Can aneroid sphygmomanometers be used at altitude? - Nature

Journal of Human Hypertension (2006) 20, 517–522 & 2006 Nature Publishing Group All rights reserved 0950-9240/06 $30.00 www.nature.com/jhh

ORIGINAL ARTICLE

Can aneroid sphygmomanometers be used at altitude? NA Kametas1, F McAuliffe1, E Krampl1, KH Nicolaides1 and AH Shennan2 1

Harris Birthright Research Centre for Fetal Medicine, King’s College Hospital, Denmark Hill, London, UK and Maternal and Fetal Research Unit, Department of Obstetrics and Gynaecology, St Thomas’ Hospital, London, UK 2

Mercury-independent devices are increasingly being used in clinical practice as mercury will soon be removed from clinical use as a result of environmental, health and safety concerns. The aim of this study was to evaluate the accuracy of a portable aneroid device in an adult population at high altitude by following the part of the protocol of the British Hypertension Society regarding comparison between device and observer. We examined 10 subjects in Cerro de Pasco, Peru, which is situated 4370 m above sea level. The aneroid device was initially calibrated at both high altitude and at sea level to ensure optimal function. Validation of the device was undertaken at high altitude by connecting it in parallel to two mercury sphygmomanometers. Eleven sequential same-arm measurements were taken from each subject by two trained observers, alternating between mercury sphygmomanometry and the aneroid

device. Simultaneous mercury readings were also recorded for additional analysis. During calibration, all 60 comparisons between the aneroid and mercury sphygmomanometers were within 3 mm Hg both at sea level and at high altitude. At validation, the device achieved an A grade for both systolic and diastolic pressures and also fulfilled the requirements of the Association for the Advancement of Medical Instrumentation. The mean and standard deviation for systolic and diastolic pressures, respectively, were 1.32 (4.3) mm Hg and 3.7 (4.7) mm Hg in sequential analysis and 0.7 (2.6) mm Hg and 3.3 (2.7) mm Hg in simultaneous analysis. We conclude that the Riester-Exacta portable aneroid device can be recommended for use in an adult population at high altitude. Journal of Human Hypertension (2006) 20, 517–522. doi:10.1038/sj.jhh.1001998; published online 13 April 2006

Keywords: mercury sphygmomanometers; aneroid sphygmomanometers; blood pressure; high altitude

Introduction High altitude, defined as altitude of more than 2500 m, is home to about 140 million people throughout the world,1 and is visited yearly by more than 34 million people.2 Furthermore, air travel exposes even more people to simulated moderate altitudes, as airplane cabins are routinely maintained at pressures equivalent to an altitude of about 2500 m. With increasing altitude, there is a reduction in barometric pressure with a concomitant drop in partial pressure of oxygen, posing significant strain to the human homeostasis. Hence, travellers at high altitude can be severely affected by highaltitude illness and concurrent pulmonary oedema.3 With a significant part of the population residing or sojourning hypobaric environments there is an increasing need for adequate provision of medical care. Accurate blood pressure measurement is one of

Correspondence: Dr N Kametas, Harris Birthright Research Centre for Fetal Medicine, King’s College Hospital, Denmark Hill, Golden Jubilee Wing, London SE5 9RS, UK. E-mail: [email protected] Received 19 April 2005; revised 29 November 2005; accepted 23 December 2005; published online 13 April 2006

the cornerstones of any comprehensive medical aid provision and a prerequisite for the appropriate set up of any attempt to confirm and understand the pathophysiology of cardiovascular disorders at high altitude. The mercury sphygmomanometer has been the basis for blood pressure measurement for more than a 100 years. However, the realization of the toxic effects of mercury on humans has led to an attempt during the last decade to replace the mercury sphygmomanometer with other devices, such as aneroid or automated mechanical devices. A plethora of such devices has been marketed during the last few years, with very few of them complying with the standards set by the Association for the Advancement of Medical Instrumentation4 and the British,5 or European Society of Hypertension.6 Moreover, there is lack of reports on the accuracy of such devices in measuring blood pressure at high altitude, where the reduced barometric pressure may interfere with normal operation of these instruments. The aim of this study was to examine the accuracy of a portable aneroid device in measuring blood pressure in a population residing at an altitude of 4400 m in the Peruvian Andes according to similar

