Measuring Blood Pressure Accurately: Title and subTitle BreakNew and Persistent Challenges

High blood pressure is a major public health issue, affecting approximately 25% of US adults, or roughly 50 million Americans.¹ The prevalence of high blood pressure increases dramatically with age, such that the lifetime risk of high blood pressure approaches 100%.² Despite the availability of effective antihypertensive agents, rates of hypertension treatment and control have remained low and static over the last decade. Control of blood pressure begins with accurate measurement leading to appropriate diagnosis and treatment decisions.

For more than 100 years, clinicians and researchers have used the mercury sphygmomanometer, a simple, gravity-based instrument, for blood pressure measurement. Because of its accuracy and reliability, the mercury sphygmomanometer is generally regarded as the gold standard against which all other devices for blood pressure measurement should be compared.³

In recent years, with the confluence of concern that mercury is contaminating the environment plus the development of electronic/digital equipment for blood pressure measurement, many health care institutions have replaced the mercury manometer with alternate equipment.^{4 - 5} Experience with the reliability of the mercury manometer has contributed to a lack of attention both to issues of accuracy and to regulation of nonmercury measurement devices. The effort to remove mercury manometers from health care facilities has created unanticipated problems and challenges to measuring blood pressure accurately.

Recently the US Environmental Protection Agency (EPA) has identified mercury and mercury compounds as persistent bioaccumulative toxins.⁴ Most of the toxic effects of mercury are associated with mercury compounds, especially methyl mercury. Long-term exposure to mercury compounds has been associated with serious health problems, but the elemental mercury enclosed in manometers is quite stable and has rarely been reported to cause health problems. This same elemental mercury has been used in dental amalgam in fillings for years with no recognized toxicity.

As part of EPA efforts, a memorandum of understanding regarding elimination of mercury waste was signed in 1998 between the EPA and the American Hospital Association.⁶ This memorandum created "Hospitals for a Healthy Environment," a voluntary project requesting all hospitals to virtually eliminate mercury waste by 2005. The memorandum and the associated project have already led to mercury manometers being removed from many hospitals, health systems, and clinics.⁴

The controversy over mercury has led to an examination of several issues related to accurate blood pressure determination, involving both the instrument used and other issues of quality. These issues, recently discussed in a working meeting convened by the National Heart, Lung, and Blood Institute and the American Heart Association,⁷ should be of interest to clinicians, researchers, health care administrators, manufacturers, and policy makers.

Indirect measurement of systolic blood pressure by palpation became possible with the development of the mercury manometer by Riva-Rocci in 1896 and the aneroid manometer by Hill and Barnard in 1897.^{8 - 9} During the first half of the 20th century, mercury, aneroid, and oscillometric manometers came into regular use.

In 1939, a committee of the American Heart Association reported on the standardization of blood pressure readings in cooperation with a similar committee of the Cardiac Society of Great Britain and Ireland.¹⁰ That report was the first of 6 reports published by the American Heart Association giving guidance to clinicians, researchers, and manufacturers.^{1 ,11 - 14} The last such report was published in 1997.¹⁵

These American Heart Association recommendations on blood pressure determination have been and remain the standards most widely cited around the world. These guidelines are the blood pressure measurement standards cited in guidelines from the National Heart, Lung, and Blood Institute's Joint National Committee of the National High Blood Pressure Education Program.¹⁵ Parallel efforts have taken place in Europe and elsewhere. At present there is little difference other than in recommendations for cuff size.^{16 - 20}

Accurate measurement of blood pressure is crucial from a public health standpoint. The risk associated with increasing blood pressure is graded and continuous and begins close to 120/80 mm Hg. The current diagnostic thresholds occur near the midpoint of a unimodal distribution curve. Therefore, even minor errors in measurement can mislabel millions of persons. A systematic error of underestimating true blood pressure by 5 mm Hg would mean that 21 million persons who would benefit from drug treatment for hypertension could be mislabeled as having high normal blood pressure level rather than hypertension. A systematic error of 5 mm Hg in the opposite direction could misclassify 27 million people as being in the hypertensive range rather than as having high normal blood pressure. This would needlessly expose many of these persons to the expense and adverse effects of treatment. This issue is compounded by the challenge of the "white coat" or "office" effect, ie, the tendency for blood pressure to increase when it is measured, particularly in the presence of a clinician.²¹

