supplement 2: progress in hypertension control || errors in assessment of blood pressure: blood...

Errors in Assessment of Blood Pressure: Blood Pressure Measuring TechniqueAuthor(s): Norman R.C. Campbell, Donald. W. McKay, Arun Chockalingam and J George FodorSource: Canadian Journal of Public Health / Revue Canadienne de Sante'e Publique, Vol. 85,SUPPLEMENT 2: Progress in Hypertension Control (SEPTEMBER / OCTOBER 1994), pp. S18-S21Published by: Canadian Public Health AssociationStable URL: http://www.jstor.org/stable/41991195 .

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ABSTRACT

This article reviews the recommended techniques for accurate and reproducible blood pressure measurements. The scientific basis underlying current recommendations for blood pressure measurement is presented. Many of the current recommendations are not followed in ambulatory care clinics and this paper will show how measurement errors in excess of 15 mm Hg or more can occur. Many patients will be misclassified and treated inappropriately when errors in blood pressure measurement of this magnitude are made. Rigorous adherence to proper blood pressure measurement is necessary to evaluate a patient's risk of cardiovascular disease, and to assess the need or efficacy of antihypertensive therapy.

ABRÉGÉ

Cet article passe en revue les techniques recommandées pour faire des mesures exactes, que l'on peut reproduire, de la tension artérielle. Les données scientifiques à la base des recommandations actuelles pour mesurer la tension artérielle y sont présen- tées. Bon nombre des recommandations actuelles ne sont pas suivies dans les cliniques de soins ambulatoires et cet article montre comment des erreurs de mesure dépassant de 15 mm Hg ou plus peuvent survenir. De nombreux patients ne sont pas classés comme il faut ni traités adéquatement quand des erreurs de cette importance surviennent. Le respect rigoureux des règles de mesure adéquates de la tension artérielle sont néces- saires pour évaluer les risques de maladie car- diovasculaire encourus par un patient ainsi que pour évaluer la nécessité d'une thérapie antihypertension efficace.

Errors in Assessment of Blood

Pressure: Blood Pressure Measuring

Technique

Norman R.C. Campbell , MD, FRCP(C),1 Donald. W. McKay, PhD,2 Arun Chockalingam, PhD,3 J George Fodor ; MD , FRCP(C), PhD?

The error that can arise when an inap- propriate technique is used to measure blood pressure can often reach 50 mm Hg.1'2 Knowledge and use of currently recommended techniques for measurement of blood pressure by stethoscope and sphygmomanometer can markedly reduce the error. This review presents a summary of recommended blood pressure measuring techniques (Table I, modified from the American Heart Association3) and discuss- es their scientific basis. Recommendations regarding the preparation of patients for blood pressure measurement and recommendations for blood pressure measuring equipment are available elsewhere3"7 and in other articles in this series.

Hearing Ability We are aware of no studies examining

the effect of hearing deficits on the ability to measure blood pressure. However, low frequency hearing loss will very likely result in consistently lower systolic and higher diastolic readings because of the low frequency of KorotkofF sounds.8 The magnitude of the errors in measurement will depend on the extent of the hearing loss.

1. Divisions of Internal Medicine and Geriatrics, Department of Medicine, Department of Pharmacology and Therapeutics, Faculty of Medicine, The University of Calgary, Calgary, Alberta, Canada.

2. Division of Basic Medical Sciences, Faculty of Medicine, Memorial University of Newfoundland, St John's, Newfoundland, Canada

3. Division of Community Medicine, Faculty of Medicine, Memorial University of Newfoundland, St. John's, Newfoundland, Canada

Dr. N. Campbell is supported by the Brenda Strafford Foundation.

Correspondence and reprint requests: Dr D.W. McKay, Faculty of Medicine, Memorial University of Newfoundland, St. John's, Newfoundland, Canada A1B3V6. 709-737-6587.

Persons with impaired hearing, including that caused by aging, who are assessing blood pressure should be tested to determine the extent of low frequency hearing loss and their ability to accurately assess blood pressure.

TABLE I Blood Pressure

Measurement Techniques • Support patient's arm with the antecubital

fossa at heart level. • Use an appropriate sized blood pressure cuff. • Check blood pressure initially by palpation

prior to auscultation. • Deflate the blood pressure cuff by 2 mm Hg/

cardiac cycle. • Use the first and last Korotkoff sounds to determine systolic and diastolic blood pressure, respectively. • Allow at least 30 seconds between blood pressure readings. • Measure blood pressure in both arms at the first visit, and if one arm has higher blood pressure than the other, use the arm with the higher blood pressure for subsequent readings. • Measure blood pressure with the patient in the lying or sitting and standing positions. • Take an average of multiple readings with the patient in the sitting or lying position to assess blood pressure at a given time.