Can aneroid sphygmomanometers be used at altitude? NA Kametas et al 518

criteria recommended by the British Hypertension Society.5 The first reason for choosing to validate an aneroid device was because we wanted to assess the affect of altitude on a pressure chamber, produced and calibrated at sea level and then taken to altitude. Would the environmental changes at high altitude influence its calibration and accuracy as compared to the Hg system where differences attributed to altitude would be very small? Furthermore, we wanted to examine a simpler and financially more affordable device than automated blood pressure measuring devices. This would therefore be a more valuable tool for everyday health care provision at a local level but also an important aid for future research at high altitude.

Subjects and methods The validation procedure was performed by two observers who were trained using a British Hypertension Society videotape and tested against an expert observer. Each observer during training achieved the standards set by the British Hypertension Society protocol, that is, 45 out of 50 of their measurements were within 5 mm Hg and 48 out of 50 within 10 mm Hg from the expert’s measurements. The examined population consisted of 10 subjects (four male and six female). As the principal aim of this investigation was to test the influence of altitude, not the performance of the device on a large cross-section of subjects (as is required in standard validation procedures), only 10 subjects were selected. To maintain sufficient power to determine meaningful differences, we obtained five paired measurements from each of the 10 subjects, providing 50 differences to analyse. This is more than 45 differences recommended by the latest validation protocol from the European Society of Hypertension.6 Subjects were seated for at least 5 min before taking the measurements in a room with standardized temperature of 22–241C. Arm circumference was measured at the approximate midpoint of the upper arm to establish the correct cuff size to be used. The ethics committee of the Peruvian Ministry of Health gave approval for the study and all subjects gave informed written consent. Three devices underwent the initial ‘before-use’ and ‘after-use’ calibration procedures but one of the three devices was randomly selected for use in the main validation. We proceeded with the calibration of the device both at sea level and at high altitude and the in field assessment because of the need to establish proper baseline functioning and calibration of the device at both altitudes. This was carried out in an attempt to eliminate any systematic error that would be corrected or deteriorated when ascending to altitude. Therefore, validating the aneroid device Exacta-Riester (Rudolf Riester GmbH Journal of Human Hypertension

Table 1 Chronological sequence of the four phases of the device assessment 1 Before-use device calibration At high altitude At sea level 2 In-use field assessment 3 After-use device calibration 4 Static device validation

& Co. KG, Jungingen, Germany) consisted of four phases (Table 1). Each phase had to be completed successfully in order to continue to the next phase. Before-use device calibration

This was performed immediately before phase 4 at high altitude and at sea level to ensure optimal calibration and function of each device before its assessment. The pressures within the device were compared with those on a well-functioning mercury column. The device was connected to a mercury sphygmomanometer, while the cuff was placed around a cylinder. Using the calibration table in the protocol,5 the widest range of pressures applicable to the device was selected. The first observer inflated the cuff and called out at five randomly selected pressure points during deflation, noting the pressure on the mercury column. The second observer noted the device reading. This procedure was repeated six times giving a total of 30 readings at a deflation rate of 2 mm Hg min1 and 30 readings at a deflation rate of 4 mm Hg min1. Readings obtained from the two observers were then compared and the difference had to be o3 mm Hg in at least 28 of the readings for each rate of deflation. As a quality control measure, the mercury column itself was also tested as a test device under the same conditions against another well-functioning mercury sphygmomanometer. In-use field assessment

The aneroid devices had already been in use locally and had also, for 1.5 months, undergone more than 400 inflations for the purposes of a large study examining physiology at high altitude. After-use device calibration

Similar methodology was used as in the before-use device calibration. Static device validation

