Download - SUPPLEMENT 2: Progress in Hypertension Control || Errors in Assessment of Blood Pressure: Sphygmomanometers and Blood Pressure Cuffs

Errors in Assessment of Blood Pressure: Sphygmomanometers and Blood Pressure CuffsAuthor(s): Norman R.C. Campbell, Donald. W. McKay, Arun Chockalingam and J. GeorgeFodorSource: Canadian Journal of Public Health / Revue Canadienne de Sante'e Publique, Vol. 85,SUPPLEMENT 2: Progress in Hypertension Control (SEPTEMBER / OCTOBER 1994), pp. S22-S25Published by: Canadian Public Health AssociationStable URL: http://www.jstor.org/stable/41991196 .

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Errors in Assessment of Blood

Pressure: Sphygmomanometers and

Blood Pressure Cuffs

Norman R.C. Campbell, MD, FRCP(C),1 Donald. W. McKay, PhD,2 Arun Chockalingam, PhD,3 J. George Fodor, MD, FRCP(C), PhD3

The equipment used to measure blood pressure is as critical as patient preparation and proper technique in obtaining accurate blood pressure readings.1"10 Many errors introduced by the sphygmomanometer and blood pressure cuff tend to be consistent and affect the readings of all patients each time blood pressure is evaluated with that equipment. A consistent error of 10 mm Hg could result in errors in the management of 40% of adult patients evaluated.6 With some types of sphygmomanometers, errors of this magnitude are not uncommon.5'10

The most common sphygmomanometers used by physicians are the mercury and aneroid sphygmomanometers.6,7 Electronic devices are generally used by patients for home monitoring, but are occasionally used in physicians' offices.6,11,12 This article reviews the basis of current recommendations for the maintenance of standard blood pressure measuring equipment.

Blood Pressure Cuffs Assessment of blood pressure is the most

common diagnostic procedure performed in the outpatient clinic.13 Most physicians have only a mid-sized adult cuff (12.5 cm

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. Author to whom all correspondence should be addressed: Dr D.W. McKay, Faculty of Medicine, Memorial University of Newfoundland, St. John's, Newfoundland, Canada A1B 3V6. 709-737-6587.

wide bladder) available for use in blood pressure measurement.6 Even the small percentage of physicians who own a variety of cuff sizes seldom use alternative sizes, despite the large range in arm circumference seen in their clinics.6

Large cuffs are not only important for the measurement of blood pressure in obese patients, but when used within an acceptable range of arm circumference they are also recommended for routine use in the place of the common mid-sized cuff.1"4 An overestimate of diastolic blood pressure by 6 mm Hg occurs when a mid-sized cuff is used on a person ideally suited for a large-sized cuff, whereas use of a large cuff on an individual ideally suited for a mid- sized cuff causes an underestimate of 3 mm Hg.2 Table I indicates the error in diastolic blood pressure that occurs when blood pressure cuffs of different widths are used on arms of different circumferences. Since the use of a large cuff on a thin arm will underestimate blood pressure, it is necessary to have smaller cuff sizes available for younger patients and others with thin arms.2 Wide cuffs will also underestimate systolic blood pressure in thin patients.15 The Systolic Hypertension in the Elderly Program (SHEP) study, which demon- strated the benefit of treating isolated systolic blood hypertension, used cuff sizes based on arm size.16 Therefore it may be best to use a cuff width selected to the patient rather than a single 'standard' cuff. The use of blood pressure cuffs with blad- ders that encircle the arm has been advo- cated in Britain17'18 and will also provide accurate results,19 but these cuffs are not widely available in Canada. Recently a cuff with a bladder that has variable expansion widths has been developed and may be beneficial if used properly.20

S22 REVUE CANADIENNE DE SANTÉ PUBLIQUE VOLUME 85, SUPPLÉMENT 2

ABSTRACT

This article reviews the current recommendations on equipment when blood pressure is measured by sphygmomanometer. The scientific rationale underlying the current recommendations for selection and maintenance of blood pressure measuring equipment is presented. The errors that can occur when the recommendations are not followed are quantified whenever the data are available. Inadequate assessment and maintenance of equipment often lead to the use of faulty equipment, and as a result errors in the assessment of patients' blood pressure are likely to be common. If followed, the current guidelines for use and maintenance of equipment would remove most of the problems noted. Physicians must ensure that properly maintained and appropriate equipment is used to measure blood pressure.