• Assess blood pressure on at least three occa- sions over three to six months in patients with mild elevations in blood pressure.

Digit Preference Many physicians record blood pressure

as a multiple of 5 or 10 mm Hg.9,10 The digit preference occurs even in physicians who are aware that digit preference is a problem (Campbell and McKay, unpublished data). This can result in the misclassification of many patients and has resulted in the recommendation to record blood pressure to the closest 2 mm Hg.3

Cuflf size selection This topic is discussed in the next article

of this series.

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BLOOD PRESSURE MEASURING TECHNIQUE

Patient Arm Position The effect of arm position on blood

pressure measurement has been extensively studied11"14 using auscultatory methods. Blood pressure changes by approximately 8 mm Hg for every 10 cm that the antecubital fossa is above or below heart level.3,1 1,13,14 When the arm is dependent the average increase is approximately 22/12 mm Hg, and when the arm is held vertical- ly the average decrease is about 20/ 1 5 mm Hg.13 The alterations in blood pressure are predominantly due to changes in hydrosta- tic pressure, and even small changes in arm position can affect readings considerably.14

Patient Arm Support Isometric muscular activity increases

peripheral vascular resistance and blood pressure.15,16 However, the changes in blood pressure that occur due to isometric exertion when an arm is not supported during blood pressure measurement have not been studied extensively. Diastolic and systolic blood pressure increased on average 4 mm Hg during 100 observations when the arm was not supported.17,18 Increases in blood pressure of 14 mm Hg systolic and 1 8 mm Hg diastolic have been found to occur in some subjects when their arm is not supported (Campbell and McKay, unpublished observations). Hypertensive patients and those being treated with beta-blockers have an exaggerated response to isometric exercise15,16 and therefore may have exaggerated increases in blood pressure when their arm is unsup- ported. It is important that the patient be in a relaxed position when blood pressure is measured, as isometric exertion in the contralateral arm17 or being seated without back support can affect blood pressure.

Cuff Inflation Inflation of the blood pressure cuff

alone can cause blood pressure to rise in some patients;18,19 the increases in blood pressure in the cases described were con- siderable, being up to 40/36 mm Hg in one case. This problem has not been reported by all investigators20 and can only be diagnosed with the aid of an intra-arterial monitor. It could be suspected in patients with long-standing high blood pressure that is resistant to treat-

ment and with little evidence of hypertensive damage on examination, or in patients in whom antihypertensive therapy causes symptoms of hypotension despite blood pressure readings in the office that remain high. The problem of cuff inflation hypertension is thought to be uncommon.

Palpation of Systolic Blood Pressure to Avoid Patient Discomfort and Auscultatory Gaps

The data supporting the recommendation to palpate systolic blood pressure before auscultation have been reviewed previously1 and are supported by more recent studies.21 Failure to recognize auscultatory gaps can lead to errors of up to 55/80 mm Hg.22 On average, palpation underestimates systolic blood pressure by approximately 10 mm Hg.1 Recommendations for auscultation to inflate the cuff to pressures 30 mm Hg above that palpated avoids a systolic auscultatory gap. Korotkoff sounds decrease in intensity and become inaudible in about 5% of hypertensive patients during phase II, creating an auscultatory gap.1,21" 23 Failure to identify systolic blood pressure by palpation could result in the erro- neous recording of systolic blood pressure as the re-initiation of phase II sounds21"23 and has resulted in underestimation of systolic blood pressure of up to 55 mm Hg.1,22 The reappearance of Korotkoff sounds occurs 10 to 50 mm Hg below the initial disappearance of sound and can also lead to overestimation of diastolic blood pressure of up to 80 mm Hg if the disappearance of sound during phase II is mistaken for diastolic blood pressure.22 Korotkoff sounds may not return during an auscultatory gap, and this uncommon phenomenon can lead to overestimation of diastolic blood pressure. Currently there is no easy way to resolve this latter problem in the clinic.21 The current recommendation to continue auscultation for 1 0 mm Hg below the last sound heard would not identify the true diastolic blood pressure of most patients who have an auscultatory gap. Auscultation should be continued until the cuff pressure is in the normotensive range or if sounds continue to zero.24