Validation of the device was undertaken at high altitude by connecting it in parallel to two mercury sphygmomanometers. Eleven same-arm sequential blood pressure measurements were taken alternating between simultaneous mercury sphygmomanometer readings (by both observers) and the device. At least 30 s were allowed between each measurement to


Results The demographic details of the studied population and the median and range of blood pressures are Table 2 British Hypertension Society grading criteria Grade

Absolute difference between standard and test device p5

p10

Cumulative percentage of readings (%) A 60 85 B 50 75 C 40 65 D Worse than C

p15

95 90 85

The device should achieve percentages greater than or equal to those in the table in order to achieve a particular grade.

shown in Table 3. The median (range) for the ‘classification’ blood pressures was 114 (76–170) mm Hg for systolic and 71 (52–90) mm Hg for diastolic blood pressure, respectively. Bland–Altman plots7 show the difference between the device and the observer readings against the mean of the device and the observer readings for systolic (Figure 1) and diastolic (Figure 2) blood pressure. Calibration (Table 4): Both at sea level and at high altitude all 30 measurements at each rate of drop of the mercury column were within 3 mm Hg. Therefore, the anaeroid Riester-Exacta passed the pre-use calibration at both altitudes. Validation (Table 5): The aneroid blood pressure measuring device Exacta-Riester is accurate for measuring blood pressure at high altitude. It achieved grade A both for systolic and diastolic blood pressure according to the recommendations of the BHS (Table 2). Furthermore, it fulfilled the AAMI recommendations as the mean (standard deviation) of the differences between the observer and the device were 1.32 (4.3) for systolic and 3.7 (4.7) for diastolic, respectively, that is, less than 5 mm Hg for the mean and less than 8 mm Hg for the standard deviation. The mean and standard deviation for systolic and diastolic pressures were 0.7

Table 3 Demographic characteristics and blood pressure for the subjects recruited (median and range of measurements) N ¼ 10

Median (range)

Age (years) Weight (kg) Height (cm) Systolic blood pressure (mm Hg) Diastolic blood pressure (mm Hg)

27.5 60.1 157 106 69

(23–61) (51–76) (154–169) (76–170) (52–90)

15 Difference of pressures measured by anaeroid and mercury sphygmomanometer

avoid venous congestion but less than 60 s to minimise variability in blood pressure. The systolic blood pressure and Korotkoff phase V were recorded. An initial mercury reading, before the subsequent 11 readings used for the validation of the device, was used only for ‘classification’ purposes and was not used in the analysis. Similarly, the first device reading was used to ‘familiarize’ the device with the patient. The remaining 11 readings (six mercury and five device) were subsequently used in the analysis according to the British Hypertension Society protocol and the AAMI. The two observers were blinded to each other’s readings. Differences between the device and the observers were calculated separately for systolic and diastolic blood pressure and for each individual observer. These differences were established by calculating the difference between each device reading and both the previous and subsequent mercury reading. The smallest set of differences for each subject (i.e. either with the previous or subsequent mercury reading) was chosen for each observer and used to calculate the mean differences and standard deviation for systolic and diastolic blood pressure. This is because the British Hypertension Society protocol suggests using the set of differences, which were more favourable to the device. The readings of the better observer were used in the final grading, as per the protocol. British Hypertension Society grading is determined by the percentage of differences p5, p10 and p15 mm Hg, the device needing to obtain A/B for both systolic and diastolic measurements to be recommended (Table 2). Furthermore, criteria set by the AAMI requires the mean to be within 5 mm Hg and the standard deviation to be within 8 mm Hg for recommendation. In addition, simultaneous measurements were made with the aneroid and mercury sphygmomanometers and five paired measurements were compared in each arm.

