Mercury, or not Mercury .... that is the question

Aneroid sphygmomanometers are comparable to mercury sphygmomanometers in cost, technique and performance. However, some medical professionals hesitate to adopt aneroid blood pressure devices because of conflicting statements they have heard about aneroid gauge performance.

 
What are the concerns about aneroid sphygmomanometers?

Obtaining accurate blood pressure measurement is a foremost concern in the selection of sphygmomanometers. Once calibrated, there is no expected difference in performance between aneroid and mercury sphygmomanometers manufactured by the reputable companies we feature in our eStore (Trimline, Riester and MDF). All sphygmomanometers need routine calibration checks and regular preventive maintenance. The procedures are different for mercury and aneroid sphygmomanometers, but are otherwise equivalent in frequency, complexity and the amount of attention required. In practice, aneroid and mercury sphygmomanometers require different calibration techniques but otherwise proportionate amounts of attention.

Some medical professionals have concerns that aneroid sphygmomanometers are easily damaged during use, resulting in inaccuracy due to the device being dropped or bumped and knocked out of calibration. This is perceived to be less of a concern for mercury sphygmomanometers due to the mercury column's rigid mounting requirements. Because the mercury column must be perfectly vertical in its mounting and the mounting perpendicular to the floor for accuracy, most mercury devices are either wall mounted or mounted on robust mobile stands, or in metal casings. Concerns about dropping aneroid devices can be alleviated by purchasing aneroid sphygmomanometers as either wall-mounted units or mounted on mobile stands, comparable to the mercury sphygmomanometers, rather than selecting portable aneroid devices – or of course many gauges (for example, Trimline’s Bainbridgeâ and HaderÔ models) can be fitted with a guard for extra protection.


Comparative Characteristics of Sphygmomanometers

Sphygmomanometers are introduced to the marketplace only after thorough testing and evaluation. Sphygmomanometers sold in the United States are regulated and must be approved by the Food and Drug Administration (FDA). The FDA approval process requires companies to show that new sphygmomanometers are substantially equivalent to models already on the market and to demonstrate accuracy through a clinical validation study. The FDA recognizes ANSI/AAMI SP-9 (a voluntary standard) as a performance standard and both aneroid and mercury sphygmomanometers meet this standard. This Standard covers functionality, accuracy and safety, including requirements and suggested tests to verify compliance.

Many United States hospitals have eliminated mercury sphygmomanometers. Hospitals interviewed in a recent survey reported that the alternatives perform satisfactorily. There are many more hospitals that have not completely eliminated mercury sphygmomanometers but have phase out programs underway. (EPA, HCWH)

Both mercury and aneroid sphygmomanometers require routine maintenance. Key issues for mercury gauges include: verify (and adjust if necessary) the zero level of mercury, replace air filter, verify that column is perpendicular in its unit and vertical to the ground, check for oxidation of mercury (making column appear dirty and difficult to read) and clean tube if necessary. Key issues for maintenance of aneroid gauges include: check needle for smooth rotation, and test accuracy at several intervals against a reference meter or mercury manometer.

In addition to managing the maintenance and calibration of the sphygmomanometers themselves, hospitals that use mercury gauges must also maintain the capability to safely handle mercury and respond to a mercury spill. Mercury spill capability includes personnel trained to respond to a spill on a 24-hour, 7 day per week basis, a mercury spill kit (for example, Trimline’s “Quick-Up Mercury Recovery Kit”), hazardous waste resources for decontaminating a spill area and removing the mercury, and the financial resources for the spill response and liability associated with mercury exposure.

Increasingly hospital Group Purchasing Organizations (GPOs) are voluntarily supporting pollution prevention efforts.


Mercury Sphygmomanometers
 
Considerations for selecting sphygmomanometers include purchase price, ease of use, accuracy, reliability, maintenance requirements, and health/environmental impact.
 
In practice, both 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. Both styles require calibration checks at regular intervals (at least annually), but 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.
  

To put this in perspective, a recent study of mercury sphygmomanometers (in a hospital setting) found many faults and concluded "the majority of the mercury sphygmomanometers...had serious problems which would give rise to major errors in blood pressure measurement".
 
In either case, mercury or aneroid, maintenance is key.

However, many hospitals have made the switch to aneroid sphygmomanometers. Once these hospitals had 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.

Tips for Procuring Aneroid Sphygmomanometers
 

Many hospitals are replacing mercury sphygmomanometers with cost-competitive aneroid sphygmomanometers. An aneroid sphygmomanometer is a mechanical gauge with a round dial and needle that rotates to indicate pressure from 0-300 mm Hg. On the inside, a bellows and spring mechanism senses and transmits the blood pressure measurementto the dial.

Aneroid units have several features that are appealing to healthcare professionals:

• The dial of an aneroid gauge is much easier to read and requires considerably less effort than the meniscus of a mercury column, which requires awkward head movement to do a good job.
  
• It is easy to see if the aneroid needle is off zero when not in use. This provides a good indication of whether the gauge has been damaged or needs maintenance, and is an easy check each time a reading is taken.
  
• It is easy to check the needle for smooth rotation, another way of verifying that the aneroid device is in good working order.
  

When procuring new sphygmomanometers, a few considerations up front can make things easier in the long run:

• Verify that all components are latex free.
  
• When you buy the aneroid devices, ask the manufacturer to accept intact mercury sphygmomanometers for recycling. If this is negotiated in the contract, it saves the expense of disposing of mercury devices.
  
• Consider the need for spare units. For offsite calibration, extra units allow a swap with instruments in current use. These spares keep the hospital running while allowing the use of more cost efficient ground shipment to and from calibration.
  
• When you receive newunits, consider bar coding them. This allows you to track reliability and performance, calibration status, and inventory status.
  

Consider the cuff size and options, provided – you will need a selection of sizes (color coding can be immensely useful), and they should have range markings, also be latex free, and comply with your facility’s infection control program.  Few clinicians also realise that traditional cuffs are arm specific – a balanced bladder cuff (such as that manufactured by Trimline) is essential for accurate blood pressure reading.  This applies to both reusable and disposable cuffs.