ABRÉGÉ

Cet article passe en revue les recommandations actuelles concernant le matériel quand la tension artérielle est mesurée par un sphyg- momanomètre. Les raisons scientifiques à la base des recommandations actuelles pour sélectionner et entretenir l'équipement de mesure de la tension artérielle y sont présen- tées. Lorsque les données sont disponibles, on y précise l'importance des erreurs qui peu- vent survenir quand les recommandations ne sont pas suivies. Lorsque l'évaluation et l'entretien de l'équipement ne sont pas adéquats, on se sert d'équipement défectueux, ce qui conduit à des erreurs fréquentes dans l'évaluation de la tension artérielle des patients. Si l'on suit les direc- tives actuelles d'utilisation et d'entretien de l'équipement, la plupart des problèmes cessent d'exister. Les médecins doivent s'assurer que l'on se serve d'un équipement adéquat et convenablement entretenu pour mesurer la pression sanguine.



SPHYGMOMANOMETERS AND BLOOD PRESSURE CUFFS

Many cuffs are marked by the manufacturer to indicate the acceptable range of arm circumference. Unfortunately, these markings may not agree with the current recommended range and need to be checked and possibly remarked on each individual cuff.2 Marking the correct size range on the cuff helps with proper cuff selection and can be performed easily using a ruler and permanent marker.4 The ideal arm circumference for a cuff is 2.5 times the cuff s bladder width. Cuffs can be used on arms that have a circumference ±4 cm of 'ideal'. When it is necessary to remark the cuff, start the measurement at the end that contains the bladder. Permanently mark the cuff at the ideal arm circumference, and then mark a line across the cuff at 4 cm on either side of the ideal circumference. The error introduced by using a cuff within the acceptable range is <3 mm Hg for a mid sized cuff and <2 mm Hg for a large sized cuff (Table I). The errors associated with the use of a cuff of the wrong size can be substantial (Table I). It is recommended that physicians who assess blood pressure have an appropriate selection of cuff sizes available for their patient population. Patients being evaluated for the presence of hypertension need to have the appropriate cuff size determined.

Cuffs and tubing that lose pressure at rates faster than 1 mm Hg per second warrant repair or replacement.2'3 Similarly, cuff repair or replacement is needed when the fastening mechanism slips or when the bladder bulges out of the cuff. Cuffs with these defects will not effectively transmit pressure to the arm, resulting in an overestimation of blood pressure.2'3

Sphygmomanometers Most mercury sphygmomanometers are

accurate when tested against a calibrated standard, as long as the meniscus of the mercury is at zero when the cuff is deflated and the mercury column is in the intended position (in most cases vertical).2'3 Studies have shown that the majority of these devices are within 4 mm Hg of zero despite many years of service.6'7*10 The device is not accurate when the mercury meniscus is above or below the zero mark when the cuff is fully deflated.2'3 This error can be correct- ed by the addition or removal of mercury.

Table I Errors in Diastolic Blood Pressure Readings (mm Hg) Associated with the Use

of a Cuff with Different Bladder Sizes in Arms of Various Mid-arm Circumference (Modified from Fröhlich and colleagues14)

Mid-arm Circumference (cm) Cuff Size (Bladder Width) Medium Large Thigh (12 cm) (15 cm) (18 cm)

26 -3 -5 -5 28 -2 -4 -5 30 0 -3 -4 32 1 -2 -4 34 3 -1 -3 36 4 -1 -3 38 6 0 -2 40 7 1 -1 42 9 2 -1 44 10 3 0 46 11 3 0 48 13 4 1 50 14 5 1