10 5 0 -5 -10 -15 70

90 110 130 150 170 Mean of pressures measured by anaeroid and mercury sphygmomanometer

Figure 1 Bland–Altman plot for systolic blood pressure for the validation of the aneroid device Exacta-Riester in the examined high-altitude population (10 subjects—each one contributing to five paired measurements). Journal of Human Hypertension

Can aneroid sphygmomanometers be used at altitude? NA Kametas et al Difference of pressures measured by anaeroid and mercury sphygmomanometer

520

(2.6) mm Hg and 3.3 (2.7) mm Hg, respectively, in simultaneous analysis. Inter-observer agreement was extremely high with 98% of systolic readings and 96% of diastolic blood pressure being within 5 mm Hg and 100% of systolic and diastolic blood pressure being within 10 mm Hg.

15 10 5 0 -5 -10 -15 35

Discussion 45 55 65 75 85 95 105 115 Mean of pressures measured by anaeroid and sphygmomanometer

Figure 2 Bland–Altman plot for diastolic blood pressure for the validation of the aneroid device Exacta-Riester in the examined high-altitude population (10 subjects—each one contributing to five paired measurements).

Table 4 Before-use calibration of the aneroid Riester-Exacta at sea level and high altitude Rate

Sea level

High altitude

2 mm Hg s1 Within 3 mm Hg Within 2 mm Hg

30/30 29/30

30/30 28/30

4 mm Hg s1 Within 3 mm Hg Within 2 mm Hg

30/30 29/30

30/30 27/30

Differences between a mercury sphygmomanometer and the RiesterExacta at a rate of 2 and 4 mm Hg/s.

The results of this study have demonstrated that the Exacta-Riester aneroid device achieved a grade A performance according to the similar criteria described by the British Hypertension Society, when used at an altitude of 4330 m. The AAMI criteria were also met. The mercury sphygmomanometer has been the mainstay for blood pressure monitoring for the last century8 but it is gradually being phased out, not because of any technological advances but because of environmental concerns. Mercury is a serious non-degradable pollutant, which eventually accumulates on the sea-bed with contamination of marine life thus entering the aquatic food chain.9 In humans, direct exposure to mercury is a major health risk via inhalation of vapour and absorption through the skin. Mercury poisoning has detrimental effects on the central nervous system, renal, hepatic and respiratory systems.10 In the clinical setting, concerns arise from the fact that each mercury sphygmomanometer contains approximately 64–85 g of elemental mercury. The actual

Table 5 Blood pressure (mm Hg), BHS grading criteria and pressure differences between mercury sphygmomanometry and the aneroid device Exacta-Riester in the examined high-altitude population Grade

Differences between standard & test device p5 (%)

p10 (%)

p15(%)

Mean (s.d.) of BP measurements (mm Hg)

Mean (s.d.) of differences

Observer 1 SBP DBP

A A

74 70

100 96

100 100

111 (25) 72.5 (15)

0.07 (4.4) 2.7 (4.7)

Observer 2 SBP DBP

A A

78 74

100 98

100 100

111 (26) 73 (15)

1.32 (4.3) 3.7 (4.7)

Overall grading SBP DBP

A A

78 74

100 98

100 100

111 (26) 73 (15)

1.32 (4.3) 3.7 (4.7)

Observer comparison SBP DBP

A A

98 96

100 100

100 100

111 (26) 72.5 (15)

0.62 (2.5) 0.32 (2.1)

94 82

100 98

100 100

111 (26) 73 (13)

0.7 (2.6) 3.3 (2.7)

Simultaneous measurement SBP A DBP A

Mean (s.d.) are shown to determine the AAMI status. SBP, systolic blood pressure; DBP, diastolic blood pressure. Journal of Human Hypertension