Indirect measurement of blood pressure is the only practical means of identifying hypertension. Of the 3 common types of instruments in use today, the mercury manometer and the aneroid manometer depend on the trained observer using the manual auscultatory technique. Compared with direct intra-arterial measurement of blood pressure, the auscultatory technique using Korotkoff sounds tends to give systolic values that are slightly lower and diastolic values that are slightly higher than intra-arterial measurements.¹⁴

The third type of instrument for indirect measurement, the automated electronic manometer, assesses the oscillations of pressure in a cuff during gradual deflation. The point of maximal oscillation corresponds to the mean intra-arterial pressure. Systolic and diastolic blood pressure are then calculated according to an empirically derived algorithm. In currently available commercial instruments, these algorithms are typically proprietary, vary from manufacturer to manufacturer, and sometimes also vary from device to device.²²

Indirect measurement of blood pressure by any of these methods involves problems of accuracy for at least 3 reasons: the inherent biological variability of blood pressure, the white coat effect, and inaccuracies related to suboptimal technique. The oscillometric technique is particularly vulnerable to error in certain clinical circumstances, such as in patients with arrhythmias and in elderly patients with stiff arteries due to atherosclerosis. However, manual auscultatory measurement of blood pressure in patients with arrhythmias also has been shown to be highly variable.¹⁴

The impact of human error on blood pressure measurement is a well-described and substantial problem. Human errors are manifold and include inaccurate cuff selection and application, incorrect cuff positioning, inadequate rest period, rapid cuff deflation rate, poor observer concentration, digit bias, and lack of repeated measurements. Use of automated devices rectifies some but not all of these problems. Retraining and recertification of blood pressure observers has been shown to reduce variability of blood pressure due to human error.¹⁵

Certification of medical devices in the United States is the responsibility of the Food and Drug Administration. For some medical devices including blood pressure manometers, the Food and Drug Administration works through the Association for the Advancement of Medical Instrumentation (AAMI).^{23 - 24} This group has a subcommittee for evaluation of blood pressure measuring devices. The subcommittee consists of clinicians, scientists, and representatives of manufacturers of blood pressure measuring instruments.

Importantly, the AAMI standards in the United States are voluntary. Successful testing by AAMI standards allows a manufacturer to note in its materials that the device has met AAMI standards. However, lack of this designation does not preclude the sale of blood pressure instruments to health systems, hospitals, clinics, or individuals. Currently, no regulatory agency requires the use of AAMI-validated instruments. Some of the most common automated blood pressure measuring devices used in clinical practice have never passed AAMI certification.

Several concerns have been raised about the current AAMI validation process: there is no requirement for testing in more challenging clinical circumstances (eg, patients with arrhythmias, elderly patients); validation procedures depend on the mean of multiple readings rather than a percentage of readings that exceed certain limits; the standards are consensus-based rather than evidence-based; and the use of a standard set of cuffs is not required. The British Hypertension Society standards are also commonly applied and at present differ from those of the AAMI.^{25 - 27} Efforts are under way to consolidate these 2 standards.

Another major factor related to the accuracy of indirect measurement of blood pressure with any type of instrument is proper cuff size. Accurate measurement of blood pressure depends on the relationship between arm circumference and the length and width of the blood pressure cuff. Typically, a cuff too small for the patient's arm circumference will overestimate blood pressure, whereas a too large cuff will underestimate blood pressure.¹⁴

Some pressure measuring devices are manufactured without the ability to interchange cuffs or with only 1 size available for routine use. The standard "adult" cuff is too small for the arm of many adult patients. Appropriate larger-sized cuffs may not be used in clinical practice due to the pejorative names of the current cuffs. "Large adult cuff" or "thigh cuff" have negative connotations that may inhibit the appropriate use of these cuffs in individuals with larger arm circumferences. Because of the increasing body size of the US population, large adult and thigh cuffs must become more widely available regardless of the device or setting.^{28 - 29}

