Volume 195 - Issue 11
Not much need for ambulatory blood pressure monitoring
Authors: Bruce C Neal and Les Irwig
Med J Aust 2011; 195 (11): 634-635. || doi: 10.5694/mja11.11304
Published online: 12 December 2011
Published online: 12 December 2011
Do you need to “know your numbers”?
“Know your numbers” was the public catchcry on World Hypertension Day 2011, and doctors were once again admonished to measure their patients’ blood pressure levels. The National Institute for Health and Clinical Excellence (NICE) in the United Kingdom has gone one step further and now recommends that all patients in whom hypertension is suspected be offered ambulatory blood pressure monitoring to confirm the diagnosis.1 A recent report suggested this approach is cost-effective,2 although other research findings suggest that the precise measurement of blood pressure may not be very important when selecting whom to treat.3-4 Indeed, a recent report suggested that age alone might be a sufficient basis for decision making.5 What then is the role of ambulatory blood pressure monitoring in clinical practice?
The past few decades have seen huge advances in our understanding of how blood pressure causes illness. At the heart of these advances has been the recognition that blood pressure causes disease in a progressive way across a broad range of blood pressure levels, and not just when an individual crosses a diagnostic threshold for “hypertension”.1 “Hypertension” cut-points, as variously set over the past half century, are now recognised as arbitrary levels on a continuous scale; and while they were a pragmatic solution at the time, their limitations are now widely understood.6
The development of risk-based approaches to allocating blood pressure-lowering treatment has been equally important.7 Under the risk-based model, the effects of blood pressure, age, sex, blood lipid levels and the presence or absence of smoking and diabetes all play a role in the decision whether or not to treat the patient with blood pressure-lowering therapy. Further, it is the blood pressure level, not the presence or absence of hypertension, that is incorporated into the decision-making process. Compared with traditional hypertension control programs, strategies that allocate blood pressure-lowering treatment based on measures of risk are projected to avert more disease for fewer people treated and at lower cost.8
Risk-based strategies are superior in this way because they direct treatment towards those most likely to gain from therapy. A diagnosis of hypertension on its own is a poor discriminator of risk, and assessment based on multiple risk factors is much better for identifying those who will benefit.8 The discriminatory power of risk-based strategies is only marginally improved by more precise measurement of the risk factors.3 The usual day-to-day variability of blood pressure for an individual is large, which means that many, many measurements are required to define an individual’s blood pressure level with real precision.9 This is a real problem when trying to categorise a patient as above or below a hypertension cut-point, but less important when blood pressure is treated continuously and is just one of half a dozen factors influencing the treatment decision.
In deciding the potential value of ambulatory blood pressure monitoring to clinical practice, it is important to understand that showing that ambulatory blood pressure is more strongly associated with risk is not in itself sufficient. To be of real clinical value, it must also more reliably select those who will benefit from intervention. Recent economic analyses2 showed that routine ambulatory blood pressure measurement was superior compared with strategies based on home or clinic measurements because it made more reliable diagnoses of hypertension and saved treatment costs. However, the analysis did not report how ambulatory blood pressure performed compared to a risk-based approach. This is a serious shortcoming, because it is in this context that NICE makes its recommendations.8 Furthermore, in those cost-effectiveness analyses, ambulatory measurements were assumed to have perfect sensitivity and specificity for the diagnosis of hypertension, and blood pressure-lowering treatment was assumed to provide no benefit to individuals classified as non-hypertensive. Both are dubious assumptions that work substantively in support of strategies based on ambulatory measurements, and, when varied, will change the conclusions drawn.2
Making more measurements to get a better estimate of an individual’s true blood pressure is an inherently appealing concept deeply rooted in clinical practice. But as the evolving evidence shows, it’s not that simple. However precisely blood pressure is measured, it is unlikely to ever be as good a discriminator as an assessment based on multiple risk factors. And while the widespread use of ambulatory blood pressure measurement will more reliably pin down someone’s true blood pressure and better define who has hypertension, it is very unlikely to deliver the clinical and economic benefits that could be achieved by switching to a risk-based strategy.8
The National Vascular Disease Prevention Alliance has released an Australian risk assessment tool10 that supports a risk-based strategy, and management guidelines will follow. Rapid nationwide uptake of this approach to deciding who to treat has the potential to revolutionise the prevention of blood pressure-related diseases. Against this background, ambulatory blood pressure measurement will remain an excellent research tool but will add little to clinical decision making for most patients.
Competing interests
Bruce Neal receives current support or has received support within the past 5 years from a number of pharmaceutical companies marketing blood pressure lowering drugs but has no support from device manufacturers.
References
- National Institute for Health and Clinical Excellence. Hypertension: clinical management of primary hypertension in adults [clinical guideline 127]. London: NICE, 2011. http://www.nice.org.uk/CG127 (accessed Nov 2011).
- Lovibond K, Jowett S, Barton P, et al. Cost-effectiveness of options for the diagnosis of high blood pressure in primary care: a modelling study. Lancet 2011; 378: 1219-1230. 0_i1139857
- Bell K, Hayen A, McGeechan K, et al. Effects of additional blood pressure and lipid measurements on the prediction of cardiovascular risk. Eur J Cardiovasc Prev Rehabil 2011; Sep 26. [Epub ahead of print.] 0_i1139859
- Law M, Morris J, Wald N. Use of blood pressure lowering drugs in the prevention of cardiovascular disease: meta-analysis of 147 randomised trials in the context of expectations from prospective epidemiological studies. BMJ 2009; 338: b1665. doi: 10.1136/bmj.b1665. 0_i1139861
- Wald N, Simmonds M, Morris J. Screening for future cardiovascular disease using age alone compared with multiple risk factors and age. PLoS One 2011; 6: e18742. 0_i1139863
- Vickers A, Basch E, Kattan M. Against diagnosis. Ann Intern Med 2008; 149: 200-203. 0_i1139865
- Jackson R. Updated New Zealand cardiovascular disease risk-benefit prediction guide. BMJ 2000; 320: 709-710. 0_i1139867
- Gaziano TA, Steyn K, Cohen DJ, et al. Cost-effectiveness analysis of hypertension guidelines in South Africa. Absolute risk versus blood pressure level. Circulation 2005; 112: 3569-3576. 0_i1139869
- Powers BJ, Olsen MK, Smith VA, et al. Measuring blood pressure for decision making and quality reporting: where and how many measures? Ann Intern Med 2011; 154: 781-788. 0_i1139873
- National Vascular Disease Prevention Alliance. Guidelines for the assessment of absolute cardiovascualr disease risk. Melbourne: National Heart Foundation of Australia, 2009. http://www.heartfoundation.org.au/information-for-professionals/Clinical-Information/Pages/absolute-risk.aspx (accessed Nov 2011).
Provenance: Not commissioned; externally peer reviewed.