amount has been found to vary considerably, as mercury is slowly lost either by direct spillage, vaporization or secondary to oxidation. Studies have indicated that between 62–87% of mercury manometers are affected in this way.11 A recent ruling by the European Union (European Council Directive 93/42/EEC) heralded the demise of the mercury sphygmomanometer and made imperative the search for accurate non-mercury-dependent blood pressure measuring devices. Traditionally, mercury sphygmomanometry has been the gold standard against which other blood pressure devices are being validated. Mercury sphygmomanometers consist of a cuff of a pneumatic band attached to a mercury open-tube manometer. The pressure on the pneumatic calf is transmitted to the manometer displacing the column of mercury by a certain level, while a small amount of air leaks through the anti-mercury leak-plugs on the top of the mercury column. In normal conditions, the air in the pneumatic band and above the mercury column and the mercury itself are in stable conditions. However, in the current study a point to be considered was whether environmental factors would influence the properties of the air and the mercury column in the sphygmomanometer to such an extent that we could no longer use it as a gold standard. With increasing altitude from sea level to about 4500 m, the atmospheric pressure drops from 760 mm Hg to approximately 432 mm Hg.3 This reduction in atmospheric pressure, however, does not influence the viscosity of the air, as the viscosity of air is independent of pressure.12 Hence, the flow of air through the hand pump and control valve and the anti-mercury leak-plugs on the top of the device would not be affected. Another parameter to be considered was the effect of altitude on the earth’s gravitational ‘pull’ to the mercury column. With increasing altitude, gravity decreases because of the increasing distance from the centre of the earth. Using the equation provided by Kaye and Laby12 to calculate the change in gravity acceleration ‘g’ according to the change in altitude, for 4400 m altitude the ‘g’ would be affected by about 0.138%; in other words 0.138 mm in 100 mm Hg, a practically and clinically insignificant effect. Finally, owing to the fact that mercury is virtually incompressible,12 its density would not be expected to change with increasing altitude. Therefore, the pressures exerted by the mercury column would not have been affected by increasing altitude. Aneroid sphygmomanometers, as mercury sphygmomanometers do, rely on the auscultatory technique. The difference between the two types of devices is that an aneroid gauge replaces the mercury manometer. In this occasion, the pressure in the inflated calf is determined by measurement of the mechanical deformation of a sensing element that undergoes elastic deformation as the pressure difference across its surfaces changes. This mechanical deformation is in turn sensed in many different

ways, most commonly by a series of mechanical levers providing a direct display of the magnitude of the deformation. The sensing element in the aneroid Exacta-Riester is a single diaphragm and operates in gauge mode. Neither the diaphragm nor the mechanical levers are affected by the small change in gravitational force and the gauge mode is insensitive to changes in ambient pressure. As this is an open system, the changes in atmospheric pressure do not make a substantial difference to the ability to detect absolute and differential pressure changes. Therefore, the functional capacity of the aneroid device would not be affected by the high-altitude changes. Our results confirm the above, as the difference between the mercury and the aneroid device is about 1 mm Hg for systolic and 3 mm Hg for diastolic blood pressure, respectively. These differences are within the range claimed by the producing company at sea level conditions. Most importantly, the differences between the aneroid and the mercury sphygmomanometer are considered to be clinically acceptable13 and can be attributed to be within the expected accuracy of any good device, independent of altitude. The portable aneroid device Riester-Exacta is accurate for measuring blood pressure at high altitude. Accurate measurement of blood pressure is the cornerstone of clinical care provision to many millions inhabitants and travellers at high altitude. Ours is the first study to consider the basic properties of mercury at high altitude and the ability of mercury sphygmomanometers to measure blood pressure accurately. The finding that aneroid sphygmomanometry is reliable provides a simple and affordable method of measuring blood pressure at high altitude. Further work is now needed to validate non-mercury-dependent blood pressure measuring devices at high altitude. What is known about this topic K High altitude, being home to about 140 million people throughout the world1 and visited yearly by more than 34 million people,2 can lead to human disease or decompensation of human homeostasis3 9,10 K Mercury has serious toxic effects on humans and this has led to an attempt to replace the mercury sphygmomanometer, which has been the basis for blood pressure measurement for more than a hundred years,8 alongwith other devices. K Aneroid blood pressure measuring devices are simpler and financially more affordable than automated blood pressure measuring devices. What this study adds Aneroid sphygmomanometry, as assessed by the validation of the Exacta-Riester aneroid device, provides a reliable, simple and affordable method of measuring blood pressure at high altitude.

K

Acknowledgements This study was funded by the Fetal Medicine Foundation (registered charity 1037116). Journal of Human Hypertension


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