Sustainable Hospitals
 

Comparing Mercury and Aneroid Sphygmomanometers

In an ideal world, considerations for selecting sphygmomanometers include purchase price, ease of use, accuracy, reliability, maintenance requirements, and health/environmental impact. One manufacturer compared three types of sphygmomanometers for these factors (1):
 
Type of blood pressure device Initial Cost Maintenance Accuracy Ease of Use Reliability Environmental Exposure Summary Score* (higher is better)
Aneroid 4 4 5 4 4 5 26
Electronic Monitor 2 4 4 5 5 4 24
Mercury 4 5 5 3 4 2 23
5 = most favorable; 1 = least favorable
 
Reference: Table excerpt from E.W.Wright, 1/11/2000, "Sphygmomanometers: Internal Analysis of Different Technologies; by Welch Allyn, Inc."
 
*Summary score added by C. Galligan, SHP, and equals the sum of the ratings
 
In reality, mercury and aneroid sphygmomanometers are widely used because of their low purchase price. When both units are in proper working order, either will give acceptable results (1). Both styles require calibration checks at regular intervals (at least annually). For aneroid devices, the procedure requires adjusting calibration at several pressure points, not just at zero like a mercury device.
 
Although simpler to calibrate, mercury sphygmomanometers have some inherent disadvantages when compared with the aneroid sphygmomanometer.
  • Mercury is a toxin that threatens humans and wildlife. As a result, spills require careful and costly cleanup.
     
  • It requires excellent technique to read the meniscus of a mercury column. Even if both types are in good working order, the aneroid dial is easier and requires less effort to read than a mercury column.
     
  • Maintenance of mercury devices is cumbersome. For accuracy, the mercury tube must be perfectly perpendicular in its unit and perfectly vertical to the ground. The more off-vertical, the greater the inaccuracy.
     
  • Each mercury sphygmomanometer has a vent or filter allowing outside air to be drawn in. Without frequent filter replacement, the mercury column experiences lag. "Lag" is a delay in the mercury response, which may result in an inaccurate reading.
     
  • Oxidized mercury can make the column appear dirty, making it difficult to read the true mercury level (2).
To put this in perspective, a recent study of 444 mercury sphygmomanometers (in a hospital setting) found the following faults. The study concluded "the majority of the mercury sphygmomanometers...had serious problems which would give rise to major errors in blood pressure measurement". (2)
 
Mercury zero level* between -10 & 20 mm Hg 55%
Dirty columns 38%
Column not vertical 20%
Air filter blocked 5%
Visible Hg droplets (outside the tube) 3 units
* This is the resting level of the mercury when the unit is not in use. When a sphygmomanometer is not in use, the mercury should be at the 0 mm Hg line. In these cases, the mercury was off (inaccurate) by 10-20 mm Hg.
 
Many hospitals have made the switch to aneroid sphygmomanometers. When these hospitals accepted the challenge of eliminating mercury, many were pleasantly surprised to find that good quality aneroid sphygmomanometers are accurate and reliable without the many hidden drawbacks of mercury sphygmomanometers.
 
For more information about aneroid sphygmomanometers, refer to our list of alternative sphygmomanometers or contact the SHP by email at shp@uml.edu or by phone at 978-934-3386.
 
 
The Sustainable Hospitals Project expresses its appreciation to Welch Allyn, Inc for providing detailed technical information on mercury and aneroid sphygmomanometers.
 
References:
 
(1) E.W. Wright, "Sphygmomanometers: Internal Analysis of Different Technologies; by Welch Allyn, Inc." Welch Allyn, Inc. publication, dated 1/11/2000
 
(2) N.K. Markandu, F. Whitcher; A. Arnold and C. Carney, "The mercury sphygmomanometer should be adandoned before it is proscribed", Journal of Human Hypertension (2000) 14, pp 31-36


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