Measurement of blood pressure (BP) is done poorly because of both human and machine errors.
To assess the difference between BP recorded in a pragmatic way and that recorded using standard guidelines; to assess differences between wrist- and mercury sphygmomanometer-based readings; and to assess the impact on clinical decision-making.
Royal Swaziland Sugar Corporation Mhlume hospital, Swaziland.
After obtaining consent, BP was measured in a pragmatic way by a nurse practitioner who made treatment decisions. Thereafter, patients had their BP re-assessed using standard guidelines by mercury (gold standard) and wrist sphygmomanometer.
The prevalence of hypertension was 25%. The mean systolic BP was 143 mmHg (pragmatic) and 133 mmHg (standard) using a mercury sphygmomanometer; and 140 mmHg for standard BP assessed using wrist device. The mean diastolic BP was 90 mmHg, 87 mmHg and 91 mmHg for pragmatic, standard mercury and wrist, respectively. Bland Altman analyses showed that pragmatic and standard BP measurements were different and could not be interchanged clinically. Treatment decisions between those based on pragmatic BP and standard BP agreed in 83.3% of cases, whilst 16.7% of participants had their treatment outcomes misclassified. A total of 19.5% of patients were started erroneously on anti-hypertensive therapy based on pragmatic BP.
Clinicians need to revert to basic good clinical practice and measure BP more accurately in order to avoid unnecessary additional costs and morbidity associated with incorrect treatment resulting from disease misclassification. Contrary to existing research, wrist devices need to be used with caution.
La tension artérielle (BP) est mal mesurée à cause de l'erreur humaine et des machines.
Evaluer la différence entre la BP mesurée d'une façon pragmatique et celle mesurée à l'aide des directives classiques; pour évaluer les différences entre les mesures prises au tensiomètre de poignet et celles prises au tensiomètre à colonne de mercure; et pour évaluer l'impact sur la prise de décision clinique.
L'hôpital Mhlume de la Royal Swaziland Sugar Corporation, au Swaziland.
Après avoir obtenu le consentement, la BP a été mesurée d'une façon pragmatique par une infirmière qui a décidé du traitement. Ensuite, on a remesuré la BP des patients avec le tensiomètre à colonne de mercure et le tensiomètre de poignet.
La prévalence de l'hypertension était de 25%. La tension artérielle systolique moyenne était 143 mmHg (pragmatique) et 133 mmHg (standard) avec un tensiomètre à mercure et 140 mmHg pour la BP standard évaluée avec un tensiomètre de poignet. La BP diastolique moyenne était 90 mmHg, 87 mmHg et 91 mmHg respectivement pour la mesure pragmatique, la standard au mercure et celle au poignet. Les analyses de Bland Altman ont montré que les mesures de BP pragmatiques et standard étaient différentes et ne pouvaient pas être cliniquement interverties. Les décisions de traitement entre les mesures basées sur la BP pragmatique et la BP standard correspondaient dans 83.3% des cas, alors que les résultats des traitements de 16.7% des participants étaient mal classés. 19.5% des patients avaient été mis sous un traitement erroné antihypertenseur basé sur la BP pragmatique.
Les cliniciens doivent revenir aux bonne pratiques cliniques de base et mesurer la BP avec plus de précision afin d’éviter les coûts supplémentaires superflus et la morbidité liée au traitement incorrect dû à la mauvaise classification de la maladie. Contrairement aux recherches existantes, les tensiomètres de poignet doivent être utilisés avec précaution.
Hypertension is a consistent, powerful and independent risk factor for cardiovascular disease, stroke and renal disease.
Hypertension is a common health burden affecting both developed and developing nations.
Control of BP begins with accurate measurement that leads to appropriate diagnosis, assessment of cardiovascular risk and treatment decisions.
The mercury sphygmomanometer, because of its accuracy and reliability, is widely regarded as being the gold standard against which all other devices for BP measurement should be compared.
Rose suggested that the observer was the most critical component of accurate BP measurement.
In a study by Roubsanthisuk, Wongsurin and Saravich, physicians and trained nurses were compared, showing that trained nurses overestimated, rather than underestimated, blood pressure, but systolic BP underestimation was very common in participants with moderate to severe hypertension.
Clinicians should also be aware that BP in human beings is affected by multiple stimuli, such as respiration, temperature, body posture, emotional or physical stress, meals, alcohol, or caffeine and smoking and hence these factors should be taken into consideration during measurement of BP.
From the literature reviewed, it is clear that BP measurement is subject to errors, thus there are still some social and scientific questions which need clarity and further research, especially in resource-limited settings. Literature review concluded that with proper measurement technique, machine variation between the gold standard mercury sphygmomanometer and the wrist is minimal.
