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Are Aneroid Sphygmomanometers Accurate in Hospital and Clinic Settings?
Vincent J. Canzanello, MD;
Patricia L. Jensen, RN;
Gary L. Schwartz, MD
Arch Intern Med. 2001;161:729-731.
ABSTRACT
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Background The aneroid sphygmomanometer is commonly used for the indirect measurement
of blood pressure despite significant concerns about its accuracy. Although
the mercury sphygmomanometer is highly accurate, there are concerns about
the environmental toxicity of mercury. In response to various external pressures
to become essentially mercury free, the Mayo Clinic, Rochester, Minn, has
replaced many mercury sphygmomanometers with aneroid devices. Since 1993,
a maintenance protocol has been in place to ensure proper function and accuracy
of these devices.
Methods We assessed the accuracy of 283 aneroid devices using as the reference
standard a digital pressure and vacuum meter that was calibrated using a mercury
sphygmomanometer.
Results The mean ± SD values from the aneroid device in millimeters of
mercury at each reference point (at 20–mm Hg intervals from 60 to 240
mm Hg defined by the reference device) were 59.9 ± 1.9 at 60; 79.9
± 1.9 at 80; 100.0 ± 1.8 at 100; 120.3 ± 1.8 at 120;
140.7 ± 1.4 at 140; 160.7 ± 1.7 at 160; 180.9 ± 1.3 at
180; 200.7 ± 5.0 at 200; 221.0 ± 1.3 at 220; and 240.8 ±
1.6 at 240 (r = 0.99; P<.001).
The values from the aneroid device underestimated those of the reference device
by a mean of 0.5 mm Hg (95% confidence interval, 0.3-0.7). Virtually 100%
of the values from the aneroid device were within the 4–mm Hg range
recommended by the Association for the Advancement of Medical Instrumentation.
Conclusion Aneroid sphygmomanometers provide accurate pressure measurements when
a proper maintenance protocol is followed.
INTRODUCTION
THE ANEROID sphygmomanometer has been used for the indirect measurement
of blood pressure since the studies of Hill and Bernard in 18971
and has been widely used in clinical practice because of its convenience and
portability. Compared with standard mercury sphygmomanometry, however, the
aneroid device has more working parts and requires more maintenance, particularly
when it is subjected to heavy use. This potential shortcoming has been emphasized
in the report of Bailey et al,2 who demonstrated
that aneroid sphygmomanometers are significantly less accurate compared with
mercury devices. These investigators concluded that "aneroid sphygmomanometers
[should] be inspected for physical damage and validated for accuracy against
a standard mercury manometer at 6-month intervals to prevent inaccurate blood
pressure measurements."
Concern about the accuracy of the aneroid sphygmomanometer notwithstanding,
apprehension regarding the potential environmental toxicity of mercury3-5 has led to increased
interest in mercury-free methods of blood pressure measurement.6
This phenomenon has occurred despite the lack of evidence of injury resulting
from a sphygmomanometer-related mercury exposure in the health care setting,7 except under very extreme or unusual circumstances.8
During 1993-1998, increasing pressure from state regulatory and licensing
agencies (regarding hazardous waste handling and removal) and a review of
our institutional experience with mercury spills led to the replacement of
mercury sphygmomanometers with aneroid devices throughout our inpatient facilities
and several outpatient clinical areas. A regular maintenance program was developed
and implemented. The focus of this study was to assess the accuracy of a sample
of aneroid sphygmomanometers since the implementation of a regular maintenance
protocol.
METHODS
Since 1993, approximately 1500 mercury sphygmomanometers were replaced
with wall-mounted aneroid devices at the 2 principal inpatient areas of the
Mayo Clinic (Saint Marys Hospital and Rochester Methodist Hospital), Rochester,
Minn. At the same time, the following protocol was developed in conjunction
with the Division of Hypertension and in accordance with the standards published
by the Association for the Advancement of Medical Instrumentation.9 This protocol was incorporated into an annual inspection
of additional patient-related medical devices in all patient care areas, including
medical and surgical nursing units, critical care units, surgical units, and
radiology procedure areas. The protocol was performed by a biomedical equipment
maintenance technician following instruction and initial observation by a
member of the Division of Hypertension (P.L.J.).
PROTOCOL
- All aneroid devices were visually inspected for
damage to the instrument case, wall mount, bracket, and extension hose.
- The sphygmomanometer needle should be set to zero
prior to inflation.
- A digital pressure and vacuum meter (Digimano,
Netech Corp, Hicksville, NY) was used as the reference standard. This device
was checked for accuracy against a mercury sphygmomanometer twice yearly by
the biomedical equipment maintenance technician and was also checked by the
manufacturer once yearly. When checked for accuracy against a mercury sphygmomanometer
set to zero throughout the pressure range of 60 to 240 mm Hg, the digital
reference device underestimated the former by 0.12 mm Hg (95% confidence interval,
0.00-0.24). Subsequently, a Y tube was used to connect the inflation bulb
to the reference and aneroid devices. The tube was then inflated to 240 mm
Hg on the reference device and the corresponding value on the aneroid device
was recorded. The system was then deflated in increments of 20 mm Hg to a
lower limit of 60 mm Hg with corresponding values from the aneroid device
taken at each interval.
- Any aneroid sphygmomanometer that appeared physically
damaged, did not read zero prior to inflation, or whose reading differed from
that of the reference device by greater than 4 mm Hg was replaced with a new,
properly functioning device.
STATISTICAL ANALYSES
Statistical analyses were performed using a database (Excel; Microsoft
Corp, Redmond, Wash) and statistical program with additional analyses according
to the method of Bland and Altman.10
RESULTS
Aneroid sphygmomanometer data were collected prospectively from Saint
Marys Hospital (n = 155) and Rochester Methodist Hospital (n = 93) from January
1, 1999, to April 30, 1999. This sample represented approximately 17% of the
1500 aneroid sphygmomanometers installed at the 2 hospitals since 1993 and
was collected as part of the routine maintenance program as described previously.