Pseudohypertension and Osier's Manoeuvre

Pseudohypertension is suggested to be common in the elderly,25,26 but due to the referral bias in most studies the true inci- dence is unknown. Pseudohypertension occurs when the brachial artery is rigid due to atherosclerosis and withstands cuff pressures above the true systolic blood pressure.25 A bedside test (Osier's manoeuvre) for detecting pseudohypertension has been described.25 Ordinarily, distal arteries will collapse and not be palpable when the cuff is inflated above systolic pressure. Palpable arteries, after the extinction of the pulse, determine a positive Osier's manoeuvre and are suggestive of the presence of pseudohypertension. Under these circum- stances indirect blood pressure measurement typically overestimates blood pressure by 15 mm Hg, but the error can be much greater.25 Unfortunately, testing with Osier's manoeuvre has both a high false- positive and false-negative rate;25,27 oscillo- metric blood pressure measurement techniques may help in these cases.28 Pseudohypertension can be suspected either in patients who have had a long- standing history of high blood pressure readings yet have little evidence of hypertensive damage upon examination, or in those on hypertensive therapy with symptoms of hypotension despite normal or ele- vated blood pressure readings.

Cuff Deflation Rate The rate of cuff deflation influences the

accuracy of the blood pressure reading. Slow deflation from high cuff pressures can result in discomfort for the patient and this may explain the small increase in diastolic blood pressure that is observed.29 Rapid deflation will result in substantial drops in cuff pressure between cardiac cycles and therefore an underestimate of systolic and an overestimate of diastolic pressure. The recommended deflation rate of 2 mm Hg per cardiac cycle (approximately 2 to 3 mm Hg per second) is recommended to minimize these errors.3,5,6 Deflation rates of 10 mm Hg/second are not uncommon in clinical practice;30 thus, in a patient whose heart rate is 60 beats per minute, cuff pressure could drop by 10 mm Hg per cardiac cycle. The maximum underestimation of

SEPTEMBER- OCTOBER 1994 CANADIAN JOURNAL OF PUBLIC HEALTH - SUPPLEMENT 2 S19




systolic and overestimation of diastolic blood pressure determination in this set- ting is 10 mm Hg. The errors in blood pressure readings are likely increased with slower pulse rates and faster rates of cuff deflation.

Venous Congestion During Repeated Measurement

Recommendations suggest waiting at least 30 seconds between readings, ostensi- bly to prevent venous congestion;3 however, we are aware of no data to support this recommendation. Venous pressure is lower than arteriole pressure, and venous congestion can occur during deflation and inflation of the blood pressure cuff as well as when the patient wears constrictive cloth- ing. It has been suggested that venous congestion causes attenuation of Korotkoff sounds and results in errors in blood pressure measurement.3 Rapid cuff inflation rates and waiting between readings can help to avoid this problem. Waiting extra time between readings also helps one to detect the occurrence of the alerting reaction.2

Bilateral Measurement Recommendations call for bilateral

blood pressure measurements on initial readings, with subsequent readings from the arm with the higher blood pressure.3"7 In unselected patients, simultaneous bilateral direct arterial tracings show consistent differences of more than 10 mm Hg in systolic blood pressure in 6% of patients and in diastolic blood pressure in 1% of patients.31 Studies using a stethoscope and sphygmomanometer with simultaneous blood pressure readings on both arms have found that consistent differences in blood pressure between the arms are slightly more common than this.31,32 Early studies that examined differences between arms did not use simultaneous blood pressure readings and found that over 50% of patients had differences of more than 10 mm Hg in blood pressure between arms.31 The differences in blood pressure between the arms in these later studies, however, were not consistent over time, and these results emphasize the lability of blood pressure. In patients with peripheral vascular disease33 or pre-left-subclavian coarctation

of the aorta, large bilateral differences in blood pressure may be more common. The arm with the higher blood pressure is more likely to reflect systemic arterial blood pressures, and its use is recommended for the assessment of blood pressure.3"6

