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National Clinical Guideline Centre (UK). Hypertension: The Clinical Management of Primary Hypertension in Adults: Update of Clinical Guidelines 18 and 34 [Internet]. London: Royal College of Physicians (UK); 2011 Aug. (NICE Clinical Guidelines, No. 127.)

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Hypertension: The Clinical Management of Primary Hypertension in Adults: Update of Clinical Guidelines 18 and 34 [Internet].

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6Measuring blood pressure

For many years blood pressure has been measured using a brachial pressure cuff and auscultation of the brachial artery to identify the appearance and disappearance of Korotkoff sounds. Increasingly, automated devices for measuring blood pressure are now used in the clinic, hospitals and by people in their homes. In addition, ambulatory blood pressure measurement devices are available that are programmed to allow blood pressure to be measured repeatedly during the day and night. Blood pressure (BP) can be highly variable and this variability is due to the inherent variability in BP itself and the influence of factors such as posture, room temperature and pain/discomfort or stress. In addition there are factors related to the process of BP measurement itself that can contribute to BP variability such as the appropriateness of the cuff size, the rate of inflation and deflation of the cuff and the accuracy of the process of measurement or the automated BP monitor being used.

6.1. Techniques for measuring blood pressure

6.1.1. Manual blood pressure measurement

The cuff is inflated to block the brachial pulse. The first sound occurring with the return of the brachial pulse is the systolic pressure (the point at which the heart pumping at its hardest overcomes the pressure exerted by the cuff to push blood past the obstruction). Intermediate sounds follow as the cuff pressure drops, with muffling and then the disappearance of sounds indicating the diastolic pressure (the point at which the heart is not pumping outward and the residual arterial pressure is sufficient to overcome the pressure exerted by the cuff). The interpretation of the sounds was later developed by Ettinger.579

Three types of error have been identified for the RRK technique. Failure to accurately identify the Korotkoff sounds can lead to over or under estimation. Digit preference refers to the tendency of clinicians to round readings up or down, often to the nearest zero. Observer prejudice occurs when clinicians alter readings toward their prior expectation, a particular concern when close to a threshold which changes management.64,482 Supervised training and reassessment may help minimise errors.

Systolic pressure is estimated by first palpating the brachial pulse with slow deflation of the cuff. The cuff is reinflated before listening for Korotkoff sounds. The first pass is important since sometimes the first sounds disappear as pressure is reduced (the auscultatory gap) leading to an underestimation of systolic pressure by auscultation alone. In a case series, 21% of 168 untreated hypertensive patients demonstrated an auscultatory gap.121 A number of summaries are available highlighting good technique: an adaptation of these is shown in Table 12.

Table 12. Estimating blood pressure by manual auscultation.

Table 12

Estimating blood pressure by manual auscultation.

There has been some controversy as to whether phase IV or phase V sounds should be used to record diastolic blood pressure. Commonly, the difference in pressure between phase IV and V is less than 5 mmHg but occasionally can be substantial. Phase V can be absent with sounds audible to zero cuff pressure notably in some children, during pregnancy, with anaemia, aortic insufficiency and with elderly people. Phase V correlates better with direct measurement, is commonly used in clinical trials of antihypertensive therapies, and is more reproducible when assessed by different observers. There is now general consensus that phase V should be taken as the diastolic pressure except when absent.27,64,99

6.2. Cuffs

Modern cuffs consist of an inflatable cloth-enclosed bladder which encircles the arm and is secured by Velcro or by tucking in the tapering end. The width of the bladder is recommended to be about 40%, and its length 80%, of the arm circumference. Manufacturers are now required to provide markings on the cuff indicating the arm circumference for which it is appropriate (BS EN 1060-1) 21; these marks should be easily seen when the cuff is being applied to an arm. When the bladder is too small (under-cuffing) it is possible to overestimate blood pressure. The existence of over-cuffing and consequent underestimation is contentious although likely to be of smaller magnitude.482,553,636

6.3. Conditions and environment

Blood pressure is maintained by a combination of mechanical, neuronal and endocrine self-regulating systems in the body. These systems can alter blood pressure in response to changes in environment. Individual readings are influenced (for example) by age, ethnicity, disease, the time of day, posture, emotions, exercise, meals, drugs, fullness of bladder, pain, shock, dehydration, acute changes in temperature and changes in altitude. These influences can be substantial, altering systolic readings by as much as 20 mmHg.65

Standardising the environment in which blood pressure measurements are made reduces variation and enhances the interpretation of a series of readings taken over time.27,99 A quiet, comfortable location at normal room temperature is optimal. Ideally, the patient should not need to pass urine, not recently have eaten, smoked or taken caffeine or exercise. Allowing the patient to rest at least five minutes before measurement is also advised.27,65,99

Blood pressure readings tend to increase as patients move from the supine to standing position. The change may not be significant, but it is traditional for measurements to be taken whilst seated. Certain patients demonstrate a significant lowering of blood pressure when standing (postural hypotension).27,65,66,99,452

