Assessing Blood Pressure Accuracy of an Aneroid Sphygmomanometer in a National Survey Environment

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1 original contributions nature publishing group Assessing Blood Pressure Accuracy of an Aneroid Sphygmomanometer in a National Survey Environment Yechiam Ostchega 1, Ronald J. Prineas 2, Tatiana Nwankwo 1 and George Zipf 1 Background The gold standard employed for obtaining blood pressure (BP) for all the National Health and Nutrition Examination Surveys (NHANES) has been the mercury sphygmomanometer (HgS). Because of environmental concerns, there is a need to explore an alternative to HgS. Methods We compared the accuracy of the Welch Allyn 767 wall aneroid sphygmomanometer (AnS) to the HgS in children and adults and by BP cuff sizes. Each participant had three BP measurements per device recorded sequentially. The order of the devices and observer were random. A total of 727 individuals participating in the NHANES participated in the study. Results The mean AnS readings were not statistically significantly different from those of the HgS with the exception of systolic BP (SBP) in aged 8 17 years (mean difference 1.10, s.d. 4.87). There were no statistically For many years, the gold standard employed for all the National Health and Nutrition Examination Surveys (NHANES) has been the mercury sphygmomanometer (HgS). However, because of the increased environmental concerns about the disposal of mercury-contaminated medical waste and the risk of spills from HgSs, the HgS is being phased out in clinical settings. In 1999, the Environmental Protection Agency and American Hospital Association agreed to eliminate mercury-containing waste from the health-care industry by an ideal not yet achieved. In that report the example of best practices cited a federal hospital s complete elimination of mercury-containing instruments including sphygmomanometers and cited multiple sources of discussion around the recommendations for and against. The most recent American Heart Association recommendations for blood pressure (BP) measurement also note that mercury use is 1 Division of Health and Nutrition Examination Statistics, National Center for Health Statistics, Centers for Disease Control and Prevention, U.S. Department of Health and Human Services, Hyattsville, Maryland, USA; 2 Wake Forest University School of Medicine, Department of Epidemiology and Prevention, Winston-Salem, North Carolina, USA. Correspondence: Yechiam Ostchega (yxo1@cdc.gov) Received 8 July 2010; first decision 28 August 2010; accepted 9 October American Journal of Hypertension, Ltd. significantly different by BP cuff sizes. Agreement for the prevalence of hypertension (BP 140 systolic or diastolic 90 mm Hg) was above chance (κ = 0. 81; sensitivity = 81%; specificity = 98%) with AnS readings underestimating by 1.66% (18.33 vs. 20%, P > 0.05) compared to the HgS reading. Conclusions With the exception of SBP in ages 8 17 years, the AnS device readings were not significantly different from HgS readings by age or BP cuff sizes selection. Agreement for hypertension classification is good. An accurate and well-calibrated AnS could therefore provide an acceptable alternative to the use of a HgS in surveys, although with appropriate caution given the 81% sensitivity with regard to hypertension thresholds that was observed. Keywords: aneroid; blood pressure; hypertension; mercury sphygmomanometer; NHANES American Journal of Hypertension, advance online publication 16 December 2010; doi: /ajh being banned in many countries. 2 Therefore, alternatives to the HgS are being increasingly used and national standards need to continue with technology that can be used by the majority of health-care providers and for home BP measurement. The aneroid sphygmomanometer (AnS), a mercury-free device employing the same auscultatory principles as those used for the HgS, has been used as a substitute for the mercury HgS. Although the AnS is ubiquitous, it is surprising to find that few studies have been published comparing aneroid to the mercury manometers. 3,4 and there are none in the public service setting. The exception is a recently published study by Ma et al. comparing an aneroid (the Welch Allyn Tyco 767) to a HgS. 5 The study involved 997 subjects but it had a number of limitations. Specifically, it took place in 24 clinics, among adults only and the same technician observed both the AnS and HgS raising the issues of observer bias. Our study comparing a HgS and an AnS, in the setting of the NHANES, had three objectives. The first was to compare the measured values of BP, systolic and diastolic, both in youth and adults. Because recent studies showed that mid-arm circumference has increased significantly in recent years 6,7 and this has implication for the accuracy of BP measurements; 322 march 2011 VOLUME 24 NUMBER AMERICAN JOURNAL OF HYPERTENSION

