Although the mercury sphygmomanometer is widely regarded as the “gold standard”
for office blood pressure measurement, the ban on use of mercury devices continues to
diminish their role in office and hospital settings. To date, mercury devices have
largely been phased out in US hospitals. This has led to the proliferation of non-
mercury devices and has changed (probably for ever) the preferable modality of blood
pressure measurement in clinic and hospital settings. In this article, the basic
techniques of blood pressure measurement and the technical issues associated with
measurements in clinical practice are discussed. The devices currently available for
hospital and clinic measurements and their important sources of error are presented.
Practical advice is given on how the different devices and measurement techniques
should be used. Blood pressure measurements in different circumstances and in special
populations such as infants, children, pregnant women, elderly persons, and obese
subjects are discussed.
The standard location for blood pressure measurement is the brachial artery.
Arm blood pressure monitor that measure pressure at the wrist and fingers have
become popular, but it is important to realize that systolic and diastolic pressures
vary substantially in different parts of the arterial tree with systolic pressure
increasing in more distal arteries, and diastolic pressure decreasing.
The auscultatory method
Although the auscultatory method using mercury sphygmomanometer is regarded as the
‘gold standard’ for office blood pressure measurement, widespread implementation of
the ban in use of mercury sphygmomanometers continues to diminish the role of this
technique.72 The situation is made worse by the fact that existing aneroid manometers,
which use this technique, are less accurate and often need frequent calibration.72 New
devices known, as “hybrid” sphygmomanometers, have been developed as replacement for
mercury devices. Basically, these devices combine the features of both electronic and
auscultatory devices such that the mercury column is replaced by an electronic
pressure gauge, similar to oscillometric devices, but the blood pressure is taken in
the same manner as a mercury or aneroid device, by an observer using a stethoscope and
listening for the Korotkoff sounds.72
The oscillometric technique
This was first demonstrated by Marey in 1876,38 and it was subsequently shown that
when the oscillations of pressure in a sphygmomanometer cuff are recorded during
gradual deflation, the point of maximal oscillation corresponds to the mean intra-
arterial pressure.32,39,97 The oscillations begin at approximately systolic pressure
and continue below diastolic (Fig. 1), so that systolic and diastolic pressure can
only be estimated indirectly according to some empirically derived algorithm. This
method is advantageous in that no transducer need be placed over the brachial artery,
and it is less susceptible to external noise (but not to low frequency mechanical
vibration), and that the cuff can be removed and replaced by the patient during
ambulatory monitoring, for example, to take a shower. The main disadvantage is that
such recorders do not work well during physical activity when there may be
considerable movement artifact. The oscillometric technique has been used successfully
in ambulatory blood pressure
monitors and home monitors. It should be pointed out that different brands of
oscillometric recorders use different algorithms, and there is no generic
oscillometric technique. Comparisons of several different commercial models with
intra-arterial and Korotkoff sound measurements, however, have shown generally good
agreement.
Devices incorporating this technique use an ultrasound transmitter and receiver
placed over the brachial artery under a sphygmomanometer cuff. As the cuff is
deflated, the movement of the arterial wall at systolic pressure causes a Doppler
phase shift in the reflected ultrasound, and diastolic pressure is recorded as the
point at which diminution of arterial motion occurs. Another variation of this method
detects the onset of blood flow at systolic pressure, which has been found to be of
particular value for measuring pressure in infants and children.18 In patients with
very faint Korotkoff sounds (for example those with muscular atrophy) placing a
Doppler probe over the brachial artery may help to detect the systolic pressure, and
the same technique can be used for measuring the ankle-brachial index, in which the
systolic pressures in the brachial artery and the posterior tibial artery are
compared, to obtain an index of peripheral arterial disease.
The finger cuff method of Penaz
This interesting method was first developed by Penaz63 and works on the principle
of the “unloaded arterial wall.” Arterial pulsation in a finger is detected by a
photo-plethysmograph under a pressure cuff. The output of the plethysmograph is used
to drive a servo-loop, which rapidly changes the cuff pressure to keep the output
constant, so that the artery is held in a partially opened state. The oscillations of
pressure in the cuff are measured and have been found to resemble the intra-arterial
pressure wave in most subjects (Fig. 2). This method gives an accurate estimate of the
changes of systolic and diastolic pressure when compared to brachial artery
pressures;63 the cuff can be kept inflated for up to 2 hours. It is now commercially
available as the Finometer and Portapres recorders and has been validated in several
studies against intra-arterial pressures.61,84 The Portapres enables readings to be
taken over 24 hours while the subjects are ambulatory, although it is somewhat
cumbersome.
