Measurement of Blood Pressure In and Out of the Office

Although hypertension can only be identified by measuring blood pressure (BP), the conventionally used methods for its detection are notoriously unreliable. There are three main reasons for this: inaccuracies in the methods, some of which are avoidable; the inherent variability of BP; and the tendency for BP to increase in the presence of a physician (white coat effect). For clinical practice, the gold standard is measurements made with the auscultatory technique by a physician using a mercury sphygmomanometer, but there is increasing evidence that this may lead to the misclassification of large numbers of individuals as hypertensive and also fail to detect others who are hypertensive outside the clinic setting. In addition, mercury is being banned in many countries, and there is still uncertainty as to what will replace it. Neither the distribution of BP in the population nor the relationship between BP and cardiovascular morbidity provide any justification for a rigid separation between normotension and hypertension, but for clinical purposes, a threshold level of BP above which antihypertensive treatment is recommended needs to be established. Thus, the accurate measurement of BP is of extreme importance. There are two general reasons for measuring BP. The first is as a “vital sign” when evaluating a critically ill patient, in which case the BP at the time of measurement is of critical interest—is it too high, normal, or too low? For the vast majority of measurements, however, we are not interested in the pressure at the time of measurement, so much as its ability to estimate the average or “true” level of BP, which is generally assumed to be responsible for the adverse effects of high BP on the circulation and for which the clinic or office BP is taken as a surrogate measure. Recent advances in the techniques of measuring BP, particularly ambulatory monitoring, have begun to provide the opportunity to examine the pathological role of other measures of BP, such as abnormalities of the diurnal rhythm and the short-term variability. The methods currently available for BP measurement in clinical practice are shown in Figure. There are two basic techniques—the auscultatory and oscillometric methods. The former can be done with mercury or aneroid sphygmomanometers, or the more recently introduced hybrid technique. Measurements are preferably taken from the upper arm, although the wrist and finger are alternative sites. In addition, measurements can be taken in the office, at home, or over 24 hours with ambulatory monitoring. The use of all three methods has been endorsed by the seventh national report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure (JNC 7) recommendations. Techniques of blood pressure measurement in clinical practice It is surprising that nearly 100 years after it was first discovered, and the subsequent recognition of its limited accuracy, the Korotkoff technique for measuring BP has continued to be used without any substantial improvement. The Korotkoff sound method tends to give values for systolic pressure that are lower than the intra-arterial pressure, and diastolic values that are higher, but there is no obvious superiority for phase 5 over phase 4. The official recommendation of organizations such as the American Heart Association is to use the fifth phase, except in children or other situations in which the disappearance of sounds cannot reliably be determined. Most of the large-scale clinical trials that have evaluated the benefits of treating hypertension have used the fifth phase. Mercury Sphygmomanometers. The mercury sphygmomanometer has always been regarded as the gold standard for clinical measurement of BP, but this situation is likely to change in the near future, as discussed below. The design of mercury sphygmomanometers has changed little over the past 80 years, except that modern versions are less likely to spill mercury if dropped. In principle, there is less to go wrong with mercury sphygmomanometers than other devices, but this should not be any cause for complacency, and in practice many mercury devices in clinical use have been found to be defective. The Future of Mercury. A growing trend throughout the world is the removal of mercury-containing devices from hospitals. This has already happened with thermometers and is now happening with sphygmomanometers. The reason is not because a more accurate device has been developed, but because of concerns about the safety of mercury. In some European countries, mercury has already been banned, and there is a growing tendency in the United States to replace mercury devices, although this is being resisted by some organizations. The unresolved issue is what should replace mercury. Currently, the three alternatives are aneroid, electronic (oscillometric), and hybrid. Neither aneroid nor electronic is regarded as satisfactory. Hybrid devices offer the possibility of retaining some of the advantages of the mercury method. Aneroid Devices. The incipient demise of the mercury sphygmomanometer has placed new interest in alternative methods, of which aneroid devices are the leading contenders. However, surveys conducted in hospitals in the past 10 years have examined the accuracy of the aneroid dials and reported a high rate of inaccuracies. Hybrid Devices. These devices combine the main advantages of the mercury technique while avoiding the use of mercury, by using an electronic transducer in place of the mercury column. 