2 Comparing Aneroid to a Mercury BP Device original contributions we compared the measured values of BP among the different BP cuff sizes. The second was to compare hypertension classifications (JNC 7) between the two sphygmomanometer types. 8 The third was to examine the influence of higher levels of BP (systolic and diastolic) on the difference between the readings obtained with the two sphygmomanometers, controlling for age, gender, body mass index, BP cuff size, and examiner effects. Methods Study population. The study participants were from the NHANES. Briefly, NHANES is a national survey of the civilian noninstitutionalized US population. In 1999, NHANES became a continuous survey. Annual samples are selected using a complex, stratified, multistage probability sampling method and the data are released in 2-year cycles. Descriptions of the sample design and data collection methods are available at the NHANES website. 9 Survey participants were interviewed in their homes and examined in mobile examination centers. After completing the mobile examination centers examination portion the participants were asked to volunteer for the validation study, which was described as an additional optional study; no randomization was attempted and the sample was a convenience sample. The NCHS Ethics Review Board approved the study. The study was designed in two parts, the first of which dealt with the accuracy of the Omron HEM-907 XL 10 and the second part with the accuracy of the aneroid manometer against the mercury manometer. This analysis addresses the aneroid mercury arm of the study. During survey years 2006 and 2007, a total of 727 individuals aged 8 years and older agreed to participate in the validation study. Among the participants 390 were male (54%) and 337 (46%) were female. For this analysis, age was categorized into three groups: 8 17, 18 49, and 50 years. Among the age groups, 127 (17%) individuals were 8 17 years old; 236 (32%) were years old; and 364 (50%) were 50 years. The mean age of the participants was 45.9 years (s.d. 23.3; range 8 92 years). Measurements. An auscultatory AnS (wall-mounted Welch Allyn model 767) was used in this study. The aneroid gauge consists of a metal bellows and a watch-like movement connected to the compression cuff. Variations of pressure within the system cause the bellows to expand and contract. Movement of the bellows rotates a gear that turns a pointer pivoted on bearings, across a calibrated dial. The pressure is read from the dial. 11 A wall-mounted HgS (Baumanometer) was used as the standard comparison device. Four cuff sizes (child, adult, large adult, and thigh) also manufactured by Baumanometer were shared by both devices. Specifically, among the participants: 3.3% used a child cuff; 31.5% used an adult cuff; 52.1% used a large adult cuff; and 13.1% used a thigh cuff. Measurement of BP. A standardized protocol was used to train each technician. The initial training was followed by a biannual retraining. The training protocol for the mercury device was similar to that used for training the NHANES physicians who measure survey BP 12 and greatly reduces digit preference. The measurements were taken in a quiet room with an ambient temperature between 58 and 83 F, average = 76 F. Participants were seated in a chair with back support, with both feet resting comfortably on the floor and right arm in which the BP readings were taken supported on a level surface with the cubital fossa at heart level. After a 5-min rest, the study participants had their systolic and diastolic BP (DBP) (onset of K1 and K5) measured, each determination 30 sec apart. The order of sphygmomanometer used was assigned at random. Additionally, observers were randomly assigned to AnS or HgS. If the AnS measurements were made first, a different observer made the HgS measurements and vice versa to avoid observer bias. Moreover, the observers were blinded to the other device values. Following the Mayo Clinic, Rochester Minnesota protocol the Netech DigiMano digital pressure and vacuum meter part no IN was used for accurate calibration of the aneroid. 11 The HgS device was calibrated using the standard NHANES calibration protocol. 