Sphygmomanometer Calibration Why, How and How Often?

CLINICAL PRACTICE Practice tip Sphygmomanometer calibration Why, how and how often? Martin J Turner BACKGROUND BSc(Eng), MSc(Eng), PhD, is Hypertension is the most commonly managed problem in general practice. Systematic errors in blood pressure Senior Research Asssociate, Departments of Anaesthetics measurements caused by inadequate sphygmomanometer calibration are a common cause of over- and under- and School of Public Health identification of hypertension. Screening and Test Evaluation Programme, University of OBJECTIVE Sydney, New South Wales. This article reviews sphygmomanometer error and makes recommendations regarding in service maintenance and [email protected] calibration of sphygmomanometers. Catherine Speechly DISCUSSION BMedSc, MBBS, FRACGP, is Most sphygmomanometer surveys report high rates of inadequate calibration and other faults, particularly in aneroid Research Officer, Projects, sphygmomanometers. Automatic electronic sphygmomanometers produce systematic errors in some patients. All Research and Development Unit, The Royal Australian sphygmomanometers should be checked and calibrated by an accredited laboratory at least annually. Aneroid College of General sphygmomanometers should be calibrated every 6 months. Only properly validated automatic sphygmomanometers Practitioners, New South should be used. Practices should perform regular in house checks of sphygmomanometers. Good sphygmomanometer Wales. maintenance and traceable sphygmomanometer calibration will contribute to reducing the burden of cardiovascular Noel Bignell disease and the number of patients overtreated for hypertension in Australia. PhD(Physics), is Manager, Mass Quantities and Acoustics, National Measurement Institute, New South Wales. Hypertension is the most commonly managed problem errors is to use the correct measurement technique and in general practice, accounting for 8.6% of encounters well maintained and calibrated instruments. and 7.9% of prescriptions.1 However, just under half the cases in Australia are untreated.1 Frequent consequences Hypertension detection and systematic errors of hypertension are stroke and cardiovascular disease, The detection of hypertension is extremely sensitive to which caused 38% of all deaths in Australia in 2002.2 systematic errors in BP measurements. Figure 1 shows Hypertension in its early stages can be diagnosed only that a consistent 5 mmHg error can more than double by measurement of blood pressure (BP). or halve the number of patients diagnosed with diastolic hypertension. Further analysis of data from the same survey3 All measurements are contaminated by errors that may be allows the effects of any systematic error on the detection divided into two types: of diastolic and systolic hypertension to be estimated.4 • random errors are different on every occasion and can A consistent 5 mmHg error in systolic pressure can be reduced by averaging a number of measurements result in systolic hypertension being underdiagnosed by (random variation caused by biological variability is 30% or overdiagnosed by 43%.4 The current Australian usually indistinguishable from random measurement Sphygmomanometer Standard allows systematic errors up error and is also reduced by averaging), and to approximately ±4 mmHg in new sphygmomanometers.5 • systematic errors, which have approximately the Sphygmomanometers same value on every occasion and are not reduced by averaging. Mercury and aneroid sphygmomanometers Inadequate sphygmomanometer maintenance and Studies of calibration errors of mercury and aneroid calibration is a common cause of systematic error sphygmomanometers in Australia6–8 have been limited in BP measurements. Systematic errors are difficult to and lacking in quality, but do suggest that all is not well. detect and correct. The only way to reduce systematic Several studies indicate that substantial proportions of 834 Reprinted from Australian Family Physician Vol. 36, No. 10, October 2007 Sphygmomanometer calibration – why, how and how often? CLINICAL PRACTICE sphygmomanometers in general practices with every patient before readings are used to several consultations and the potential costs of and hospitals exhibit clinically significant diagnose or manage hypertension.21 the additional visits and the adverse effects of (>3 mmHg) systematic pressure errors and incorrect treatment of a number of patients have Nonautomatic electronic sphygmomanometers other faults.9–12 Some guidelines implicitly to be weighed up against the cost of calibration. assume that mercury sphygmomanometers The anticipated demise of the mercury There may also be medicolegal consequences never require calibration.13 While aneroid sphygmomanometer has prompted the of not calibrating sphygmomanometers at sphygmomanometers