Blood pressure readings also tend to increase as the patient's arm is lowered below the horizontal and decrease when the arm is raised. When blood pressure is measured in the clinic setting, the patient’s arm should be out-stretched, level with their heart and in line with their mid sternum, and supported by a table or some other means.27,65,66,99,452 Blood pressure is usually measured in the non-dominant arm, especially when using home or ambulatory monitoring. Differences in readings may occur between arms. A BP difference of <10mmHg can be considered normal, however, a difference of more than 20mmHg between arms is unusual, occurring in <4% of people and is usually associated with underlying vascular disease. Clinicians are advised to take readings in both of the patient's arms initially, and use the arm with the higher reading for subsequent measurements of blood pressure. . Consistent inter-arm differences of over 20/10 mmHg may suggest pathology warranting specialist referral.27,65,99

6.4. White Coat Hypertension

The observation that clinicians (signified by their white coats) can cause spuriously high blood pressure readings in patients was first described in the 1940s.58 Additionally, sympathetic symptoms such as sweating, tachycardia and palpitation sometimes occur. The effect is short-lived with blood pressure dropping to normality after or near the end of the consultation. Consequently, a patient may present as hypertensive in clinic (in a primary or secondary care setting) but be normotensive otherwise.

White Coat Hypertension (WCH) is reported to occur in as many as 15% to 30% of the population,448 although this may be inflated due to inadequate evaluation of patients. It is more common in pregnancy and with increasing age although poorly understood otherwise.569 The size of white coat effect in individuals can vary over time and a small proportion (4%) may demonstrate atypical very high clinic readings.27 Failing to identify WCH makes inappropriate treatment for hypertension in normotensive patients a possibility. Similarly, hypertensive individuals can also exhibit WCH and may receive inappropriate dose titrations or additional antihypertensive agents.490,506,635 Patients have historically been enrolled in trials using clinic BP values, and these trials will almost certainly have included a proportion of patients with WCH. It is unknown whether benefits of treatment differ substantially in those with or without WCH.

“White Coat” Hypertension: A difference between clinic BP and home or ambulatory blood pressure averages is expected. This difference has been reported to average approximately 10/5mmHg but this will vary considerably and is usually greater in people with a higher baseline blood pressure and as people age. White coat hypertension is defined when a patient has a persistently elevated clinic BP and a normal home or ambulatory BP day time average, i.e. <135/85mmHg.

“White coat Effect” in people with hypertension: People with true hypertension, treated or untreated, can also exhibit a “White Coat Effect”, for example a clinic BP reading that is disproportionately greater than their home or ambulatory BP averages, but their home or ambulatory BP averages are in a hypertensive range. Such patients are at risk of receiving more BP medication than they need and will require out of office measurement to monitor the efficacy of their BP treatment.

6.5. Blood pressure measurement devices

There is considerable guidance about the range of appropriate devices for measuring blood pressure.100,171,446 and about their maintenance and periodic recalibration [172 Local medical physics and biomedical/clinical engineering departments can often give further advice.

6.5.1. Mercury sphygmomanometer

The mercury sphygmomanometer has been used for the traditional measurement of blood pressure. It is reliable and provides the reference standard for indirect measurement. However it is bulky, fragile and there are particular safety and economic concerns about the toxic effects of mercury. Mercury is being phased out of clinical use and mercury sphygmomanometers have already been removed from clinical areas in hospitals and primary care. Thus, alternatives to mercury sphygmomanometry are now required for routine clinical use.

Non-mercury devices that operate in a similar way to the traditional mercury column devices are available and provide a suitable alternative to mercury devices when manual auscultation is required to measure blood pressure.

6.5.2. Aneroid sphygmomanometers

Aneroid sphygmomanometers measure pressure using a lever and bellows system. They may be less accurate than mercury sphygmomanometers and their alternatives (see above), especially over time. Using the manual auscultation technique they are subject to the same sources of observer error.64

6.5.3. Automated devices

Automated devices are increasingly being used in hospitals and primary care. All sphygmomanometers need regular maintenance. Rubber tubing can crack and leak making cuff deflation hard to control, underestimating systolic and overestimating diastolic readings. Faulty valves can cause similar problems.64

6.6. Ambulatory blood pressure monitors

Ambulatory Blood Pressure monitoring (ABPM) involves a cuff and bladder connected to electronic sensors which detect changes in cuff pressure and allow blood pressure to be measured oscillometrically. The cuff is inflated by a battery powered compressor and sensors within the cuff detect changes in pressure oscillations during cuff deflation. Systolic and diastolic pressure readings are deduced from the shape of these oscillometric pressure changes using an algorithm built into the measuring device. Developed as a research tool in the 1960s, these devices have considerably reduced in size and now can be described properly as ambulatory. Thus a patient's blood pressure can be automatically measured at repeated intervals (commonly every 30 minutes) throughout the day and night, while they continue routine activities. Systolic and diastolic pressure can be plotted over time, with most devices providing average day, night and 24 hour pressures.448 (see Figure 2, page 41) An advantage of ABPM is the removal of observer error with automated reading. However, oscillometric measurement may be difficult in the presence of arrhythmias, particularly rapid atrial fibrillation, and in a subgroup of the general population in whom oscillometric readings are inaccurate for unknown reasons.445,448

