DAVE JONASON

A 9-year-old boy was brought to the Royal Hospital for Sick Children in Glasgow (Scotland). For three weeks, he suffered from abdominal pain, constipation, lethargy, pain in his limbs, and unsteadiness. He also showed signs of facial weakness, diminished reflexes, an inability to control his voluntary muscles, and constant restlessness. His mother indicated that his handwriting and schoolwork had deteriorated. The doctors suspected heavy metal poisoning, but no likely exposure source could be found.

Upon further investigation, however, the doctors learned that the boy’s younger brother had recently undergone renal transplantation and had been sent home from the hospital with a mercury sphygmomanometer for blood pressure monitoring. The 9-year-old played with and broke the sphygmomanometer, causing a spill, but continued playing with the instrument for a few days before alerting his mother. Exposure to the mercury caused the boy’s symptoms.

Mercury can be found in sphygmomanometers, thermometers, weather barometers, and dental amalgams. Although it is useful for the precise measurement of blood pressure and temperature, it is a dangerous toxin. The acute lethal dose for mercury compounds for an adult is 14–57 mg/kg or 1–4 g for a 70-kg person. The minimum lethal dose of methyl mercury, the organic form of mercury, is 20–60 mg/kg.

Mercury from broken medical equipment causes severe reactions, such as those seen in the 9-year-old in this story. Although most homes do not have instruments that contain large amounts of mercury, when a spill occurs, it’s better to be safe than sorry. Spillage accidents occur more often than most people know. The U.S. Environmental Protection Agency and public health officials across the country report a steady stream of concerned calls from parents who have accidentally broken mercury thermometers.

Under pressure
As a blood pressure cuff is inflated, it begins to occlude the flow of arterial blood, which then becomes turbulent and noisy (Korotkoff sounds) as it spurts through the artery. Eventually, the blood can flow past only when its pressure exceeds that of the cuff. Thus, to measure blood pressure, the cuff is inflated (and the mercury in the column rises) until it eliminates arterial flow, usually at 180–200 mm Hg. As the cuff pressure is slowly released, the user notes the pressure at which the Korotkoff sounds first appear (the systolic pressure). As the pressure continues to be released, arterial flow becomes easier and less noisy. The pressure at which the noise ceases is the diastolic pressure.

A sphygmomanometer is arguably the most accurate method of blood pressure measurement. Because blood pressure is measured against the height (and therefore weight) of a column of mercury, it can only be made inaccurate by changes in gravity. According to the American Heart Association’s Council for High Blood Pressure Research, mercury sphygmomanometers remain the “gold standard” for measuring blood pressure. But are there mercury-free alternatives?

Both aneroid and electronic manometers are safer than sphygmomanometers in that they do not contain mercury, but are they just as reliable? The short answer to that question is no. Aneroid, meaning “no liquid”, was first used to describe barometers that operated by the effect of outside air pressure on a diaphragm forming one wall of an evacuated container rather than a column of mercury. This term carried over into manometers that do not contain mercury. Aneroid manometers are not as reliable as sphygmomanometers but are considerably less expensive (see Table 1). Both devices, however, rely on the manual dexterity and hearing acuity of the user. Electronic devices, on the other hand, have digital readouts, which makes them easier to read. In all cases, regular calibration and appropriate cuff size are the keys to accuracy.

Fevered discussion
And what about mercury thermometers? In 1998, more than 18,000 calls were made in the United States to poison control centers and emergency rooms because of broken mercury thermometers. The city council of Duluth (MN) recently passed an ordinance banning the sale of mercury thermometers. Several other cities have followed suit, including Boston, San Francisco, Ann Arbor (MI) and Freeport (ME). Several national store chains, including Wal-Mart and Kmart, have agreed to stop selling mercury thermometers.

Alternatives to traditional mercury thermometers include Teflon-coated mercury thermometers that are resistant to breakage but are slightly more expensive. The traditional alternative to mercury-free thermometers is the alcohol thermometer, which is less hazardous but also less accurate. Thermometers filled with petroleum-based mineral spirits are even safer. Finally, microprocessor-based thermometers offer a digital readout.

Alternative thermometers are available from common lab equipment suppliers and drugstores. Do you have a mercury thermometer in your medicine cabinet? If so, you should be aware of the possible dangers associated with a mercury thermometer and be prepared to act immediately in case of an accidental spill (see box, “Spill ready”).

Physician, heal thyself
According to a study performed by the Mayo Clinic (Rochester, MN) hospitals and physicians are as concerned about the possible environmental hazards of mercury as they are about accuracy when conducting blood pressure checks. If mercury sphygmomanometers have been abandoned and one of the alternatives is being used, physicians must take the initiative to have the instrument regularly calibrated because of the elevated usage. If the sphygmomanometer is being used in the physician’s office, the doctor and office staff must take precautions to prevent mercury spills and be prepared to handle accidental spills. Some doctors’ offices are prepared with spill kits, which are available from lab suppliers.

Further reading

• Centers for Disease Control and Prevention; ATSDR and EPA warn the public about continuing patterns of metallic mercury exposure. www.cdc.gov/od/oc/media/pressrel/mercury.htm.
• Global Perspective on Lead, Mercury & Cadmium Cycling in the Environment; Hutchinson, T. C., Ed.; Franklin Book Co.: Elkin Park, PA, 1994.
• Kulig, K. A tragic reminder about organic mercury. New Engl. J. Med. 1998, 338, 1692– 1694.
• Lead, Mercury, Cadmium and Arsenic in the Environment; Hutchinson, T. C., Meema, K. M., Eds.; John Wiley & Sons: New York, 1987.
• O’Brien, E. Replacing the mercury sphygmomanometer. Brit. Med. J. 2000, 320, 815–816.
• Rennie, A. C.; et al. Mercury poisoning after spillage at home from a sphygmomanometer on loan from hospital. Brit. Med. J. 1999, 319, 366–367.
• U.S. Environmental Protection Association; Mercury Poisoning. www.epa.gov/r02earth/health/mercury.htm.