Nearly all the articles found on literature review are from developed countries with a good patient-to-health-worker ratio. In a developing country setting, where the patient-to-health-worker ratio is low and resources limited, the potential for BP measurement errors may be worse. One obvious question was on assessment of the reliability of BP measurement methods, looking at both sphygmomanometer and observer differences in resource-limited settings. In so doing, such research will further enlighten health workers about the trustworthiness of BP readings and ensure that health workers are treating BP optimally. Problems related to over- or under-treatment may be serious and, if identified early, could reduce unnecessary morbidity and mortality. Most of the prior studies have focused mainly on sphygmomanometer-related differences.
An analysis of variations between pragmatic or ‘real-life’ and standard BP measurement based on the ‘gold standard’ would be useful in improving chronic disease management and ensuring effective use of already-strained resources in primary care. This study will have an impact on increasing awareness of human-induced variation in BP measurement and its impact on therapeutic decisions; hence, it may motivate clinicians to follow protocol. In the long run, it may have some economic advantages in saving cost of drugs erroneously prescribed to those who, if BP had been recorded properly, would not need treatment.
Is there a difference between pragmatic and standard BP measurement in primary care?
To ascertain variations between standard and pragmatic BP measurements and comparison of wrist BP and mercury sphygmomanometer-based BP. To assess the impact of any differences on treatment decision.
To quantify the existence of any differences between BP recorded in a pragmatic way and that recorded using standard BP measurement protocols. To quantify any discrepancy between BP measurements done by wrist sphygmomanometer when compared to mercury sphygmomanometers. To assess if the differences in BP measurement have impact on treatment decisions: whether or not to treat, to start anti-hypertensive treatment or to adjust hypertension treatment.
A cross-sectional study design was used.
This study was done at Royal Swaziland Sugar Corporation (RSSC) Mhlume hospital, targeting outpatients. RSSC Mhlume hospital is a rural primary care facility in the eastern part of Swaziland The facility has a turnover of about 5000 patients per month and offers mostly primary care with minor office procedures. It serves a catchment area of about 30 000.
The study population comprised adult (> 18 years) patients, with or without hypertension, who accessed primary care at the RSSC hospital during the study period June 2011 to December 2011 and who gave consent to participate in the study.
Every fourth patient who had attended the outpatient clinic was eligible for selection. A sample size of 60 was used, based on statistical calculations and sample size from similar studies.
Informed consent was obtained from eligible patients. Participants had BP assessed in a pragmatic way by nurse practitioners who would give their therapeutic decision based on their readings. Participants had BP re-assessed according to the standard protocol, using mercury sphygmomanometer and wrist sphygmomanometer alternately. To reduce bias, the order of measurement for pragmatic or standard BP measurements was alternated for successive patients. Finally, demographic and relevant clinical data were collected into a ‘Data Collection’ form, which was subsequently entered into a Microsoft® Excel spreadsheet for analysis.
To improve internal validity, the potential biases were handled as laid out below.
For the reduction of selection bias, a systematic random sample (every fourth patient) was used.
This level of bias could occur at any stage during the measurement, recording, management or analysis of the data. Notable biases were the Hawthorne effect (nurses could change their BP measurement routine because they were aware of the investigation underway) and observer diagnostic suspicion bias. These were reduced by blinding the nurse researcher to results from the nurse practitioners and nurse practitioners were blinded to the ongoing study. Use of validated, standardised and calibrated sphygmomanometers reduced instrument variation. Batteries for the wrist devices were replaced regularly. To reduce subject physiologic variation, as well as the known regression to mean with repeated BP measurement phenomenon,
Time between performing the BP measurements was an important confounder. Blood pressure tends to come down with time, which is known as regression to the mean. The time between pragmatic and standard BP assessment was kept at a minimum so as to reduce the possibility of confounding bias. Previous studies indicate that a time lag of less than 10 minutes does not have any significant effect on the BP result.
Microsoft® Excel was used to capture the data and the data analysis software system, STATISTICA version 9 (StatSoft Inc., 2009), was used to analyse the data. The statistical analysis comprised both descriptive and analytical statistics. For descriptive statistics, summary statistics were used to describe the variables. The Wilcoxon sign rank test was used to assess differences between means of BP. For analytical statistics, simple logistic regression, Pearson correlation (
Ethical approval for the study was granted by the University of Stellenbosch Human Research Ethics Committee (reference number N10/11/394) on 13 May 2011. Institutional ethical approval was also obtained.