Notwithstanding the robustness of the mercury sphygmomanometers when com- pared with a calibrated standard, these devices still require routine maintenance. A sluggish response or bouncing of the mercury column during inflation and deflation usually indicates a blocked vent.2'3,21 Blocked vents cause differences between the pressure in the cuff and those indicated by the sphygmomanometer and result in overestimation of blood pressure.21 The magnitude of error depends on the extent of vent blockage and the rate of cuff deflation. At the recommended rate of deflation, approximately 30% of devices had errors of 4 mm Hg or more. At a rate of deflation of 10 mm Hg/second, 65% of the mercury sphygmomanometers have been found to have errors of 4 mm Hg or more, and 6% were off by 10 mm Hg.21 The errors caused by the blocked vent are not readily apparent and underscore the need for annual maintenance.21 Additionally, mercury oxidation occurs and obscures visibility of the column and meniscus. The mercury and column can be cleaned easily, but precautions must be taken to avoid mercury contami- nation and poisoning. Always wear a face mask and gloves and work on a tray that can contain mercury spillage. Never clean a mercury spill using a vacuum cleaner. Always dispose of contaminated cleaning materials according to local regulations. Mercury poisoning has occurred because adequate precautions were not taken in the cleaning of mercury sphygmomanometers.

Aneroid sphygmomanometers can lose accuracy with time, and in some cases may not be accurate even when purchased.6'7'9'10 In several studies, 30 to 40% of aneroid sphygmomanometers were out of calibration by at least 4 mm Hg, and more than 10% were out by over 10 mm Hg.6,7'9'10 If an aneroid sphygmomanometer does not zero at atmospheric pressure, then it is not properly calibrated. Unfortunately, even if it does zero this does not necessarily mean that it is accurate,6,7 and it must be checked for accuracy throughout the range of the aneroid device.

Assessment of calibration is quick and easy to perform by means of a simple connection between the aneroid device and a well-maintained mercury sphygmomanometer.6 With some aneroid devices, use of a 'Y' or 'T'-tube connec- tor may be necessary. A check of calibration is necessary any time the device is dropped or damaged in addition to regu- lar testing at six month intervals. 2*4'719 Unlike mercury sphygmomanometers, the calibration errors of aneroid devices are not necessarily consistent throughout the range of the device; thus it is appropriate to check calibration every 20 mm Hg throughout its complete range. Aneroid sphygmomanometers are not recommended for hospital use or for use by physicians who do not have a mercury sphygmomanometer that can be used for calibration checks.7 Inaccurate aneroid sphygmomanometers warrant replacement or return to the manufacturer for repair.

SEPTEMBER - OCTOBER 1994 CANADIAN JOURNAL OF PUBLIC HEALTH - SUPPLEMENT 2 S23




Electronic devices are increasingly popular with patients, but fortunately are rarely used by physicians.2'6'11 Like aneroid sphygmomanometers, these devices can lose accuracy,2'11'22 and routine' checks of calibration can be complicated by auto- matic inflation/deflation mechanisms.23 Accurate testing and calibration can be done by the manufacturer or by trained technicians. Furthermore, on equipment using microphones, placement relative to the brachial artery is critical. We and others have found the placement of microphones especially problematic when used on obese patients.11'20'22'24 Small changes in microphone placement over the brachial artery can produce substantial changes in pressure.9'19,22 In our experience, 6 to 8 mm Hg errors are common. We do not recom- mend these devices for routine office use.

