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Measuring Blood Pressure Accurately
New and Persistent Challenges
Daniel
W. Jones, MD; Lawrence J. Appel, MD, MPH;
Sheldon G. Sheps, MD; Edward J. Roccella,
PhD; Claude Lenfant, MD
JAMA. 2003;289:1027-1030.
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.
Blood Pressure Measurement
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
The Importance of
Accuracy
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
Sources of Error in Blood Pressure
Measurement
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
Validation of Blood Pressure Measuring
Devices
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.
Appropriate Cuff Size
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
Calibration and Maintenance of Blood
Pressure Devices
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.
Conclusions
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.
AUTHOR INFORMATION
Corresponding Author and
Reprints: Daniel W. Jones, MD, Department of
Medicine, University of Mississippi Medical Center, 2500
N State St, Jackson, MS 39216-4505 (e-mail: djones@ovc.umsmed.edu).
Author Affiliations: Department of Medicine,
University of Mississippi Medical Center, Jackson (Dr Jones);
Department of Epidemiology and International Health, Johns Hopkins
Medical Institutions, Baltimore, Md (Dr Appel); Mayo Clinic,
Rochester, Minn (Dr Sheps); and National Heart, Lung, and Blood
Institute, Bethesda, Md (Drs Roccella and Lenfant).
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