fare worse than mercury development of electronic pressure indicators appropriate intervals.22 The calibration interval instruments, many studies have found significant that can be used with manual auscultation also depends on the robustness of the errors in mercury sphygmomanometers.9,10,12 of the Korotkov sounds. These ‘hybrid’ instrument and the conditions under which it Rouse and Marshall14 found that nearly 100 sphygmomanometers are available with is used. If an instrument proves to be stable of 1462 sphygmomanometers were in such segmented displays that mimic the linear after several calibration cycles it is possible to poor condition that their tester suggested and circular scales of mercury and aneroid increase the calibration interval with caution they be withdrawn from ser vice, and manometers. Some versions, which have buttons and due consideration of the risks of erroneous Knight et al10 found that none of the 472 that the operator presses at the systolic and measurements. Conversely, if large calibration sphygmomanometers they tested complied diastolic pressure points, should reduce some errors are found, the interval should be reduced fully with the British Sphygmomanometer operator dependent errors such as terminal digit or the instrument replaced. S t a n d a r d c u rr e n t i n 2 0 01. A n e r o i d preference.21 Recommended test and calibration sphygmomanometers provided as promotional Maintenance and calibration of methods gifts by pharmaceutical companies have been sphygmomanometers shown to be less accurate than others12 and Formal calibration of the pressure indicator should be avoided. All sphygmomanometers sold in Australia • T h e p r e s s u r e i n d i c a t o r s o f a l l are required to comply with the Australian sphygmomanometers should be calibrated Automatic oscillometric sphygmomanometers Standard AS EN 1060 2002 Noninvasive by a laboratory accredited by the National M o s t a u t o m a t i c oscillometric Sphygmomanometers Parts 1, 2 and 35 at the Association of Testing Authorities (NATA) to sphygmomanometers measure cuff pressure time of sale. Although these standards are calibrate pressure gauges or transducers electronically and use proprietary algorithms primarily intended for assessing and licensing over the range 0–40 kPa (0–300 mmHg). to estimate systolic and diastolic pressures new instruments, they do contain limited NATA publishes searchable lists of calibration by analysing the pulsations in cuff pressure as performance and quality clauses against laboratories on its website (www.nata.com. the cuff deflates or inflates. Systematic errors which sphygmomanometers in service can be au). Use the keyword 'pressure' to search can be caused by both lack of calibration of assessed and calibrated. the measurement science and technology the electronic pressure sensing system and field of testing for a laboratory. The least How often should sphygmomanometers be by the algorithm that estimates diastolic and checked and calibrated? uncertainty of measurement included in the systolic pressures. Because the algorithms are scope of each laboratory is the best accuracy confidential and differ between instruments, There are three criteria to consider when selecting that laboratory can offer. Look for a least protocols have been developed to validate a calibration interval: uncertainty of measurement of 0.05 kPa oscillometric sphygmomanometers against • the probability that the sphygmomanometer (0.4 mmHg) or less. manual auscultatory measurements.15,16 The will go out of calibration to a clinically • The laboratory should be requested dabl Educational Trust (www.dableducational. significant extent between calibrations to calibrate the indicator from zero com) assesses each validation report and makes • the consequences of discovering that to the maximum pressure on the recommendations according to the results and a sphygmomanometer has a clinically sphygmomanometer scale at pressure quality of the validations.16 Sphygmomanometers significant calibration error increments not greater than 6 kPa can pass validation tests despite producing • the cost of calibration. (50 mmHg). clinically significant errors that can be If a clinician is notified by a medical testing • Calibration intervals should not be greater greater than 15 mmHg in some individuals.17 laboratory of systematic errors in cholesterol test than those indicated in Table 1. Oscillometric sphygmomanometers perform results, he/she would advise patients to have the Performance and condition poorly in pregnant women,18 diabetics19 and measurement repeated. Similarly, if a clinically in patients with stiff arteries,20 but the causes significant BP error is discovered, the clinician The general condition of sphygmomanometers

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