Ambulatory Blood Pressure Monitoring The noninvasive monitoring of blood

pressure over 24 hours is becoming more popular.25 Ambulatory blood pressure measurements taken under routine daily living circumstances are reproducible and are a better reflection of some of the conse- quences of hypertension than a measurement at a single clinic visit.26"31 Furthermore, ambulatory measurements can be used in the prediction of cardiovascular morbidity.32 On average, the 24-hour diastolic blood pressure is 5 to 10 mm Hg below clinic measurement,26 and over 30% of patients classified as hypertensive on a single clinic visit will have normal 24-hour blood pressure readings.28 However, ambulatory monitors can be annoying and uncomfortable for the patient and expen- sive, and erroneous data may be record- ed.24'33 These devices, if microphone- equipped, require meticulous placement and assessment of accuracy on every patient. Rarely, complications from noninvasive ambulatory monitoring can occur with trauma to neurovascular structures if the cuff fails to deflate or if it repeatedly inflates and the patient does not remove it.33 The devices are very useful, however, in delineating "white coat" hypertension, paroxysmal hyper- and hypotension, loss of diurnal variation of blood pressure, and the effectiveness of antihypertensive therapy over 24 hours.34 Ambulatory measurements

can influence the decision to initiate antihypertensive therapy in certain patients.28,29,35 Although 24-hour blood pressure monitors have great promise for the future, they currendy share many of the accuracy and calibration problems of home electronic devices, and their role at present is still probably in the realm of the hypertension specialist or clinical investigator.

CONCLUSIONS

The use of defective and deficient equipment to assess blood pressure is common- place and likely leads to frequent and sig- nificant errors. Guidelines for the use and maintenance of sphygmomanometers have been established2 and if followed would resolve most of the problems noted. Care must be exercised in the use of new technology for blood pressure measurement as many devices require maintenance and calibration checks beyond the immediate resources of most physicians and patients. It is the responsibility of each physician assessing blood pressure to ensure that properly maintained and appropriate equipment is always used.

ACKNOWLEDGEMENTS

We thank Dr. P. Magner for reviewing this manuscript and Ms. Heather Arcari and Ms. Shirley Atkins for their expert sec- retarial assistance.

REFERENCES 1. American Society of Hypertension.

Recommendations for routine blood pressure measurement by indirect cuff syphygmomanome- try. Am J Hypertens 1992;5:207-9.

2. Perloff D, Grim C, Flack J, et al. Human blood pressure determination by sphygmomanometry. Circulation 1993; 88:2460-70.

3. Petrie JC, O'Brien ET, Littler WA, De Swiet M. Recommendations on blood pressure measurement. BMJ 1986; 293:611-15.

4. Campbell NRC, Chockalingam A, Fodor JG, McKay DW. Accurate reproducible measurement of blood pressure. Can Med Assoc J 1990;143:19-24.

5. O'Brien ET, O'Malley K. ABC of blood pressure measurement: The sphygmomanometer. BMJ 1979;2:851-3.

6. McKay DW, Campbell NRC, Parab LS, et al. Clinical assessment of blood pressure. J Hum Hypertens 1990;4:639-45.

7. Burke MJ, Towers HM, O'Malley K, et al. Sphygmomanometers in hospital and family practice: Problems and recommendations. BMJ 1982;285:469-71.

8. Conceicao S, Ward WK, Kerr DNS. Defects in sphygmomanometers: An important source of error in blood pressure recording. BMJ 1976;1:886-8.

9. Bowman CE. Blood pressure errors with aneroid sphygmomanometers (letter). Lancet 1981;1:1005.

10. Perlman LV, Chiang BN, Keller J, Blackburn H. Accuracy of sphygmomanometers in hospital practice. Arch Intern Med 1970;125:1000-3.

11. Canadian Coalition for High Blood Pressure Prevention and Control. Recommendations on self-measurement of blood pressure. Can Med Assoc J 1988;138:1093-6.

12. Hunt JC, Fröhlich ED, Moser M, et al. Devices used for self-measurement of blood pressure. Revised statement of the National High Blood Pressure Education Program. Arch Intern Med 1985;145:2231-4.

13. National Center for Health Statistics, McLemore T. & Delozier J. 1985 Summary: National ambulatory medical care survey. Advance data from Vital and Health Statistics, No. 128. DHHS Pub. No. (PHS) 87-1250. Hyattsville, MD: Public Health Service, Jan. 23, 1987.

