ABC of hypertensionBlood pressure measurementPart I—Sphygmomanometry: factors common to all techniquesGareth Beevers, Gregory Y H Lip, Eoin O’Brien

Methods of blood pressuremeasurementMost devices for measuring blood pressure are dependent onone common feature, namely, occluding the artery of anextremity (arm, wrist, finger, or leg) with an inflatable cuff tomeasure blood pressure either oscillometrically, or by detectionof Korotkoff sounds. Other techniques, which are notdependent on limb occlusion, such as pulse-waveform analysis,can also be used, but these have little application in clinicalpractice. The array of techniques available today owe theirorigins to the conventional technique of auscultatory bloodpressure measurement, and these new techniques must indeedbe shown to be as accurate as the traditional mercurysphygmomanometer. Since the introduction of sphygmo-manometry, mercury and aneroid sphygmomanometers havebeen the most popular devices for measuring blood pressures.

Factors affecting blood pressuremeasurementNo matter which device is used to measure blood pressure, itmust be recognised that blood pressure is a variablehaemodynamic phenomenon, which is influenced by manyfactors, not least being the circumstances of measurement itself.These influences on blood pressure can be significant, oftenaccounting for rises in systolic blood pressure greater than20 mm Hg, and if they are ignored, or unrecognised,hypertension will be diagnosed erroneously and inappropriatemanagement instituted. These factors have to be carefullyconsidered in all circumstances of blood pressuremeasurement—self measurement by patients, conventionalmeasurement, measurement with automated devices whether ina doctor’s surgery, an ambulance, a pharmacy, or in hospitalusing sophisticated technology.1 2

Variability of blood pressureThe observer must be aware of the considerable variability thatmay occur in blood pressure from moment to moment withrespiration, emotion, exercise, meals, tobacco, alcohol,temperature, bladder distension, and pain, and that bloodpressure is also influenced by age, race, and circadian variation.It is usually at its lowest during sleep. It is not always possible tomodify these many factors but we can minimise their effect bytaking them into account in reaching a decision as to therelevance or otherwise of a particular blood pressuremeasurement.1

Insofar as is practical the patient should be relaxed in aquiet room at a comfortable temperature and a short period ofrest should precede the measurement. When it is not possibleto achieve optimum conditions, this should be noted with theblood pressure reading—for example, “BP 154/92, R arm, Vphase (patient very nervous).”

Mercury sphygmomanometer

Important factors affecting measurementx The inherent variability of blood pressurex The defence reactionx The limitations of the device being usedx The accuracy of the devicex Blood pressure is not as easily measured in some groups, such as

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Example of a normal ambulatory blood pressure pattern plotted by theDABL® Program showing a marked variability of blood pressure

This article has been adapted from the newly published 4th editionof ABC of Hypertension. The book is available from the BMJbookshop and at www.bmjbooks.com

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“White coat” hypertensionAnxiety raises blood pressure, often by as much as 30 mm Hg.This may be regarded as a physiological reaction, often referredto as the “fight and flight” phenomenon, or “defence” or “alarm”reaction. It is commonly seen in the accident and emergencydepartments of hospitals when patients are frightened andextremely anxious, but it may also occur in family doctors’surgeries and in the outpatients department. It may occur innormotensive and hypertensive subjects. The degree of thisreaction varies greatly from patient to patient, being absent inmany, and it is usually reduced or abolished altogether withreassurance and familiarisation with the technique andcircumstances of blood pressure measurement. Its importancein practice is that decisions to lower blood pressure, andespecially to administer drugs, should never be made on thebasis of measurements taken in circumstances where thedefence reaction is likely to be present.

White coat hypertension is a condition in which anormotensive subject becomes hypertensive during bloodpressure measurement, but pressures then settle to normaloutside the medical environment. It is best demonstrated byambulatory blood pressure measurement (ABPM).

No one group seems to be exempt from the white coatphenomenon; it may affect the young, the elderly, normotensiveand hypertensive subjects, and pregnant women. In youngsubjects with borderline elevation of conventional bloodpressure, identification of white coat hypertension can be ofconsiderable importance in avoiding undue penalties forinsurance and employment. Moreover, there are nocharacteristics that allow for the identification of thephenomenon, other than by obtaining blood pressures awayfrom the medical environment, either by self measurement inthe home or with ABPM, which is the technique of choice.Patients diagnosed as “hypertensive” with conventionalmeasurement in whom white coat hypertension is considered apossibility should have ABPM performed before they arelabelled “hypertensive,” and certainly before treatment isinstigated.

