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U.S. Food and
Drug Administration
FDA Consumer magazine
September-October 2002
Table
of Contents
by Carol Lewis
Measuring a person's blood pressure is a routine part of every physical exam. The results can predict long-term health risks, assess suitability for certain physical activities, help manage many types of medical problems, and determine eligibility for insurance. The procedure is done to screen for high blood pressure (hypertension), a major risk factor for serious conditions, such as stroke, kidney failure, and the leading killer in the United States--cardiovascular disease.
The safety of the current "gold standard" instrument used to measure blood pressure--the mercury-filled sphygmomanometer--however, is being called into question due to the environmental health risks associated with mercury. At the same time, medical experts fear that the mercury gauges may be replaced by less accurate devices without consideration for the health risks that could follow.
Although the environmental concerns are serious, the Food and Drug Administration believes that mercury sphygmomanometers are still useful medical devices.
The most accurate means for measuring blood pressure is directly within an artery (intra-arterial) using a catheter. But because this method is invasive, it is neither practical nor appropriate for repeated measurements in non-hospital settings, or for large-scale public health screenings. In addition, different methods for measuring blood pressure can produce different readings. The guidelines for diagnosing and treating hypertension are based upon measurements made using the mercury-filled sphygmomanometer, not upon intra-arterial measurement of blood pressure.
The usual method of measurement, therefore, is a noninvasive means that uses a sphygmomanometer, which includes either a column of mercury or pressure-registering gauge. With this technique, the flow of blood is temporarily stopped by an inflated cuff that is wrapped around the upper arm and that puts pressure on the main artery in the arm. Blood flow is then gradually restarted as the user slowly deflates the cuff.
An examiner uses a stethoscope to listen for sounds, called Korotkoff sounds, that can be heard when the blood begins flowing again through the artery and that change in tone and volume while the cuff is deflated. Blood pressure is typically measured in units of millimeters of mercury, and represents the force of blood against the blood vessel wall. The first number, called the systolic pressure, represents the highest blood pressure that occurs each time the heart beats. The second number, called the diastolic pressure, is the lowest pressure that occurs when the heart relaxes between two beats. The Korotkoff sounds are used to identify a person's systolic and diastolic blood pressure readings.
Both numbers are important because when either is elevated, so is the risk of developing heart and blood problems. According to the National Heart, Lung, and Blood Institute, a blood pressure reading consistently higher than 140/90 is a sign that the blood pressure needs to be brought under control. The typical adult blood pressure is 120/80 or lower, but readings vary depending on age and other factors.
The mercury sphygmomanometer is simple, easy to read, and requires no readjustment. It has been validated in many clinical circumstances against the direct method of measurement through the artery.
The push to replace mercury sphygmomanometers began in June 1998, when the Environmental Protection Agency and the American Hospital Association agreed to limit the amount of mercury waste from hospitals as much as possible by 2005. Other organizations, over time, have joined the effort.
Mercury is a silver-colored metallic element that is liquid at room temperature and tends to break into tiny, highly mobile droplets when spilled. These droplets vaporize and can contaminate the atmosphere. Precautions must be taken to limit the inhalation, ingestion or absorption of mercury in case of a spill or breakage. Exposure to mercury from sphygmomanometers used in health-care settings is extremely rare. Modern mercury sphygmomanometers are available in models that prevent accidental spillage of mercury. And, there have been only a few isolated cases of illness in children from mercury toxicity related to broken glass thermometers.
The FDA, which regulates blood pressure devices, requires companies to show that new monitors are substantially equivalent to models already on the market. They also must demonstrate accuracy through a clinical validation study.
There are two alternative types of blood pressure measuring instruments being marketed. Aneroid devices, which have no liquid, use metal that acts like a spring to measure blood pressure. These have a round compass-like face that is attached to a cuff and accompanied by a stethoscope, and are commonly used in physicians' offices. Electronic devices measure pressure by converting the readings into measurable electronic waves.
Electronic instruments include in-home blood pressure monitoring devices as well as the small stations often seen at drug stores where people place their arms through a mechanical cuff. These use physical measurements and mathematical formulas to calculate pressure. Electronic monitors were originally designed for use during surgery and in emergency room settings. They are not commonly used by U.S. physicians to diagnose or to monitor hypertension.
The two crucial considerations for substituting aneroid and electronic units for mercury instruments are calibration and validation. Calibration is a way to make sure that measurements begin from zero--much like when a scale is balanced before it is stepped on to measure body weight. If the starting mark is above or below zero, the final measurement will be inaccurate. Validation ensures that the instrument can take accurate measurements over a wide range of blood pressures, ages and clinical conditions.
The FDA also is concerned that aneroid and electronic devices may not be regularly calibrated, potentially making these devices prone to erroneous readings.
Regardless of the type of device used to measure blood pressure, selecting appropriately sized cuffs is critical. The appropriate cuff width is based on the diameter of the upper arm. Taking blood pressure measurement with a cuff that's too narrow could overestimate blood pressure, while too wide a cuff can underestimate the pressure. Inappropriately low blood pressure, or clinical shock, is a medical emergency. Inappropriately high blood pressure can indicate hypertension.
The American Heart Association (AHA) says that physicians who are involved in the management of patients with blood pressure problems must accept responsibility for ensuring that adequate instruments are available. They need to educate themselves on the instruments available for use in their clinics, and encourage the general use of mercury sphygmomanometers as the instrument of choice until others have been better validated.
Where aneroid or electronic devices are used, the AHA recommends validation through the Association for the Advancement of Medical Instrumentation or a similar organization and a program of regular maintenance.
The FDA recommends the following guidelines for in-home monitors:
FDA experts say it's important to remember that home monitors are not an appropriate substitute for the regular measurement of blood pressure during physician visits.