12,13 Both instruments were calibrated at the beginning of the study and on a weekly basis throughout the duration of the study which totaled 1.5 years. (In addition, there were daily visual checks that were performed on each device.) A total of eight trained technicians were involved in the BP comparative study. The observers zero end digit preference was 22% for the HgS observed systolic; 25% for the HgS observed DBP; 24% for the AnS observed systolic; and 26% for the AnS observed DBP. Other measurements. Individual age was obtained at interview time. Height in cm and weight in kg were measured in the mobile examination center following the standard NHANES anthropometric protocol. Body mass index was calculated as weight in kg divided by the height in m 2 (kg/m 2 ). Mid-arm circumference is determined by having the examinee stand erect with feet together and the right arm flexed 90 at the elbow with the palm facing up. On the right scapula, the observer locates and marks with a horizontal line the uppermost edge of the posterior border of the acromion process. The observer holds the zero end of the measuring tape at this mark and extends the tape down the posterior surface of the arm to the tip of the olecranon process. The observer makes a horizontal mark at the midpoint at the posterior aspect of the arm and measures the arm circumference. 12 Statistical analyses. We estimated that a sample size of 127 participants, at an α = 0.05 and given a s.d. of 17.7 mm Hg, would provide 85% power to detect a 3 mm Hg mean difference between the 2 sphygmomanometers. Each subject contributed three paired observations for each device. The data from these three observations were averaged. Paired t-test was used to test the difference between the two AMERICAN JOURNAL OF HYPERTENSION VOLUME 24 NUMBER 3 march

3 original contributions Comparing Aneroid to a Mercury BP Device Table 1 Mean and s.d. and difference (Δ) (AnS-HgS) and 95% confident interval (CI) by age groups for systolic and diastolic readings in mm Hg Device N Systolic, X (s.d.) device-readings for each participant, overall and for three age groups: 8 17, 18 49, and 50+ years. Also, a graphic display of averaged device reading differences against corresponding averaged systolic and diastolic paired measurements separately (Bland and Altman graph) is presented to visually assess the relationship between the two devices. 14 We calculated sensitivity and specificity as well as the κ statistic to assess the agreement between the devices whereas HgS was considered the gold standard for systolic defined hypertension (BP 140 mm Hg), diastolic defined hypertension (BP 90 mm Hg), and stage one hypertension ( 140 systolic or 90 diastolic mm Hg). 8 In addition to κ, sensitivity, and specificity statistics we used McNemar s test to assess the proportion of individuals classified as hypertensive by the two devices. McNemar s test assesses the significance of the difference between two correlated proportions where the two proportions are based on the same sample of subjects. 15 The hypertension classification values were based on the HgS readings obtained during the validation study and were estimated for adults only (individuals 18 years and older). The correlation between systolic and diastolic readings by each device was assessed using the Pearson correlation coefficient. In addition, intraclass correlation coefficient (2.1), was calculated. Conceptually, intraclass correlation coefficient 2.1 assesses raters agreement; in the present study the raters were the two devices. 16 Multiple linear regression analyses were conducted to model the association of HgS systolic and diastolic measured BP readings on the magnitude of the difference in readings between the two devices for systolic and DBP controlling for age, gender, body mass index, observers (dummy coded), and cuff size: child, adult, large, and thigh cuff (dummy coded). Finally, the effect of the order of reading (HgS/AnS or AnS/ HgS) was tested using a paired t-test by age group categories. The α-level for a significant test was considered to be P < Diastolic, X (s.d.) HgS (s.d.) AnS (s.d.) Δ (CI) HgS (s.d.) AnS (s.d.) Δ (CI) Overall (18.23) (17.85) 0.08 ( 0.54 to 0.38) (11.55) (11.71) 0.22 ( 0.71 to 0.27) Age group (years) 8 17 years (9.64) * (9.79) 1.10 (0.25 to 2.0) (9.88) (10.74) 0.26 ( 2.08 to 1.56) years (11.6) (12.12) 0.08 ( 0.66 to 0.82) (10.62) (10.82) 0.45 ( 1.17 to 0.28) 50+ years (19.45) (19.06) 0.60 ( 1.33 to 0.13) (11.57) (11.49) 0.06 ( 0.65 to 0.53) Cuff size Child 9 17 cm ( (18.32) 0.39 ( 3.19 to 2.41) (12.93) (11.61) 1.5 ( 7.36 to 4.36) Adult cm (19.69) (19.81) 0.44 ( 0.37 to 1.25) (11.57) (12.34) 0.59 ( 1.56 to 