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Principles and techniques of blood pressure measurement
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
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.
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.