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. The oscillations begin at approximately systolic pressure and continue below diastolic, so that systolic and diastolic pressure can only be estimated indirectly according to some empirically derived algorithm. One advantage of the method is that no transducer need be placed over the brachial artery, so that placement of the cuff is not critical. Other potential advantages of the oscillometric method for ambulatory monitoring are that 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, 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 BP 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. However, comparisons of several different commercial models with intra-arterial and Korotkoff sound measurements have shown generally good agreement. Validation of Oscillometric Monitors. The increasing use of electronic monitors for both self-monitoring and ambulatory monitoring has necessitated the development of standard protocols for testing them. The two most widely used have been developed by the British Hypertension Society and Association for the Advancement of Medical Instrumentation (AAMI) in the United States, but there is now an international protocol that requires a smaller number of subjects. 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. An up-to-date survey of validated monitors is available on the dabl Educational Trust Web site (http:www.dableducational.com/). There are important potential sources of error with measurements from the upper arm, which are listed below. Effects of Posture. BP measurement is most commonly made in either the sitting or supine position, but the two positions give different measurements. Diastolic pressure measured while sitting is higher than when measured supine (by about 5 mm Hg), but systolic pressure is the same. Other considerations include the position of the back and legs. If the back is not supported (as when the patient is seated on an examination table as opposed to a chair), the diastolic pressure may be increased by 6 mm Hg. Crossing the legs may raise systolic pressure by a similar amount. When measurements are taken in the supine position, the arm should be supported with a pillow. BP measurements are also influenced by the position of the arm. There is a progressive increase in the pressure of about 5 mm Hg as the arm is moved down from the horizontal to vertical position. These changes are exactly what would be expected from the changes of hydrostatic pressure. Cuff Size. The size of the cuff relative to the diameter of the arm is critical. The commonest mistake is to use a cuff that is too small, which will result in an overestimation of the pressure. In general, the error can be reduced by using a large adult-sized cuff for all except the skinniest arms. Rate of Cuff Inflation and Deflation. The rate of inflation has no significant effect on the BP, but with very slow rates of deflation (≤2 mm Hg/sec), the intensity of the Korotkoff sounds was diminished, resulting in slightly higher diastolic pressures. This effect has been attributed to venous congestion reducing the rate of blood flow during very slow deflation. The recommended deflation rate is 2–3 mm Hg/sec. Although the mercury method is still considered the gold standard for measurement in the clinic, there is a huge gulf between the ideal measurements and those made in real life. The recent interest in alternative methods of measuring BP has served to emphasize some of the potentially correctable deficiencies of the routine clinic measurement of BP. By increasing the number of readings taken per visit and the number of visits, as well as by attempting to eliminate sources of error such as terminal digit preference, the reliability of clinic pressure for estimating the true BP and its consequences can be greatly increased. However, a number of factors relating to the physician, the patient, or their interaction may lead to either over- or under-estimation of the true BP. Some of these are listed inTable I. Observer Error. Observer error and observer bias are important sources of error when conventional sphygmomanometers are used. Differences of auditory acuity between observers may lead to consistent errors, and terminal digit preference is very common, with most observers recording a disproportionate number of readings ending in 5 or 0. Patient-Related Factors. If the patient is very anxious, talks during the BP measurement, or has just smoked a cigarette or drunk coffee, the recorded pressure may overestimate the true pressure. Conversely, if the patient smokes during the day, is very physically active, or has a stressful job, the clinic pressure may underestimate the true pressure. The White Coat Effect. One of the main reasons for the growing disillusion of the value of traditional office BP readings is the white coat effect, which is conceived as the increase of BP that occurs at the time of a clinic visit and dissipates soon thereafter. It has been known for more than 50 years that BPs 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. Physicians also record higher pressures than nurses or technicians. The white coat effect is usually defined as the difference between the clinic and daytime