The increasing use of wrist blood pressure monitor for both
self-and ambulatory monitoring has necessitated the development of standard protocols
for testing them. The two most widely used have been developed by the BHS52 and
Association for the Advancement of Medical Instrumentation (AAMI) in the United
States.2 Both require the taking of three blood pressure readings in 85 subjects
(chosen to have a variety of ages and blood pressures) by trained observers and the
device being tested. The BHS protocol requires that a device must give at least 50% of
readings within 5 mm Hg and 75% within 10 mm Hg with the two methods (grade B), and
the AAMI requires that the average difference between the two methods not exceed 5 mm
Hg with a standard deviation of less than 8 mm Hg. One of the limitations of the
validation procedures is that they analyze the data on a population basis and pay no
attention to individual factors. Thus, it is possible that a monitor will pass the
validation criteria and still be consistently in error in a substantial number of
individuals.23
Devices for clinic and hospital measurement
Mercury sphygmomanometers
The design of mercury sphygmomanometers has changed little over the past 50 years,
except that modern versions are less likely to spill mercury if dropped. As indicated
earlier, although the use of mercury sphygmomanometer is widely regarded as the ‘gold
standard’ for office blood pressure measurement, widespread implementation of the ban
in use of mercury devices continues to diminish their role in office and hospital
settings. To date, mercury devices have largely being phased out in US hospitals.43
The reason is not because any more accurate device has been developed but because of
concerns about the safety of mercury. Currently the two alternatives for replacement
of mercury are aneroid sphygmomanometer and electronic (oscillometric) devices.
Aneroid devices
The ban on mercury sphygmomanometer has placed new interest in alternative
methods, of which aneroid devices are the leading contenders. The error rates reported
with regards to accuracy of aneroid devices in older hospital surveys range from 1% in
one survey,8 to 44% in another.44 Validation studies conducted a decade ago indicated
that they could be accurate.4,96 A most recent study, which compared the use of
mercury versus aneroid device in the setting of a large clinical trial across over 20
clinical sites, also found it to be accurate.36 This is the best evidence yet
attesting to the accuracy of aneroid devices.
Sources of error with the auscultatory method
Some of the major causes of a discrepancy between the conventional clinical
measurement of blood pressure and the true blood pressure are listed in Table 2. The
measurement of blood pressure typically involves an interaction between the patient
and the physician (or whoever is taking the reading), and factors related to both may
lead to a tendency to either overestimate or underestimate the true blood pressure or
to act as a source of bi-directional error. As shown in Table 2, there may be
activities that precede or accompany the measurement that make it unrepresentative of
the patient’s “true” pressure. These include exercise and smoking before the
measurement as well as talking during it.
The white coat effect and white coat hypertension
One of the main reasons for the growing emphasis on blood pressure readings taken
outside the physician’s office or clinic is the white coat effect, which is conceived
as the increase of blood pressure that occurs at the time of a clinic visit and
dissipates soon thereafter. Recent studies indicate that the mechanisms underlying the
white coat effect may include anxiety, a hyperactive alerting response, or a
conditioned response29,55 In one of these studies, we assessed office blood pressure,
ambulatory blood pressure, and anxiety scores on three separate occasions one month
apart in 238 patients. We found the largest white coat effect occurred in the
physician’s presence, and the noted white coat effect was a conditioned response to
the medical environment and the physician’s presence rather than a function of the
patients’ trait anxiety level (See Figure 4). The white coat effect is seen to a
greater or lesser extent in most if not all hypertensive patients but is much smaller
or absent in normotensive individuals. It usually has been defined as the difference
between the clinic and daytime ambulatory pressure.91 A closely linked but discrete
entity is white coat hypertension, which refers to a subset of patients who are
hypertensive according to their clinic blood pressures but normotensive at other
times. Thus, white coat hypertension is a measure of blood pressure levels, whereas
the white coat effect is a measure of blood pressure monitor with extra large cuff.
What distinguishes patients with white coat hypertension from those with true or
sustained hypertension is not that they have an exaggerated white coat effect but that
their blood pressure is within the normal range when they are outside the clinic
setting. White coat hypertension is important clinically because it appears to be a
relatively low-risk condition compared to sustained hypertension (defined by an
elevated blood pressure in both the clinic and ambulatory settings).19 It can only be
diagnosed reliably by accurate automatic
home digital blood pressure monitor and home self-monitoring as described later.
Observer error and observer bias are important sources of error when sphygmomanometers
are used. Differences of auditory acuity between observers may lead to consistent
errors, and digit preference is very common, with most observers recording a
disproportionate number of readings ending in 5 or 0.60 An example is shown in Fig. 5
of readings taken by hypertension specialists, who are clearly not immune to this
error. The average values of blood pressure recorded by trained individual observers
have been found to vary by as much as 5 to l0 mm Hg.17 The level of pressure that is
recorded may also be profoundly influenced by behavioral factors related to the
effects of the observer on the subject, the best known of which is the presence of a
physician. It has been known for more than 40 years that blood pressures recorded by a
physician can be as much as 30 mm Hg higher than pressures taken by the patient at
home, using the same technique and in the same posture.3 Physicians also record higher
pressures than nurses or technicians.37,73 Other factors that influence the pressure
that is recorded may include both the race and sex of the observer.
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