Sixty outpatients consented to participate in the study, of which 32 were men. The mean age of the participants was 42.6 years, the mean weight 77.8 kg and the mean height 1.6 metres. The prevalence of hypertension was 25%. Twenty-eight per cent of the participants had co-morbid diseases.
The mean systolic BP was 143 mmHg for pragmatic BP, 133 mmHg for standard BP using mercury sphygmomanometer and 140 mmHg for standard BP assessed using wrist device. The mean diastolic BPs were 90 mmHg, 87 mmHg and 91 mmHg for pragmatic, standard mercury and wrist, respectively. It took an average of 4.2 minutes between pragmatic and standard BP measurement.
Three participants reported either having a full bladder or having eaten within 30 minutes before BP assessment, five had exercised, one had smoked and taken coffee and seven reported some degree of psychological stress.
Demographic characteristics of participants.
Characteristics | Variables | s.d. |
---|---|---|
Males ( |
32 | - |
Females ( |
28 | - |
Mean age in years (standard deviation) | 43 | 14.2 |
Mean weight in kilograms (standard deviation) | 78 | 19.4 |
Mean height in centimetres (standard deviation) | 164 | 8.5 |
Mean body mass index | 29 | - |
Prevalence of hypertension | 25% | - |
Co-morbid conditions | 28% | - |
Mid-upper arm circumference (in centimetres) | 32 | - |
Treatment decision | No treatment | Treat | Change treatment | |||
---|---|---|---|---|---|---|
|
% |
|
% |
|
% | |
Based on pragmatic blood pressure | 32 | 53 | 18 | 30 | 10 | 17 |
Based on standard blood pressure | 41 | 68 | 11 | 18 | 8 | 13 |
Clinical characteristics of participants.
Blood pressure | Method | Observation | Mean | 25th percentile | 50th percentile | 75th percentile |
---|---|---|---|---|---|---|
Systolic blood pressure in mmHg | Pragmatic | 60 | 143 | 120 | 140 | 163 |
Standard using mercury device | 60 | 133 | 110 | 130 | 151 | |
Standard using wrist device | 60 | 140 | 123 | 138 | 155 | |
Diastolic blood pressure in mmHg | Pragmatic | 60 | 90 | 73 | 90 | 105 |
Standard using mercury device | 60 | 87 | 75 | 85 | 102 | |
Standard using wrist device | 60 | 91 | 77 | 86 | 106 | |
Mean time between pragmatic and standard blood pressure measurements (minutes) | 4 | - | - | - | - |
The Pearson correlation coefficient (
Pearson (
Blood pressure | Methods in comparison | Pearson coefficient, |
Intra-class coefficient ( |
Regression equations for relationship between blood pressure methods ( |
---|---|---|---|---|
Systolic blood pressure | Standard/pragmatic | 0.9 (good association) | 0.8 (almost perfect) | SBPMc = −10.7 + 1.2 SBPPr (gradient 1.2; intercept 10.7) |
Standard/wrist | 0.9 (good association) | 0.9 (almost perfect) | SBPMc = 20 + 0.8 SBPWr (gradient 0.8; intercept 20) | |
Pragmatic/wrist | 0.9 (good association) | 0.9 (almost perfect) | SBPPr = −2.5 + 1.0 SBPWr (gradient 1.0; intercept −2.5) | |
Diastolic blood pressure | Standard/pragmatic | 0.9 (good association) | 0.9 (almost perfect) | DBPPr = −0.7 + 1.0 DBPMc (gradient 1; intercept −0.7) |
Standard/wrist | 0.9 (good association) | 0.9 (almost perfect) | DBPMc = 10.6 + 0.8 DBPWr (gradient 0.8; intercept 10.6) | |
Pragmatic/wrist | 0.9 (good association) | 0.9 (almost perfect) | DBPPr = 2.5 + 1.0 DBPWr (gradient 1; intercept 2.5) |
Interpretation based on: −1.0 to −0.7 strong negative association; −0.7 to −0.3 weak negative association; −0.3 to +0.3 little or no association; +0.3 to +0.7 weak positive association; +0.7 to +1.0 strong positive association.
Interpretation based on: ICC can be interpreted as follows: 0–0.2 indicates poor agreement: 0.3–0.4 indicates fair agreement; 0.5–0.6 indicates moderate agreement; 0.7–0.8 indicates strong agreement; and > 0.8 indicates almost perfect agreement.
Abbreviations: SBPMc, Sytolic BP Mercury; SBPPr, systolic BP Pragmatic; SBPWr, systolic BP wrist; DBPPr, diastolic BP Pragmatic; DBPMc, diastolic BP Mercury; DBPWr, diastolic BP wrist.
Bland Altman analyses: Results and interpretation.