All devices for blood pressure measurement require regular calibration and maintenance. Among currently available instruments, the mercury manometer has been thought to be the least dependent on calibration and maintenance. Its simple gravity-based design leaves little room for mechanical errors. The aneroid instrument generally is a spring-based device. This spring is typically made of metal and can fatigue over time. Manufacturers of aneroid devices typically recommend calibration against a known standard (mercury manometer or nonmercury pressure meters) at least every 6 months. If the aneroid device fails the calibration test, it often must be returned to the manufacturer. Likewise, electronic instruments usually come with recommendations for calibration, but devices that fail calibration tests sometimes cannot be corrected without return to the manufacturer.

In many health systems, there is regular calibration and maintenance of all electronic instruments, typically at 6- to 12-month intervals. However, nonelectronic devices frequently have no standards for calibration and maintenance. To a large extent, this lack of calibration reflects the absence of mandated standards for calibration of blood pressure measuring devices in hospitals or other health care facilities in the United States. Several studies done with instruments in common use have demonstrated problems of calibration, maintenance, and accuracy.^{30 - 31}

Recently the Mayo Clinic removed mercury manometers from its Rochester, Minn, facilities and instituted a low-cost system of regular maintenance and calibration for its aneroid instruments. A study assessing these devices in regular use documented high accuracy, with an instrument failure rate of less than 0.5%.³² Aggressive programs of maintenance and calibration with any instrument and re-education of the observers can likely overcome many of the problems associated with inaccurate determination of blood pressure.

Removal of mercury sphygmomanometers from health care settings requires focused attention on the quality of blood pressure measurements in the office setting, specifically on the accuracy of the alternative devices that do not use mercury. Considerations for clinicians, researchers, health care administrators, manufacturers, and policy groups include

1. The mercury manometer remains the gold standard for blood pressure measurement. Alternate blood pressure measuring devices are available, but until equipment monitoring and surveillance have been implemented, medical institutions should have the option to continue to use mercury manometers.

2. Efforts of the EPA and other agencies to ensure appropriate management of mercury should be supported. This support should include careful attention to safety in the manufacture and use of mercury manometers and in handling of mercury spills.

3. Additional research should be conducted to assess the accuracy of aneroid and automated devices under conditions of routine and frequent use. Such studies should include patients with arrhythmias and elderly patients with noncompliant arteries—that is, those with a greater chance of inaccurate measurements.

4. When further accuracy and validation research results are available, validation guidelines should be adjusted to reflect the new knowledge base. These guidelines should be evidence-based rather than consensus-based. Validation guidelines would be most useful if commonly adopted rules were used in all countries. Validation standards should be based, in part, on the percentage of individual readings within an accurate range.

5. Device manufacturers should provide users with the option to purchase sets of appropriately sized cuffs for the setting of intended use, that is, a set of pediatric cuff sizes, a set of adult cuff sizes, or both. Likewise, different-sized cuffs should be readily available for use with devices designed for home use. These cuffs should be renamed with a nonpejorative naming system. Health care professionals should advise on the appropriate cuff sizes and validated devices to be purchased.

6. Manufacturers of devices for blood pressure measurement should provide package insert material for consumers noting any limitations of the instrument.

7. Regulatory agencies should consider establishing standards to ensure the use of validated devices, routine calibration of equipment, and the training and retraining of observers using manual devices. Because the use of automated devices does not eliminate all major sources of human error, the training of observers should be required even when automated devices are used.

8. Ideally, clinicians should use the auscultatory technique to diagnose hypertension and either auscultatory or oscillometric devices to monitor blood pressure. Clinicians and researchers should keep in mind that the oscillometric technique is not based on the same physiological observation as auscultation. The currently available cardiovascular disease risk prediction data come from studies that typically used a mercury manometer and auscultation. Clinicians can likely assume that blood pressure measurement with a well-calibrated aneroid manometer should yield the same results as measurement with a mercury manometer. However, measurement with an oscillometric device may not yield the same results.