Posture of subjectPosture affects blood pressure, with a general tendency for it toincrease from the lying to the sitting or standing position.However, in most people posture is unlikely to lead tosignificant error in blood pressure measurement provided thearm is supported at heart level. None the less, it is advisable tostandardise posture for individual patients and in practice bloodpressure is usually measured in the sitting position. Patientsshould be comfortable whatever their position. No informationis available on the optimal time that a subject should remain ina particular position before a measurement, but three minutesis suggested for the lying and sitting positions and one minutestanding. Some antihypertensive drugs cause posturalhypotension, and when this is expected blood pressure shouldbe measured both lying and standing.1

Arm supportIf the arm in which measurement is being made isunsupported, as tends to happen if the subject is sitting orstanding, isometric exercise is performed raising blood pressureand heart rate. Diastolic blood pressure may be raised by asmuch as 10% by having the arm extended and unsupportedduring blood pressure measurement. The effect of isometric

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exercise is greater in hypertensive patients and in those takingâ blockers. It is essential, therefore that the arm is supportedduring blood pressure measurement and this is best achieved inpractice by having the observer hold the subject’s arm at theelbow, although in research the use of an arm support on astand has much to commend it.1

Arm positionThe arm must also be horizontal at the level of the heart asdenoted by the midsternal level. Dependency of the arm belowheart level leads to an overestimation of systolic and diastolicpressures and raising the arm above heart level leads tounderestimation. The magnitude of this error can be as great as10 mm Hg for systolic and diastolic pressures. This source oferror becomes especially important in the sitting and standingpositions, when the arm is likely to be dependent by thesubject’s side. However, it has been demonstrated that even inthe supine position an error of 5 mm Hg for diastolic pressuremay occur if the arm is not supported at heart level.1 2 Armposition has become an important issue for self measurementof blood pressure with the manufacture of devices formeasuring blood pressure at the wrist, which are proving verypopular because of the ease of measurement. Many of thesedevices are inherently inaccurate, but measurement is extremelyinaccurate if the wrist is not held at heart level duringmeasurement.

Which armThis topic remains controversial as some studies, but not all,using simultaneous measurement have demonstrated significantdifferences between arms.1 However, the fact that bloodpressure differences between arms are variable makes the issueeven more problematical. A reasonable policy is to measureblood pressure in both arms at the initial examination, and ifdifferences greater than 20 mm Hg for systolic or 10 mm Hgfor diastolic pressure are present on three consecutive readingsthe patient should be referred to a cardiovascular centre forfurther evaluation.

The cuff and bladderThe cuff is an inelastic cloth that encircles the arm and enclosesthe inflatable rubber bladder. It is secured around the arm mostcommonly by means of Velcro on the adjoining surfaces of thecuff, occasionally by wrapping a tapering end into the encirclingcuff, and rarely by hooks. Velcro surfaces must be effective, andwhen they lose their grip the cuff should be discarded. It shouldbe possible to remove the bladder from the cuff so that thelatter can be washed from time to time.1

“Cuff hypertension”However sophisticated a blood pressure measuring device maybe, if it is dependent on cuff occlusion of the arm (as are themajority of devices), it will then be prone to the inaccuracyinduced by miscuffing, whereby a cuff containing a bladderthat is either too long or too short relative to armcircumference is used.

A review of the literature on the century-old controversyrelating to the error that may be introduced to blood pressuremeasurement by using a cuff with a bladder of inappropriatedimensions for the arm for which it is intended has shown thatmiscuffing is a serious source of error, which must inevitablylead to incorrect diagnosis in practice and erroneous

Recommended bladder dimensions. Data reproduced fromO’Brien E, Petrie J, Littler WA et al. Blood PressureMeasurement: Recommendations of the BritishHypertension Society. London: BMJ Books, 1997

Dimensions(cm) Subject

Maximum armcircumference (cm)

4 × 13 Small children 1710 × 18 Medium sized children

Lean adults 2612 × 26 Majority of adult arms 3312 × 40 Obese adults 50

Accurate readings may be obtained in adults with arm circumferences greaterthan 50 cm by placing a cuff with a 40 cm bladder so that the centre of thebladder is over the brachial artery. All dimensions have a tolerance of ±1 cm.