Korotkoff Sounds and Diastolic Blood Pressure

Phase V (the pressure at which Korotkoff sounds cease) is a closer measure of diastolic blood pressure than phase IV (the muffling of sound) under most cir- cumstances. Phase V was used to determine diastolic blood pressure in the studies on mild hypertension that have demonstrated reductions in cardiovascular mor- bidity and mortality. Therefore, the use of phase V is strongly recommended to determine diastolic blood pressure in order to initiate and alter therapy.3 Phase IV sounds are approximately 5 to 10 mm Hg higher on average than phase V sounds3'34 and will overestimate the recommended levels at which to treat diastolic blood pressure.The determination of diastolic blood pressure using phase V is inaccurate, however, in young children, patients in whom sounds can be heard without the application of pressure, patients in whom there is a wide discrepancy between phase IV and phase V sounds, and patients with very low peripheral vascular resistance. For these patients the use of phase IV alone will likely provide a better estimate of diastolic pressure, but both Phase IV and Phase V should be recorded.3'34

Patient Positions for Blood Pressure Measurement

Several studies have demonstrated 2 to 4 mm Hg changes in blood pressure between the sitting and lying position.35,36 These differences may be due to alterations in arm position and support or possibly to the changes in patient position.5,35,36 However, certain patients (e.g., pregnant women and hypovolemic patients) can have larger differences in blood pressure from the lying to the sitting position. Particulary in pregnancy the supine position for blood pressure measurement is to be avoided, as significantly reduced blood pressure readings occur when the gravid uterus impinges on the inferior vena cava

and reduces venous return to the heart. Pregnant women should be in the seated position or lying tilted slightly (15-30°) to their left side for blood pressure measurement.37 Most therapeutic trials in hypertension have used blood pressure readings taken with the patient in the sitting position,38"42 and we recommend this position. As postural hypotension is a common adverse effect of antihypertensive medica- tions and can be a sign of secondary hypertension, however, it is also recommended that patients' blood pressure be measured immediately upon standing and again after two minutes.

Repeated Blood Pressure Measurement on One Visit

Phasic or cyclic changes in blood pressure occur in all people, even under basal conditions;21 differences of 10 mm Hg can occur between repeat readings.2,36 These phasic changes are more likely to be missed when cuff deflation rates are rapid.21 Phasic changes in systolic blood pressure occur in about 20% of patients, and phasic changes in diastolic blood pressure occur in about 10% of patients. The use of an average of two or preferably three blood pressure readings taken under identical conditions (i.e., in the same arm, in the same position and by the same observer) should provide a better estimate of blood pressure.36 This recommendation is disputed by the find- ings of a single study that suggested that a single rigorously performed blood pressure measurement produced equivalent results to triplicate measurements.43 The results of the latter study have not been confirmed.36

Repeated Blood Pressure Measurement over Several Visits

Blood pressure readings on a single occa- sion often are not representative of the usual blood pressure.44'45 Therefore, in patients with high-normal or mildly elevat- ed blood pressure, it is recommended that three or more blood pressure readings be taken over a period of three to six months before diagnosis or treatment of hypertension.

Other Recommended Techniques It is recommended that the patient's arm

be pronated for measurement, that the

S20 REVUE CANADIENNE DE SANTÉ PUBLIQUE VOLUME 85, SUPPLÉMENT 2




lower edge of the blood pressure cuff be placed 3 cm above the antecubital fossa to allow room for auscultation, that the bell of the stethoscope exert light pressure and that the readout of the sphygmomanometer be at eye level and clearly visible. In our experience, few physicians use the bell of the stethoscope because of the difficulties involved in holding the bell over the brachial artery while supporting the arm and manipulating the deflation rate of the cuff. Phase I and Phase IV Korotkoff sounds can be heard 1 to 2 mm Hg earlier using the bell over the brachial artery rather than the stethescope diaphragm on the antecubital fossa.46

CONCLUSIONS

Non-standard technique can be performed consistently or inconsistently. Inconsistent deviation from recommended blood pressure measuring techniques results in fluctuation in blood pressure readings, and confusion regarding the level of basal blood pressure and the effective- ness of any instituted therapy. If variation in a technique is performed consistently this will result in a systematic (consistent) error that could affect the diagnosis of many patients and that will affect the management of all patients classified as hypertensive.

Variations in blood pressure measuring technique can result in 1 5 mm Hg or more changes from basal blood pressure. The errors that result in higher than basal blood pressure can lead to the misclassification of many patients as hypertensive. Variation in several of the techniques that produce lower than basal blood pressure readings leads to underestimation of cardiovascular risk and the withholding of indicated therapy.

ACKNOWLEDGEMENTS

We thank Dr. P. Magner for reviewing these manuscripts and Ms. Heather Arcari for her expert secretarial assistance.

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SEPTEMBER - OCTOBER 1994 CANADIAN JOURNAL OF PUBLIC HEALTH - SUPPLEMENT 2 S21



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