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27 September 2011
A small study1 in 83 patients has found that serial, automated blood pressure (BP) measurement over 30 minutes, with the patient left alone in the examination room, resulted in lower readings than gold standard office-based BP measurement. Thirty minute BP measurements also appeared to be more reproducible. However, more studies are necessary before this method is widely used.
Level of evidence:
Level 3 (other evidence) according to the SORT criteria.
Action
Healthcare professionals should follow NICE guidance on the management of hypertension, which was updated in August 2011. This now advises that ambulatory BP monitoring (ABPM) should be used to confirm a diagnosis of hypertension if the office-based BP measurement is 140/90mmHg or higher on two or more readings. Home BP monitoring is an alternative for those who can’t tolerate ABPM. Ambulatory or home BP monitoring may also be used as an adjunct to office-based BP measurement to monitor the response to antihypertensive treatment in people identified as having a ‘white coat effect’. Although the results of this small study suggest that 30 minute office-based BP measurement could potentially reduce measurement bias and ‘white coat effect’, and the need for ambulatory or home BP monitoring, more information is needed before existing recommendations are changed.
Active plans need to be made locally to move practice towards the new NICE hypertension guidance. During this transitional period, and until arrangements to introduce ambulatory and/or home BP monitoring have been put in place, it seems reasonable to either continue to follow the previous, 2006 guidance and ask patients (who do not have signs/symptoms of malignant hypertension) to return at least twice for further measurement in clinic, normally at monthly intervals. In future, if more data becomes available, thirty minute office-based BP measurement may become a valid alternative for those patients who are unsuitable for ambulatory and home BP measurement.
What is the background to this?
Many organisations, such as NICE, BHS and MHRA have provided detailed guidance on obtaining accurate BP measurements. In spite of this, BP measurement is rarely carried out to the appropriate standard in everyday practice, generally resulting in overestimation of BP. Also, the ‘white coat effect’ contributes to unreliable readings in up to 20% of patients. Overestimation of BP is likely to lead to unnecessary treatment with avoidable adverse effects and cost.
This study 1 compared serial, automated BP measurement over 30 minutes, without a doctor or nurse present in the examination room, with gold standard office-based BP measurement in 83 adult patients in two general practices in the Netherlands. BP was measured by two trained researchers using the same validated device for both methods of BP measurement. The level of agreement between the methods was compared to determine whether 30 minute BP measurement could potentially reduce measurement bias and ‘white coat effect’, and the procedure was repeated after two weeks to assess repeatability.
What does this study claim?
Mean 30 minute BP measurement readings were significantly lower than gold standard office-based BP measurement readings, with a mean (absolute) difference of 7.6/2.5mmHg. They also appear to be more reproducible: the mean difference between the first and second visit of 30 minute BP measurement readings was about half the mean difference with gold standard BP measurement (3.0/1.0mmHg vs. 6.0/2.3mmHg, respectively). However, the confidence intervals overlap meaning the difference between the two methods may not be statistically significant.
So what?
This small study1 suggests that 30 minute BP measurement may reflect patients’ true BP better than gold standard office-based BP measurement and may have the potential to reduce measurement bias and ‘white coat effect’ in GP practices. However, more research is necessary to confirm its place in practice. Although another small study (n=84) by the same author concluded that 30 minute BP measurement appears to agree well with daytime ABPM, more evidence is needed comparing these two methods. In addition, it is not currently clear, for example, how 30 minute BP measurement compares with home BP monitoring, whether 30 minutes is the optimal duration of measurement, what the reference BP values should be, or whether it is cost-effective.
NICE guidance on hypertension advises that ABPM should be undertaken to confirm a diagnosis of hypertension if office-based BP is 140/90 mmHg or higher on two or more readings. If a person is unable to tolerate ABPM, NICE states that home BP monitoring is a suitable alternative. NICE also advises that ambulatory or home BP monitoring may be used as an adjunct to office-based BP measurement to monitor the response to antihypertensive treatment in people identified as having a ‘white coat effect’.
The study authors suggest that ambulatory and home BP monitoring have some drawbacks. For example, ABPM is not very patient friendly and is associated with disturbed sleep, and home BP monitoring may be inaccurate because of poor technique and report bias. In future, if more data supports it, 30 minute BP measurement may be a viable alternative for confirming a diagnosis of hypertension in some patients for whom other methods are unsuitable.
This study used a common ambulatory device to record serial automated BP measurements, which many practices are likely to own. Although it requires some organisation and a spare examination room, the study authors suggest that 30 minute BP measurement takes less time from a healthcare professional than the 8–12 minutes required for gold standard BP measurement. However, the practice setting, may still contribute to the ‘white coat effect’.
- Scherpbier-de Haan N, et al. Thirty-minute compared to standardised office blood pressure measurement in general practice. Br J Gen Pract 2011;61:e590–597 (subscription needed to view paper)
Study details
Design: Method comparison study in two general practices in the Netherlands.
Patients: 105 patients agreed to participate in the study. However, 22 were excluded from the analysis, mainly because not all the serial BP measurements were valid (which may have skewed the results in favour of 30 minute BP measurement) or due to medication changes between visits. Mean age 62.1 years, 61% female, mean BP last noted in GP records 152.8/82.0mmHg, 83% on antihypertensive drugs.
Intervention and comparison: BP was measured by two trained researchers following a protocol and using the same type of validated device for both methods of BP measurement. During the first visit, gold standard office-based BP measurement was carried out after a five minute rest period in the absence of the observer. The measurement consisted of three readings. Thirty minute BP measurement followed immediately afterwards and consisted of 11 measurements, recorded automatically every three minutes; 10 in the absence of the researcher. The first measurement obtained by each method was discarded. After 2 weeks, the measurements were repeated by the same researcher in the same room at the same time of the day. To assess whether the measurement order influenced the results, an additional gold standard BP measurement was performed after the second 30 minute BP measurement.
Outcomes and results: At the first visit, mean 30 minute BP measurement readings were 7.6/2.5mmHg (95% confidence interval [CI] 6.1 to 9.1/1.5 to 3.4mmHg) lower than gold standard office-based BP measurement readings. A difference of 5mmHg or more was considered to be clinically relevant. The difference was less pronounced at the second visit (4.6/1.2mmHg, 95% CI 3.2 to 6.1/0.3 to 2.0mmHg).
The mean difference between 30 minute BP measurement readings at the first and second visits (mean difference 3.0/1.0mmHg, 95% CI 0.9 to 5.1/–0.1 to 2.2mmHg) was lower than that of gold standard BP measurement readings (mean difference 6.0/2.3mmHg, 95% CI 3.6 to 8.3/1.0 to 3.7mmHg). However, the confidence intervals overlap meaning the difference may not be statistically significant.
The order of measurement did not influence the results.
Sponsorship: Not funded.
Further information can be found on NHS Evidence and in the Cardiovascular disease – Hypertension e-learning section of the NPC website.
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