Placement of cuff

Placement of stethoscope

Mismatching of bladder and armx Bladder too small Overestimation of BP

Undercuffingx Bladder too large Underestimation of BP

OvercuffingUndercuffing more common than Overcuffing

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conclusions in hypertension research.3 There is unequivocalevidence that either too narrow or too short a bladder(undercuffing) will cause overestimation of blood pressure, socalled “cuff hypertension,” and there is growing evidence thattoo wide or too long a bladder (overcuffing) may causeunderestimation of blood pressure. Undercuffing has the effectin clinical practice of overdiagnosing hypertension andovercuffing leads to hypertensive subjects being diagnosed asnormotensive. Either eventuality has serious implications forthe epidemiology of hypertension and clinical practice.

A review of the literature shows that a number ofapproaches have been used over the years to cope with thedifficulty of mismatching and none has been ideal. These haveincluded application of correction factors, a range of cuffs,cuffs containing a variety of bladders, and a cuff for themajority of arms.

Blood pressure measurement inspecial subjectsCertain groups of people merit special consideration for bloodpressure measurement, either because of age, body habitus, ordisturbances of blood pressure related to haemodynamicalterations in other parts of the cardiovascular system. Althoughthere is evidence that many subgroups of the hypertensivepopulation may have peculiarities affecting the accuracy ofmeasurement, such as patients with renal disease, patients withdiabetes mellitus, women with pre-eclampsia, and youths with“spurious” hypertension, discussion will be confined to children,the elderly, obese subjects, and pregnant women.

ChildrenBlood pressure measurement in children presents a number ofdifficulties and variability of blood pressure is greater than inadults, and thus any one reading is less likely to represent thetrue blood pressure. Also increased variability confers a greatertendency for regression towards the mean. Conventionalsphygmomanometry is recommended for general use, butsystolic pressure is preferred to diastolic pressure because ofgreater accuracy and reproducibility. Cuff dimensions are mostimportant and three cuffs with bladders measuring 4 × 13 cm,10 × 18 cm, and the adult dimensions 12 × 26 cm are requiredfor the range of arm sizes likely to be encountered in the agerange 0-14 years. The widest cuff practicable should be used.Korotkoff sounds are not reliably audible in all children underone year and in many under five years of age. In such casesconventional sphygmomanometry is impossible and moresensitive methods of detection such as Doppler, ultrasound, oroscillometry must be used.4

Elderly peopleIn epidemiological and interventional studies blood pressurepredicts morbidity and mortality in elderly people as effectivelyas in the young.5 The extent to which blood pressure predictsoutcome may be influenced by various factors that affect theaccuracy of blood pressure measurement and the extent towhich casual blood pressure represents the blood pressure loadon the heart and circulation.6

The elderly are subject to considerable blood pressurevariability, which can lead to a number of circadian bloodpressure patterns that are best identified using ambulatoryblood pressure measurement. The practical clinicalconsequence of these variable patterns in the elderly is thatblood pressure measuring techniques can be inaccurateand/or misleading.

A proposal for the future—the “Adjustable Cuff”On the basis of a thorough examination of the literature and aware ofthe advances in cuff design, the design features for an “AdjustableCuff,” which would be applicable to all adult arms, have beenproposed,4 and one such cuff is presently undergoing testing (ACCosor and Sons Ltd (Surgical), London, UK).

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Mean systolic (top) and diastolic (bottom) blood pressures of boys and girlsfrom birth to 18 years. Diastolic blood pressure reflects the use of phase IVKorotkov sounds. Reproduced with permission from de Swiet M, Dillon MJ,Littler W, O’Brien E, Padfield PL, Petrie JC. Measurement of blood pressurein children. Recommendations of a working party of the BritishHypertension Society. BMJ 1989;299:469-70

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Epidemiological graph for the risk of hypertension in the elderly.Reproduced with permission from Dahloff B, Lindholm LH, Hansson L,Schersten B, Ekbom T, Wester PO. Morbidity and mortality in the SwedishTrial in Old Patients with Hypertension (STOP-Hypertension). Lancet1991;394:405-12

Recommended bladder length

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PseudohypertensionIt has been postulated that as a consequence of the decrease inarterial compliance and arterial stiffening with ageing, indirectsphygmomanometry becomes inaccurate. This has led to theconcept of “pseudohypertension” to describe patients with alarge discrepancy between cuff and direct blood pressuremeasurement.7 The significance of this phenomenon has beendisputed,8 but in elderly patients in whom blood pressuremeasured with the conventional technique seems to be out ofproportion to the clinical findings, referral to a specialistcardiovascular centre for further investigation may beappropriate.Overweight peopleThe association between obesity and hypertension has beenknown since 1923. The link has been confirmed in manyepidemiological studies, and has at least two components.9

Firstly, there appears to be a pathophysiological connection andit may well be that in some cases the two conditions are causallylinked, and secondly, if not taken into account, it may result ininaccurate blood pressure values being obtained by indirectmeasurement techniques.