14. Fröhlich ED, Grim C, Labarthe DR, et al. Recommendations for human blood pressure determination by sphygmomanometers. Report of a special task force appointed by the Steering Committee, American Heart Association. Circulation 1988; 77: 502A-514A.

15. Russell AE, Wing LMH, Smith SA, et al. Optimal size of cuff bladder for indirect measurement of arterial pressure in adults. J Hypertens 1989;7:607-13.

16. Labarthe DR, Blaufox MD, Smith WM, et al. Systolic Hypertension in the Elderly Program (SHEP) Part 5. Baseline blood pressure and pulse rate measurements. Hypertension 1991;17:1162-1176.

17. O'Brien ET, O Malley K. ABC of blood pressure measurement: Reconciling the controversies: A comment on "the literature". BMJ 1979;2:1201-2.

18. O'Brien ET, O Malley K. ABC of blood pressure measurement: The sphygmomanometer. BMJ 1979;2:851-3.

19. Van Montfrans GA, Van Der Hoeven GMA, Karemaker JM, et al. Accuracy of auscultatory blood pressure measurement with a long cuff. BMJ 1987;295:354-5.

20. Stolt M, Sjonell G, Astrom H, Hansson L. The reli- ability of auscultatory measurement of arterial blood pressure. A comparison of the standard and a new methodology. Am J Hypertens 1990;3:697-703.

21. Shaw A, Deehan C, Lenihan JMA. Sphygmomanometers: Errors due to blocked vents. BMJ 1979;1:789-90.

22. Carroll KK, Latman NS. Evaluation of electronic, digital blood pressure monitors: Failure rates and accuracy. Medlnstrum 1984;18:263-6.

23. Nash ČA. How do you test a digital sphygmomanometer? Am J Nursing 1992;92:66-70.

24. Chockalingam A, Fodor JG, Drover A, Campbell N. Practical experience with 24 hour ambulatory BP monitoring in outpatient setting (abstract). Clin Invest Med 1988; 1 1(4):C62.

25. Reeves RA, Myers MG. Introduction: Ambulatory blood pressure monitoring - an emerging technology. Clin Invest Med 1991;14:198-201.

26. James GD, Pickering TG, Yee LS, et al. The reproducibility of average ambulatory, home, and clinic pressures. Hypertension 1 988; 1 1:545-9.

27. Van Egeren LF. Repeated measurements of ambulatory blood pressure. J Hypertens 1988;6:753-5.

28. Weber MA. Automated blood pressure monitoring: A new dimension in diagnosis. Mayo Clin Proc 1988;63: 1151-3.

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29. Weber MA. Evaluating the diagnosis and progno- sis of hypertension by automated blood pressure monitoring: Outline of a symposium. Am Heart J 1988;116:1118-23.

30. Weber MA. Whole-day blood pressure. Hypertension 1988;11:288-98.

31. White WB. Ambulatory blood pressure and tar- get organ involvement in hypertension. Clin

Invest Med 1 99 1 ; 1 4:224-30. 32. Perloff D, Sokolow M, Cowan R. The prognostic

value of ambulatory blood pressure monitoring in treated hypertensive patients. / Hypertens (Suppl.) 1 99 1 ;9( 1 ) :S33-S39.

33. Bottini PB, Rhoades RB, Carr AA, Prisant LM. Mechanical trauma and acute neuralgia associated with automated ambulatory blood pressure

monitoring (letter). Am J Hypertens 1991;4:288. 34. Pickering TG. (editor). Ambulatory Monitoring

and Blood Pressure Variability. Science Press, London. 1991.

35. Krakoff LR, Eison H, Phillips RH, et al. Effect of ambulatory blood pressure monitoring on the diagnosis and cost of treatment for mild hypertension. Am Heart /1988;116:11 52-4.

SEPTEMBER - OCTOBER 1994 CANADIAN JOURNAL OF PUBLIC HEALTH - SUPPLEMENT 2 S25

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