A number of ABPM devices are available varying in size, weight, noise level, data manipulation and cost.450,452 Devices should be independently validated to one or both of two internationally accepted standards from the British Hypertension Society and the Association for the Advancement of Medical Instrumentation.41,447,451 See British Hypertension Society website www.bhsoc.org for a list of validated monitors.

When using ABPM, patients need some understanding of how the device works and instruction about manual deflation, missed readings, arm position, and machine location: fitting takes 15–30 minutes. An appropriately sized cuff is necessary as with non-ambulatory monitoring and if one arm gives a higher reading at baseline then this should be used subsequently. Patients may be asked to make diary records of events that are known to affect blood pressure so that readings can be related to them, for example, periods of sleep. Sleeping times can be recorded or fixed times may be predefined, including preparing for sleep (e.g. 9pm – midnight) and waking up (e.g. 6am – 9 am).448,450

6.7. Home blood pressure monitors

Home monitoring devices are oscillometric, measuring BP on the upper arm, the wrist or the finger. Home monitoring potentially offers some similar benefits to ABPM. Frequent measurement produces average values that may be more reproducible and reliable that traditional clinic measurement. Potentially, white coat hypertension, systematic error, terminal digit preference and observer prejudice can be removed.104,449,556 Home monitoring allows patients to assess their own response to antihypertensive medication, which may increase compliance with treatment. It has been argued that better evaluation provided by home monitoring may reduce unnecessary treatment, increase compliance and thus deliver cost savings.490,556 Home blood pressure devices are thought by some professionals to cause anxiety or obsessive self interest.449,452,556,569

Potential disadvantages stem from the need for appropriate training to avoid biased measurement. Use of inappropriately sized cuffs, isometric exercise when not resting the arm, measurement after or during exercise and observer prejudice (for non-automated recording) are possible.27 One study found that only 30% of patients using a manual home blood pressure monitor correctly adhered to the protocol. Further, less than 70% of the self-reported measurements were identical to those simultaneously recorded by the machine.303 Observer bias was more apparent in those patients who were more hypertensive or whose readings showed more variation. As with ABPM, home monitoring devices are oscillometric and may have difficulty measuring pressure in cases of arrhythmias, and in certain patients for no apparent reason.

See British Hypertension Society website www.bhsoc.org for a list of validated monitors.

6.8. Recommendations

  1. Healthcare professionals taking blood pressure measurements need adequate initial training and periodic review of their performance. [2004]
  2. Because automated devices may not measure blood pressure accurately if there is pulse irregularity (for example, due to atrial fibrillation), palpate the radial or brachial pulse before measuring blood pressure. If pulse irregularity is present, measure blood pressure manually using direct auscultation over the brachial artery. [new 2011]
  3. Healthcare providers must ensure that devices for measuring blood pressure are properly validated, maintained and regularly recalibrated according to manufacturers’ instructions. [2004]
  4. When measuring blood pressure in the clinic or in the home, standardise the environment and provide a relaxed, temperate setting, with the person quiet and seated, and their arm outstretched and supported. [new 2011]
  5. If using an automated blood pressure monitoring device, ensure that the device is validatedf and an appropriate cuff size for the person’s arm is used. [new 2011]
  6. In people with symptoms of postural hypotension (falls or postural dizziness):
    • measure blood pressure with the person either supine or seated.
    • measure blood pressure again with the person standing for at least a minute prior to measurement. [2004, amended 2011]
  7. If the systolic blood pressure falls by 20 mmHg or more when the person is standing:
    • review medication
    • measure subsequent blood pressures with the person standing
    • consider referral to specialist care if symptoms of postural hypotension persist. [2004, amended 2011]

6.9. Research recommendation

  1. Which automated blood pressure monitors are suitable for people with hypertension and atrial fibrillation?

Atrial fibrillation is common in older people and may prevent accurate blood pressure measurement with automated devices. It would be valuable to know if this can be overcome.

A list of validated blood pressure monitoring devices is available on the British Hypertension Society’s website (see www​.bhsoc.org). The British Hypertension Society is an independent reviewer of published work. This does not imply an endorsement by NICE.

Footnotes

f

A list of validated blood pressure monitoring devices is available on the British Hypertension Society’s website (see www​.bhsoc.org). The British Hypertension Society is an independent reviewer of published work. This does not imply an endorsement by NICE.

Copyright © 2011, National Clinical Guideline Centre.

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