Obesity may affect the accuracy of blood pressuremeasurement in children, young people, the elderly, andpregnant women.

The relationship of arm circumference and bladderdimensions has been discussed above. If the bladder is too short,blood pressure will be overestimated—”cuff hypertension”—and, iftoo long, blood pressure may be underestimated.3

ArrhythmiasThe difficulty in measuring blood pressure in patients witharrhythmias is that when cardiac rhythm is irregular there is alarge variation in blood pressure from beat to beat. Thus inarrhythmias, such as atrial fibrillation, stroke volume and as aconsequence blood pressure vary, depending on the precedingpulse interval. Secondly, in such circumstances, there is nogenerally accepted method of determining auscultatoryendpoints. Furthermore blood pressure measuring devices varygreatly in their ability to accurately record blood pressure inpatients with atrial fibrillation, indicating that devices should bevalidated independently in patients with arrhythmias.10

In bradyarrhythmias there may be two sources of error.Firstly, if the rhythm is irregular the same problems as withatrial fibrillation will apply. Secondly, when the heart rate isextremely slow, for example 40 beats per minute, it is importantthat the deflation rate used is less than for normal heart rates astoo rapid deflation will lead to underestimation of systolic andoverestimation of diastolic pressure.PregnancyClinically relevant hypertension occurs in more than 10% ofpregnant women in most populations, and in a significantnumber of these raised blood pressure is a key factor in medicaldecision making in the pregnancy. Particular attention must bepaid to blood pressure measurement in pregnancy because ofthe important implications for patient management, as well asthe fact that it presents some special problems.11

There has been much controversy as to whether themuffling or disappearance of sounds should be taken fordiastolic blood pressure. The general consensus fromobstetricians based on careful analysis of the evidence is thatdisappearance of sounds (fifth phase) is the most accuratemeasurement of diastolic pressure, with the proviso that inthose rare instances in which sounds persist to zero the fourthphase of muffling of sounds should be used.12 13

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ECG showingatrial fibrillation

Pressure recordingshowing varyingpressure

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1 O’Brien E, Petrie J, Littler WA, de Swiet M, Padfield PD, Dillon MJ. Blood pressuremeasurement: recommendations of the British Hypertension Society. 3rd ed. London: BMJPublishing Group, 1997.

2 The British Hypertension Society. Blood pressure measurement CD ROM. London:BMJ Books, 1998.

3 O’Brien E. A century of confusion: which bladder for accurate blood pressuremeasurement? J Hum Hypertension 1996;10:565-72.

4 De Swiet M, Dillon MJ, Little W, O’Brien E, Padfield PL, Petrie JC. Measurement ofblood pressure in children. Recommendations of a working party of the BritishHypertension Society. BMJ 1989;299:497.

5 Dahloff B, Lindholm LH, Hansson L, Schersten B, Ekbom T, Wester PO. Morbidityand mortality in the Swedish Trial in Old Patients with Hypertension(STOP-Hypertension). Lancet 1991;394:405-12.

6 O’Brien E, O’Malley K. Blood pressure measurement in the elderly with specialreference to ambulatory blood pressure measurement. In: Leonetti G, Cuspedi C,eds. Hypertension in the elderly. Dordrecht: Kluwer, 1994:13-25.

7 Spence JD, Sibbald WJ, Cape RD. Pseudohypertension in the elderly. Clin Sci MolBiol 1978;55:399s-402s.

8 O’Callaghan WG, Fitzgerald D, O’Malley K, O’Brien E. Accuracy of indirect bloodpressure measurement in the elderly. BMJ 1983;286:1545-6.

9 Staessen J, Fagard R, Amery A. The relationship between body weight and bloodpressure. J Hum Hypertens 1988;2:207-17.

10 Stewart MJ, Gough K, Padfield PL. The accuracy of automated blood pressuremeasuring devices in patients with controlled atrial fibrillation. J Hypertens1995;13:297-300.

11 Shennan AH, Halligan AWF. Measuring blood pressure in normal and hyperten-sive pregnancy. Baillière’s Clinical Obstetrics and Gynaecology 1999;13:1-26.

12 Shennan A, Gupta M, Halligan A, Taylor DJ, de Swiet M. Lack of reproducibility inpregnancy of Korotkoff phase IV as measured by sphygmomanometry. Lancet1996;347:139-42.

13 de Swiet M. K5 rather then K4 for diastolic blood pressure measurement in preg-nancy. Hypertension in Pregnancy 1999;18:iii-v.

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