0.38) Large adult cm (17.12) (16.72) 0.36 ( 1.01 to 0.29) (11.00) (11.02) 0.08 ( 0.48 to 0.63) X-large adult cm (14.02) (13.34) 0.14 ( 1.40 to 1.10) (11.86) (10.42) 0.19 ( 1.58 to 1.21) AnS, aneroid sphygmomanometer; HgS, mercury sphygmomanometer. *P = Mean difference systolic BP readings Mean aneroid and mercury systolic BP The vertical lines represent mean and 2SD Figure 1 Bland Altman 14 graph aneroid vs. mercury, systolic. BP, blood pressure. Results Between sphygmomanometer agreement Table 1 presents the mean and s.d. for the HgS and AnS readings for systolic and DBP. With the exception of mean systolic BP (SBP) readings among individuals aged 8 17 years (SBP: HgS = ± 9.64 mm Hg, AnS = ± 9.79 mm Hg; mean difference = 1.1 ± 4.87 mm Hg, P = 0.011) there were no statistically significant differences between HgS and AnS readings overall or for each age group or cuff size category. Absolute mean differences for systolic and DBP readings were calculated. Fifty-two percent of SBP reading differences were 3 mm Hg and 69% were 5 mm Hg. Also, for DBP, 49% of diastolic reading differences were 3 mm Hg and 69% were 5 mm Hg. Figures 1 and 2 displays the Bland Altman plots; the plots show the mean differences in systolic and DBP device readings against the corresponding averaged BP readings of both 324 march 2011 VOLUME 24 NUMBER 3 AMERICAN JOURNAL OF HYPERTENSION

4 Comparing Aneroid to a Mercury BP Device original contributions devices. All figures show some extreme values beyond two s.d. but no discerning linear relationship can be ascertained between the y- and the x-axes (systolic correlation r = 0.06, P > 0.05; diastolic correlation r = 0.03, P > 0.05). Table 2 presents the agreement in the classification of an elevated SBP ( 140), DBP ( 90), and hypertension (BP 140 systolic or /90 diastolic mm Hg) by the two devices. All agreements were above those to be expected by chance (κ = 0.82, 0.75, and 0.81) with agreement lowest for DBP. Consistently more individuals were correctly identified as nonhypertensive in all hypertensive subcategories (a notably high specificity) than were correctly identified as hypertensive (relatively lower sensitivity). For all hypertension subcategories the HgS device identified a slightly higher proportion of hypertensives than did the AnS (device difference for SBP = 0.67%, for DBP = 0.84%, and for hypertension = 1.66%). The difference in the proportion of individuals classified as hypertensive by the two devices across all hypertension subcategories were not statistically significant (McNemar s test P > 0.05). Mean difference diastolic BP readings Table 2 Consistency of the classification of high BP levels by HgS and AnS ages 18 years and older Classification HgS AnS Systolic BP 140 mm Hg Diastolic BP 90 mm Hg Systolic/diastolic BP ( 140/90 mm Hg) Mean aneroid and mercury diastolic BP The vertical lines represent mean and 2SD Figure 2 Bland Altman 14 graph aneroid vs. mercury, diastolic. BP, blood pressure. Multivariate analyses The BP values obtained using the two devices were highly correlated (r = 0.94 for SBP, P < 0.01 and r = 0.83 for DBP, P < 0.01). The intraclass correlation coefficient was (95% confidence interval, ) for SBP and (95% confidence interval, ) for diastolic showing a high between-device agreement, in essences, suggesting that the devices can be interchangeable. Multiple linear regression analyses of the between-device difference in systolic values adjusted for age, gender, body mass index, cuff size, observer effect, and SBP (measured by HgS) and a similar model for between-device difference in DBP (measured by HgS) were calculated. The results show that HgS measured systolic and diastolic values were significantly associated with the difference in device readings for SBP and DBP (β-coefficient = , P < 0.01; β-coefficient = , P < 0.01, respectively). Because both coefficients were negative, the results suggest that 1 mm Hg increase in SBP corresponded to 0.09 mm Hg decrease in the differences in systolic device readings. Similarly, 1 mm Hg increase in DBP corresponded to 0.2 mm Hg decrease in the differences between-device diastolic readings. Neither observers nor cuff sizes were significantly associated with the discrepancy between the two devices for either systolic or diastolic readings (data not shown). Finally, we analyzed the effect of the order of randomization on device difference by age groups (8 17, 18 49, and 50+). There was a statistically significant difference (P < 0.05) between the device readings for SBP in individuals aged 8 17 where AnS was the first condition (e.g., AnS/HgS) (systolic: AnS mean = s.d. 9.13; HgS mean = , s.d mm Hg, P = 0.003). All other randomization conditions were not statistically significantly different. Discussion The overall results of the study demonstrate that BP readings obtained by an AnS, a non-hgs, were not significantly different from readings obtained by an HgS. There was a statistically significant difference between-device readings in SBP only for youth (ages 8 17), however, the difference is not clinically % Difference (AnS% HgS%) Sensitivity (%) Specificity (%) κ-statistics No Yes ( ) No Yes ( ) No Yes ( ) HgS is the gold standard. AnS, aneroid sphygmomanometer; BP, blood pressure; HgS, mercury sphygmomanometer. AMERICAN JOURNAL OF HYPERTENSION VOLUME 24 NUMBER 3 march

5 original contributions Comparing Aneroid to a Mercury BP Device significant (1.1 mm Hg). The few previous studies did not include youth in their samples, so we have no data to compare with the results in our study. BP cuff size did not significantly affect the difference in device readings. Regrettably, we did not have enough statistical power to generalize from small cuff size (N = 24) affect. As for X-large cuff size category (N = 95), accepting a mean difference of 4 mm Hg, at an α = 0.05 and a s.d. of 17.7 mm Hg would provide 90% power to detect difference between the two sphygmomanometers in paired N of 85 individuals, hence we do have enough power to generalize; provided we accept a slightly larger mean difference (from 3 to 4 mm Hg). The HgS device identified a slightly higher proportion of hypertensives than did the AnS but these differences were not statistically significant. Crudely applying these findings to the national prevalence estimates of hypertension for years would have resulted in a nonsignificant change in the hypertension prevalence estimate. Stated differently, if an AnS device was used in the NHANES for survey years , the prevalence rate of hypertension would have been 1.66% lower than the reported rate of 29% but still within the 95% confident interval (29% confidence interval %). This lower estimate of hypertension prevalence by the AnS though modest and a result of the 81% sensitivity of the AnS could present a potential problem in comparison of national prevalence estimates in the future if the HgS replacement is made with a similar AnS. However, it is problem that will have to be faced given a mandatory need to replace HgS in the future with whatever replacement sphygmomanometers are chosen. It is also a problem faced by all disease-tracking technologies, whether by new laboratory measures of disease markers and or changes in nomenclature as with ICD coding changes that have resulted in mortality steps up or down of specific disease classifications. That said, the AnS step of 1.66% is a modest one and can easily be incorporated with the very high specificity into trackable hypertension estimates in the future The multivariate adjusted regression model findings suggested that differences between the two devices were larger at higher BP values. However, although statistically significant, the multivariate regression models for both systolic and DBP explained very little of the variability of the difference betweendevice readings (adjusted R 2 = 0.06 and 0.09, respectively). Similar results were reported very recently by Ma et al. 5 The aneroid manometer has moving parts that are subject to fatigue and breakage. The metal diaphragms and the coiled spring are the most vulnerable to such problems but when they are properly maintained they can provide accurate BP readings. 11 We followed the Mayo Clinic, Rochester Minnesota protocol for regular calibration of the aneroid. Indeed, during the 2 years of the study we had to replace only one aneroid. In that setting it should also be remembered that HgSs are readily imprecise if not maintained regularly. In two different hospital surveys it was noted that as many as 50% of HgSs tested defective. 17,18 The study had a number of limitations. We were unable to validate the aneroid manometer using either the Association for the Advancement of Medical Instrumentation or British Hypertension Society standard protocols. 