In addition, aneroid sphygmomanometers (n = 35) from several outpatient areas
not covered by the above maintenance protocol were randomly chosen to be inspected.
With the exception of 1 device, all the devices from the inpatient areas read
zero prior to inflation and demonstrated smooth needle movement throughout
the range of measurements from 240 mm Hg to 60 mm Hg. The 1 device was replaced
during the study. In the outpatient departments, 3 devices (9%) were replaced
because 1 or more readings differed from the reference device by more than
4 mm Hg (mean ± SD, 4.9 ± 0.9 mm Hg). When these latter 3 devices
were excluded, there were no significant differences in accuracy between the
inpatient and outpatient aneroid sphygmomanometers (data not shown).
Mean ± SD pressure values in millimeters of mercury from the
aneroid devices at each reference point (defined by the reference device)
were 59.9 ± 1.9 at 60; 79.9 ± 1.9 at 80; 100.0 ± 1.8
at 100; 120.3 ± 1.8 at 120; 140.7 ± 1.4 at 140; 160.7 ±
1.7 at 160; 180.9 ± 1.3 at 180; 200.7 ± 5.0 at 200; 221.0 ±
1.3 at 220; and 240.8 ± 1.6 at 240. The values from the aneroid device
were virtually identical in the intervals between 60 mm Hg and 240 mm Hg (r = 0.99; P<.001). The values
from the aneroid device underestimated those of the reference device by a
mean of 0.5 mm Hg (95% confidence interval, 0.3-0.7). Virtually 100% of the
values from the aneroid device were within 4 mm Hg of the reference device
(Figure 1).
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Cumulative percentage of values using an aneroid sphygmomanometer
that were within 0 to 4 mm Hg of values using a digital pressure and vacuum
meter as a reference (n = 283).
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COMMENT
The results of this study demonstrate that aneroid sphygmomanometers
provide accurate pressure determinations when compared with a digital pressure
and vacuum meter. The performance of the aneroid sphygmomanometers was well
within the accuracy guidelines of 3 to 4 mm Hg as recommended by several authors.11-14 This
finding contradicts the results of several previous studies.2, 13
The contribution of a routine maintenance regimen in assuring accuracy of
aneroid sphygmomanometers is suggested by better performance of the inpatient
devices compared with the outpatient devices that were not part of the maintenance
protocol (1 of 248 inpatient devices were replaced vs 3 of 35 outpatient devices
replaced). A potential strength of this study is that the data were collected
as part of an ongoing maintenance program and were not a random prospective
sample that could have been biased by the inclusion or exclusion of recently
inspected aneroid devices.
Routine maintenance of sphygmomanometers may not be widespread. In a
survey15 conducted in general practitioners
in England, only about 50% had serviced their devices within 1 year and 24%
of devices had never been serviced (over a mean of 6 years). While the present
study was limited to the evaluation of fixed, wall-mounted aneroid sphygmomanometers,
there is no a priori reason to suspect that a similar maintenance protocol
could not apply to more portable devices, such as those that are handheld
or attached to the blood pressure cuff. Indeed, these latter devices are more
susceptible to damage from everyday use and are more likely to need periodic
inspection.
The mercury column sphygmomanometer has remained the gold standard of
indirect blood pressure measurement for many years. In our experience and
we suspect in that of most clinicians, it has provided trouble-free service,
other than the occasional need for calibration and replacement of worn tubing
or cuffs. Unfortunately, the potential environmental toxicity of mercury,
whether real or imagined, has led to increased interest in mercury-independent
measurement of blood pressure. We were somewhat surprised when we reviewed
the Mayo Clinic experience with mercury spills from October 1, 1993, to November
30, 1995. During this time, 50 spills were documented related to leakage from
sphygmomanometers. Several of these spills resulted from physical abuse to
the device by unruly patients, children, or patients with dementia or other
psychiatric problems. The cleanup costs were $26 000, not including time
lost from temporary unavailability of the clinical area. Use of a digital
pressure and vacuum meter as a reference device has also reduced the need
to maintain a large number of mercury sphygmomanometers for maintenance checks
of the aneroid devices. We have found that these reference devices are accurate,
easy to use, and relatively inexpensive at $250 to $500, depending on the
specific model.
Additional pressures on health care institutions have come from state
regulatory agencies that can restrict access to or needed expansion of biomedical
waste disposal facilities if an institution is not mercury free. In addition,
these agencies have mandated that hospitals provide detailed inventories of
existing mercury supplies in any form. However, these proposed restrictions
provide for the retention of a small number of mercury sphygmomanometers for
calibration purposes. Financial considerations may limit conversion to aneroid
devices and development of a regular maintenance program in some institutions
currently using mercury sphygmomanometers. In these situations, the risk of
mercury spills could be minimized by the selective use of aneroid devices
in those settings associated with an increased risk of damage to equipment,
such as pediatric areas and emergency departments.
In conclusion, a carefully maintained aneroid sphygmomanometer is an
accurate and clinically useful means of indirect blood pressure measurement.
AUTHOR INFORMATION
Accepted for publication September 21, 2000.
We thank Joel Worra for his statistical assistance and Sheldon G. Sheps,
MD, for his thoughtful review.
Corresponding author and reprints: Vincent J. Canzanello, MD, Division
of Hypertension, Mayo Clinic, 200 First St SW, Rochester, MN 55905 (e-mail: canzanello.vincent{at}mayo.edu).
From the Division of Hypertension, Mayo Clinic, Rochester, Minn.
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