ambulatory pressure. The underlying mechanisms are not well understood, but may include anxiety, a hyperactive alerting response, or a conditioned response. The white coat effect is seen to a greater or lesser extent in most if not all hypertensive patients, but is much smaller or negative in normotensive subjects or those with masked hypertension. A closely linked but discrete entity is white coat hypertension, which refers to a subset of patients who are hypertensive according to their clinic BPs but normotensive at other times. Use of Oscillometric Monitors in the Office. One way of reducing observer error and increasing the number of readings is to use an oscillometric device that can be programmed to take multiple readings while the patient is seated in the waiting room. These devices may also reduce the white coat effect to some extent. The BP levels tend to be lower than the physician's readings, but it is not yet clear how well they correlate with other measures. The potential advantages of having patients take their own BP are two-fold: the distortion produced by the white coat effect is eliminated, and multiple readings can be taken over prolonged periods of time. SMBP plays an increasing role in the diagnosis of hypertension. It may be used as a first step in the evaluation of patients with suspected white coat hypertension, as recommended in JNC 7. There are two studies that have compared the predictive value of clinic and home measurements, and both have shown that home measurements are potentially superior. In the first, which was conducted as a population survey in the town of Ohasama, Japan, 1789 people were evaluated with home, clinic, and 24-hour BP measurements. Over a 5-year follow-up, it was found that the home pressure predicted risk better than the clinic readings. The second study, which was conducted in France and recruited 4939 elderly hypertensives who were currently on treatment, found that morbid events observed over a were predicted by the home BP at but not by the clinic pressure. One particularly of this was that patients who clinic pressures but high home pressures were at increased a known as masked hypertension. There is also evidence that SMBP can BP a recent of trials SMBP with found that BP was by about mm Hg in the SMBP One of the for using SMBP to the to antihypertensive treatment from the on of BP and study, which used three methods of BP measurement and to the changes in BP resulting from treatment with an to the of The changes of clinic pressure no significant with the changes in both SMBP and ambulatory monitoring The of this is that when there is a between the effects of antihypertensive treatment on clinic and BP, the may be more on readings is not they can provide a to clinic readings, both for the evaluation of patients and for monitoring their to The relative advantages of clinic, and ambulatory monitoring are shown inTable When home monitoring was first the majority of studies used aneroid sphygmomanometers. In the past years electronic devices have and and are now The standard of monitor for home use is now an oscillometric which pressure from the brachial Oscillometric monitors have the advantage of being to cuff placement is not as critical as with devices that use a Korotkoff sound and in practice the oscillometric method has been found to be as as the Korotkoff sound method. There is now a large number of monitors on the but not all have been that have are on the dabl Web site and only those that have the criteria should be used. The advantages of electronic monitors have begun to be by who have always been greatly about the accuracy of clinical BP measurement and have much attention to the of observer digit preference, and the other of devices that can take BP from the upper arm, or finger are now the use of the more may be more measurement of BP from the arm has always been the standard method and is likely to so for the Monitors. monitors have the advantages of being smaller than the arm devices and can be used in the wrist diameter is little by A potential with wrist monitors is the error introduced by the hydrostatic effect of in the position of the wrist relative to the This can be if the wrist is always at level when the readings are but there is no way of this was with when a of readings are monitors have but need to be evaluated Monitors. monitors are but have so found to be and are not developed more than years ambulatory blood pressure monitoring is only now to as a technique. advances over the past years have to the of monitors that are small, and can take to 100 readings of BP over 24 hours while patients go about their are accurate while the patient is at but less so during physical In they can provide about the three main measures of average the diurnal and short-term variability. the currently available monitors take readings than and are during they can only give a very estimate of the short-term variability of BP. in hypertensive patients that in the majority of patients the average ambulatory pressure is lower than the clinic pressure, and in some may be the leading to a diagnosis of white coat hypertension, below. There is a diurnal rhythm of BP, with a of mm Hg during and a increase on and in the The BPs are usually seen between 6 and which is the time at which the of many cardiovascular morbid events tends to be The of BP during the is on the of