19,20 To approximate these protocols we would have required that two trained observers be used to obtain BP measurements for each device, for a total of four observers, which although desirable was not feasible in our survey environment. Furthermore, unlike traditional validation studies the BP observers were not continuously monitored for observer agreement within 4 or 5 mm Hg, repeating any measure >4 or 5 mm Hg. No exclusion criteria were used in the study to exclude individuals having arrhythmias or on pharmacological therapy for hypertension. The AnS was easy to use and differences in readings compared to the HgS were not statistically significant with the exception of a small (1.1 mm Hg) over-read of SBP in youth ages 8 17 years. However, some caution needs to be considered given the 81% sensitivity with regard to hypertension classification. This is the first field study to include individuals in a varied age range in which the observers were blinded to the mercury results and in which both subjects and observers were randomly assigned to mercury or aneroid device for the first readings. Moreover, all observers were rigorously trained and closely followed for quality assurance and control. The field setting of the study provided a true measure of the device performance in a survey environment and also shows that an accurate, well-calibrated AnS could replace a HgS in the quest to remove mercury from the environment. Acknowledgments: We would like to express our gratitude to all involved and who made it possible to complete this validation study. We would like to thank Carlene Grim from Shared Care, Inc., Dr Grace Willard, Doe Knight, Gunda Kube, Ruth Pressley, Olga Guererro, Jamie Saltsman, and Belma Ybarra all from Westat, Inc. and Jeffery Hughes from the NHANES program NCHS, CDC. Disclosure: The authors declared no conflict of interest. 1. Messelbeck J, Sutherland L. Applying environmental product design to biomedical products research. Environ Health Perspect 2000; 108 Suppl 6: Pickering TG, Hall JE, Appel LJ, Falkner BE, Graves J, Hill MN, Jones DW, Kurtz T, Sheps SG, Roccella EJ; Subcommittee of Professional and Public Education of the American Heart Association Council on High Blood Pressure Research. Recommendations for blood pressure measurement in humans and experimental animals: Part 1: blood pressure measurement in humans: a statement for professionals from the Subcommittee of Professional and Public Education of the American Heart Association Council on High Blood Pressure Research. Hypertension 2005; 45: Reinders A, Jones CR, Cuckson AC, Shennan AH. The Maxi Stabil 3: validation of an aneroid device according to a modified British Hypertension Society protocol. Blood Press Monit 2003; 8: Dorigatti F, Bonso E, Zanier A, Palatini P. Validation of Heine Gamma G7 (G5) and XXL-LF aneroid devices for blood pressure measurement. 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6 Comparing Aneroid to a Mercury BP Device original contributions 8. Chobanian AV, Bakris GL, Black HR, Cushman WC, Green LA, Izzo JL Jr, Jones DW, Materson BJ, Oparil S, Wright JT Jr, Roccella EJ. The Seventh Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure: the JNC 7 report. JAMA 2003; 289: National Center for Health Statistics Analytic Guidelines (article online). < guidelines> (2004). 10. Ostchega Y, Nwankwo T, Sorlie PD, Wolz M, Zipf G. Assessing the validity of the Omron HEM-907XL oscillometric blood pressure measurement device in a National Survey environment. J Clin Hypertens (Greenwich) 2010; 12: Canzanello VJ, Jensen PL, Schwartz GL. Are aneroid sphygmomanometers accurate in hospital and clinic settings? Arch Intern Med 2001; 161: Ostchega Y, Prineas RJ, Paulose-Ram R, Grim CM, Willard G, Collins D. National Health and Nutrition Examination Survey : effect of observer training and protocol standardization on reducing blood pressure measurement error. J Clin Epidemiol 2003; 56: National Center for Health Statistics, Centers for Disease Control and Prevention. National Health and Nutrition Examination Survey (NHANES) Questionnaire and Exam Protocol. < htm> 14. Bland JM, Altman DG. Measuring agreement in method comparison studies. Stat Methods Med Res 1999; 8: McNemar s Test for Correlated Proportions in the Marginals of a 2x2 Contingency Table. < 16. Shrout PE, Fleiss JL. Intraclass correlations: uses in assessing rater reliability. Psychol Bull 1979; 86: Markandu ND, Whitcher F, Arnold A, Carney C. The mercury sphygmomanometer should be abandoned before it is proscribed. J Hum Hypertens 2000; 14: Mion D, Pierin AM. How accurate are sphygmomanometers? J Hum Hypertens 1998; 12: Association for the Advancement of Medical Instrumentation. American National Standard for Electronic or Automated Sphygmomanometers. AAMI: Arlington, VA, O Brien E, Waeber B, Parati G, Staessen J, Myers MG. Blood pressure measuring devices: recommendations of the European Society of Hypertension. BMJ 2001; 322: AMERICAN JOURNAL OF HYPERTENSION VOLUME 24 NUMBER 3 march