Quotes

 

“The aneroid manometer is also widely used and can provide accurate measurements if properly calibrated. However, because of its construction, it is prone to mechanical alterations that can affect its accuracy. The aneroid manometer consists of a metal bellows, which expands as the pressure in the cuff increases, and a mechanical amplifier that transmits this expansion through a lever to the indicator needle, which rotates around a circular, calibrated scale. The needle should rest at the zero point before the cuff is inflated and return to that point after the cycle of inflation and deflation. Aneroid manometers require maintenance every 6 months and should be handled gently to avoid decalibration. The accuracy of the calibration should be checked regularly.

Recalibration is required when the readings differ from the standard mercury manometer by more than 4 mm Hg. When decalibrated, aneroid manometers tend to under read the pressure, especially at higher levels, but may be inconsistent in their variation from the mercury standard at any blood pressure level.”

Product Code: 88:2460-2467
AHA Medical/Scientific Statement
“Human Blood Pressure Determination by Sphygmomanometry”
http://www.americanheart.org

Since this statement was published in 1993, there have been, according to manufacturers, improvements in technology making the newer aneroid manometers more shock resistant (no gears) and more reliable (elimination of the zero resting point).

These relatively simple devices perform critical tests for which there is no simple substitute (like pressing a fingernail to ascertain cardiac output).

Q: Even if it is easy to see that a unit has gone “off zero,” will staff notice and respond?

One way to address maintenance of these devices is to include them in “environmental rounds.”

“Introducing Welch Allyn DuraShock: the only completely gear-free aneroid sphygmo-manometer. By creating a gear-free design, we’ve created a gauge that is shock resistant, lighter, and much thinner than traditional gauges. Until now, a dropped sphygmomanometer required recalibration or replacement. Not so for the DuraShock; it can fall 30 inches onto a hard surface and still remain accurate.”

“Welch Allyn recommends that the calibration of aneroid sphygmomanometers be checked on an annual basis.”

“Should the Welch Allyn DuraShock integrated aneroid sphygmomanometer deviate from the ±3 mm Hg accuracy specification during the warranty period, Welch Allyn will recalibrate the sphygmomanometer at no charge.”
http://www.welchallyn.com

“it can fall 30 inches onto a hard surface and still remain accurate.” Notice it says “CAN” not WILL. Notice also that 2.5 feet is not as high as many IV pole clamps. But bottom line, if it did fall a distance 30″ or less (the nurse measured?), the warranty is not a guarantee that this device will remain accurate, only that it will be repaired if defective.

Wall mounting aneroids would assure that inaccuracies are not produced by falls, but are no insurance from knocks, over pressurization, or from leaking hoses and cuffs.

“Dr. Grim then discussed problems with aneroid instruments. Five studies (from 1970 to 1997) found inaccuracies in an average of 35 percent of aneroids, suggesting that no quality assurance (QA) measures have been implemented. He described a calibration check in 86 practices in Green Bay, Wisconsin, in which no mercury manometers were used. When the aneroids were checked against a mercury manometer (using a Y tube), 35 percent were off by at least 6 mmHg; the average error was –10 mmHg; and 2 percent leaked excessively. Only 7 of 13 clinics had an equipment maintenance schedule, and none of the nursing homes or health care services knew that aneroid manometers should be inspected every 6 months. Correcting the problem would require calibrating all aneroid devices and removing all those that are off >1 mmHg. Regular annual quality control (QC) could detect 2,500 errors, while daily QC could detect 10 errors.

Dr. Grim also made the following recommendations to address the lack of mandated ent or training for equipment operators. •Routine inspection and calibration of office BP manometers should be implemented. How often, by whom, and at what cost remain to be decided. •Careful training of those who use BP devices must be done and kept current.
•Legislation may be needed to assure compliance (as it does for glucose-monitoring equipment).

•Ongoing QA must be implemented.”

NATIONAL HIGH BLOOD PRESSURE EDUCATION PROGRAM (NHBPEP)/NATIONAL HEART, LUNG, AND BLOOD INSTITUTE (NHLBI) AND AMERICAN HEART ASSOCIATION (AHA)

WORKING MEETING ON BLOOD PRESSURE MEASUREMENT
Natcher Conference Center
National Institutes of Health (NIH)
Bethesda, Maryland
April 19, 2002
http://www.nhlbi.nih.gov

These results are from tests several years old. That notwithstanding, aneroid manometers are arguably the most widely used, critical diagnostic tool.

There exist industry standards for accuracy, but none for assuring that accuracy will be maintained.

“They have virtually no maintenance costs as the manufacturer certifies their accuracy for the lifetime of the manometer and will repair or replace them at no charge.”

“Mercury Vs Aneroid,” April, 2001 newsletter, The Institute of Biomedical Engineering Technology (IBET)

http://ibet.asttbc.org/

This excerpt is shown only to illustrate a misnomer regarding the reliability of aneroids: “They have a lifetime warranty, therefore the accuracy is assured.”

They may be other things, but lifetime warranties are also marketing devices–designed to sell–the cost of replacement being relatively lower compared with the financial gain achieved through increased sales.

“Although current aneroid gauges are generally accurate and reliable, users need to remember that the effective function of these instruments depends greatly on periodic inspection. Aneroid gauges register pressure by the deflection of a diaphragm within the meter. Overpressurization, mechanical vibration, and shock received during normal use can all cause these gauges to register erroneously. Clinical personnel should be advised to have the units checked following any abuse (e.g., an accidental drop) that might have caused damage. This will reduce the risk of inaccurate readings.”

ECRI, Aneroid Gauges versus Mercury Manometers for Blood Pressure Measurements. FAQ [Health Devices Apr-May 1998;27(4-5):176-7]
http://www.mdsr.ecri.org

Who represents the “failsafe” mechanism in your institution? Nursing? Physicians? Clinical engineering department? An outside vendor?

And what is that mechanism? A policy stating that the clinician is responsible for the effectiveness of treatment? An in-house periodic maintenance inspection? An unscheduled vendor survey? A JCAHO cleanup?

JCAHO can ask this question, but does not have to–leaving it to the department head to interpret the relative importance of the issue.

   

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