activity, with pressures to be higher during the hours of work and lower while at In hypertensive patients, the diurnal BP is at a higher level of pressure, with of the in the The short-term BP variability is increased when in Hg), but the changes are no hypertension can be regarded as a of the point or level of BP with short-term treatment these by the normal, with little effect on short-term variability. that there is a between the clinic and ambulatory pressure, it is to that the of risk will be There are now many studies relating the extent of cardiovascular to both clinic and ambulatory pressures. all have shown that the are higher for ambulatory pressure, although in many the were The superiority of ambulatory pressure in this may be attributed at in to the greater number of readings, and to their more There are now 10 studies the of cardiovascular events using clinic or ambulatory BP, and the is that ambulatory pressure is a better of risk than clinic pressure. Thus, when the ambulatory pressure is in to the clinic pressure (white coat the is This of data was what to the by The for and to for in patients with suspected white coat hypertension because they a that not need antihypertensive One at patients with hypertension, defined as a diastolic pressure mm Hg while on three or more antihypertensive were in three according to their daytime ambulatory those in the mm a lower rate of morbidity over the years, similar clinic pressures. An important in this was the Hypertension in a large of the effects on cardiovascular morbidity of treating systolic hypertension of the elderly with a A of patients used and found that was a much more of risk than office BP. This is the only that has examined the effects of treating patients with white coat hypertension because there was a as well as an treatment There was a significant of events in the treatment in patients with hypertension but not in the with white coat hypertension. The main use of is for the diagnosis of a BP as shown inTable It is of limited value for the evaluation of Some is are shown inTable the use of BP measurement in and out of the office, BP can be from the traditional method on clinic pressures. shown in the for clinic and daytime ambulatory BPs are mm Hg for the former and for the There are normotension by both true hypertension by white coat hypertension by clinic normotensive by and masked hypertension in the clinic and hypertensive during ambulatory also shown in the the evidence that the of cardiovascular risk with the ambulatory than the clinic pressure. of blood pressure according to office and measurements. White coat hypertension is defined as a clinic pressure mm with a daytime ambulatory pressure It is important to emphasize that it requires several clinic to the diagnosis because there may be a of clinic pressure with multiple White coat hypertension is not a discrete most hypertensive patients a white coat patients with white coat hypertension from those with true or hypertension is not that they have an white coat effect, but that their BP is the when they are outside the clinic setting. with white coat hypertension do not or have a while in the Thus, it can only be reliably by ambulatory monitoring. If patients with white coat hypertension are with antihypertensive there is a of clinic pressure but little or no change of ambulatory pressure, which by is to there are some patients in the white coat effect is negative and in clinic pressure may underestimate the BP measured The of which is to as masked hypertension, is that there is increasing evidence that such individuals have more and higher risk of cardiovascular than patients who are normotensive both in and out of the way of patients using ambulatory monitoring is by the change of BP during the The is defined as a difference between the average daytime and BP of at 10 mm in about of hypertensive patients the of BP is smaller or is more in than in and is a of a number of pathological hypertension, the and such as that are with The has been with increased cardiovascular risk in several treatment may to in some Most cardiovascular events in the hours between 6 and and there is recent evidence that the of BP that occurs on and may be to an increased risk of Thus, it is important that antihypertensive be used to that there is BP during the The use of BP readings taken on and at may give a good to the For the future, measurement of clinic BP by conventional will continue to be the method of clinical A is that the the BP is to the threshold level at which treatment will be the more readings should be taken over more visits, the is In patients who have clinic pressure and evidence of it is usually to the clinic readings with other of measurement a When an BP is the only however, the possibility that the clinic pressure may overestimate the true pressure should be This can be done either by self-monitoring or by ambulatory monitoring. A for the use of the different procedures for measuring BP when evaluating a hypertensive patient is shown in Figure. If self-monitoring is and pressures to the clinic, value treatment may be but if the home readings are much lower than the clinic readings, it not out the possibility that the BP may be at This is the advantage of ambulatory monitoring, which the estimate of the of BP during life. for the evaluation of hypertensive patients using office and pressure