Blood pressure | Measurements | Limits of agreement between methods under study | Do the methods agree clinically? | ||
---|---|---|---|---|---|
Bias (95% CI) | Lower (95% CI) | Upper (95% CI) | |||
Systolic blood pressure | Pragmatic/ideal | -9.6 (-13.2 to -6.1) | -36.6 (-42.7 to -30.5) | 17.4 (11.2 to 23.5) | No |
Wrist/ideal | 7.1 (4.1 to 10.0) | -15.4 (-20.5 to -10.3) | 29.6 (24.5 to 34.7) | No | |
Pragmatic/wrist | -2.6 (-5.8 to 0.7) | -26.9 (-32.4 to -21.4) | 21.8 (16.3 to 27.3) | No | |
Diastolic blood pressure | Pragmatic/ideal | -3.0 (-5.6 to -0.4) | -22.6 (-27.0 to -18.1) | 16.6 (12.1 to 21.0) | No |
Wrist/ideal | 3.7 (1.6 to 5.7) | -11.7 (-15.1 to -8.2) | 19.0 (15.5 to 22.5) | No | |
Pragmatic/wrist | 0.7 (-1.8 to 3.2) | -18.4 (-22.7 to -14.1) | 19.8 (15.4 to 24.1) | No |
CI, confidence interval.
Interpretation based on comparison of limits of agreement to clinically-acceptable range of blood pressure (BP), within 10 mmHg for diastolic BP and 20 mmHg for systolic BP.
For systolic BP, the regression relationship was summarised as SBPMc (systolic BP, mercury) = –10.7 + 1.2 SBPPr (systolic BP, pragmatic). For agreement, the bias was 9.6 mmHg with limits of agreement of –17.4 mmHg to 36.6 mmHg. Using the bias alone, 9.6 mmHg, this would equate to excellent clinical inter-changeability based on a clinically-significant BP range of within 20 mmHg. However, the limits of agreement were too wide for the two methods to be regarded as agreeing clinically.
Bland Altman plot for (a) systolic and (b) diastolic blood pressure (BP): standard mercury compared with pragmatic BP.
For systolic BP, the corresponding regression equation was SBPMc = −2.5 + 1.0 SBPWr (systolic BP, wrist). The BA analysis showed a bias of 7.1 mmHg and limits of agreement, −15.4 mmHg (lower) and 29.6 mmHg (upper), which were outside the clinical reference range for inter-changeability, within 20 mmHg. For diastolic regression the equation was linear, with DBPMc = 10.6 + 0.7 DBPWr (diastolic BP, wrist), a sign of good positive association. The limits of agreement, −19.0 mmHg (lower) to 11.7 mmHg (upper), confirmed poor clinical agreement when compared to the clinically-acceptable range of agreement, within 10 mmHg.
Bland Altman plot for (a) systolic and (b) diastolic blood pressure (BP): standard mercury compared with wrist BP.
Finally, pragmatic BP and wrist-based standard BP were also compared for completeness. For systolic BP,
Bland Altman plot for (a) systolic and (b) diastolic blood pressure (BP): standard wrist compared with pragmatic BP.
Scores for treatment decisions (whether to start anti-hypertensive = 1; alter anti-hypertensive treatment = 2; or defer treatment = 0) were subsequently compared between decisions based on pragmatic BP and those based on standard mercury-based BP. The Kappa score was 0.7 which equates to ‘good agreement’ based on the widely-accepted Byrt's criteria (see Note under B). Overall (without stratifying), the treatment outcomes concurred in 83.8% of the cases, hence 16.7% were misclassified when compared with the standard BP. For the decision not to start treatment 78% of instances concurred; for the decision to start treatment, 90.9% agreed; and for the decision to adjust treatment, the agreement was 100%. Of the patients who were not supposed to start treatment (basing on the standard mercury-based BP), 19.5% (
Comparison of treatment decisions between pragmatic and standard blood pressure measurements.
Agreement | Expected agreement | Kappa | Lower 95% CI | Upper 95% CI | |
---|---|---|---|---|---|
83.8 | 44.2 | 0.7 | 0.5 | 0.9 | 0 |
CI, confidence interval.
Byrt criteria: Excellent agreement 0.93–1.00; very good agreement 0.81–0.92; good agreement 0.61–0.80; fair agreement 0.41–0.60; slight agreement 0.21–0.40; poor agreement 0.01–0.20; no agreement ≤ 0.00.
Contingency table for per-stratum treatment outcomes comparing pragmatic to standard blood pressure.
Treatment plan based on standard mercury blood pressure | |||||
---|---|---|---|---|---|
0 | 1 | 2 | Total | ||
32 | 0 | 0 | |||
78.10% | 0% | 0% | |||
8 | 10 | 0 | |||
19.50% | 90.90% | 0% | |||
|
1 | 1 | 8 |
|
|
2.40% | 9.10% | 100.00% | |||
|
|
|
|
|
0, no treatment; 1, treatment; 2, treatment changed.
With hypertension defined as BP 140/90 mmHg, one in five (20%) South Africans have hypertension,
The next step was a comparison of pragmatic and standard BP measurements. Health workers generally do not follow BP measurement guidelines. In a study on the BP measurement behaviour of clinicians done by Villegas et al., none of the physicians tested followed all the recommendations of the American Heart Association when measuring BP and a few recommendations were only followed by a minority of the physicians studied.
The clinical consequences of poor BP measurement are well documented in literature: consistent overestimation of diastolic BP by as little as 5 mmHg may more than double the number of patients with hypertension in a physician's practice.
In this study, 19.5% of patients who were started on anti-hypertensive therapy based on pragmatic BP actually did not need any treatment. This trend was similar to many studies which showed increased diagnosis of hypertension if BP was not measured according to guidelines.
Comparison of wrist and mercury BP measurements was subsequently performed. Standard mercury diastolic and systolic BPs were consistently higher when using a wrist device. For systolic BP, the difference was as much as 20 mmHg, whilst it was approximately 10 mmHg for diastolic BP, a sharp contrast to previous studies which found similarities between mercury and wrist devices.
The mercury sphygmomanometer is generally regarded as the gold standard against which all other devices for BP measurement should be compared.
Finally, a statistical lesson! Statistical methods for comparison methods have been subject of discussion amongst clinicians. The BA method is regarded as the gold standard.
The main strength was that this study design was fast and inexpensive and was done in a resource-limited setting approximating most third-world institutions. It gave a useful initial overview of the problem, including the community prevalence. The statistical methods used were appropriate for studies of this nature.
There was very limited potential to make causal inference of any differences, an obvious weakness of this study. Secondly, we could not claim success with minimising regression to mean with the serial BP measurements as the exact time to ensure that regression to mean is rectified, is unknown. In addition, it was impossible to totally eliminate observer bias despite ‘blinding’ the nurses as there was always room for discussion when they meet outside the study centre; hence, the ‘pragmatic’ BPs might not have been as pragmatic as we expected. The other potential confounder was that the pragmatic BP was done by different nurse practitioner. No adjustments were made for this as surely their BP measurement techniques would differ. The other problem was diagnostic on the part of nurses: the clinical decision to start treatment. Usually, a number of readings are required to start treatment unless there are risk factors, significant target organ damage or BP was markedly elevated. The nurse practitioners might have over-diagnosed hypertension as they relied erroneously on one reading, even when BP was mildly elevated.
There are differences between pragmatic and standard BP but wrist and mercury BP readings are usually comparable.
This study further confirmed the existence of differences between pragmatic and standard BP measurements in a resource-limited setting. The difference leads to 16.7% disease status misclassification. Wrist and mercury devices potentially lead to conflicting results, which is contrary to earlier studies. Pearson and Intra-class correlation coefficients are weak statistical methods in studies of this nature.
There is a difference between pragmatic and standard BP measurements which affect the decision to start treatment and the decision to initiate treatment, but not the decision regarding alteration of regime for those already on treatment. There are also marked differences between wrist- and standard mercury-based BP devices which also affect treatment decision-making. In future, when assessing agreement between clinical methods, the BA method is more conclusive than correlation coefficients. Clinicians need to revert to basic good practice and measure BP more accurately so as to avoid unnecessary additional costs and morbidity associated with incorrect treatment resulting from disease misclassification. Wrist devices need to be used with caution.
Our thanks go to University of Stellenbosch statistician, Professor Justin Harvey, for verification of statistical analyses presented in this article.
The authors declare that they have no financial or personal relationship(s) that may have inappropriately influenced them in writing this article.
G.M. (University of Stellenbosch; RSCC) conceived the study, formulated the study design, did literature research, compiled the study protocol, performed the data collection statistical analysis, originated figures and graphs and was responsible for overall manuscript design. M.P. (University of Stellenbosch) was an active academic supervisor, strongly motivated the author, edited and actively contributed relevant input to this research from conception to publication, approved the final manuscript, closely followed up with the author to meet deadlines and submitted the manuscript on behalf of author and communicated with journal. S.G. (Khayelitsha Community Health Centre) was an academic co-supervisor and authorised ethical approval. Both G.M and M.P. approved the final manuscript for publication.