What constitutes controlled hypertension? Patient based comparison of hypertension guidelines

Introduction

Treatment of hypertension has been shown to reduce the relative risk of cardiovascular mortality and morbidity by 30%.1 National experts have produced recommendations on detecting and treating hypertension.2 3 4 5 6 However, despite the wealth of evidence from randomised controlled trials these guidelines differ both in content and in emphasis.7

Recently, more emphasis has been put on targeting treatment at people who have a higher absolute risk of cardiovascular disease (box 1).8 A recent systematic review showed that 58 elderly people need to be treated for five years to prevent one cardiovascular death compared with 205 people in middle age.9 Similarly, history of a cardiovascular event, presence of diabetes, left ventricular hypertrophy, and continued smoking increase absolute risk from 5.4% to 32.9% over 10 years.10 Guidelines on managing hypertension have tried to reflect this variation in risk. The attempts vary between an explicit risk matrix according to age, sex, blood pressure, and the presence of major and minor risk factors2 to a guideline mentioning that “several factors other than diastolic and systolic blood pressure…may influence the decision to begin drug treatment” followed by a list of the factors that should be taken into account.5

Population studies on detection and control of hypertension in middle aged and elderly people have used blood pressure alone for assessment.12 13 Since the risk factors for a cardiovascular event and their emphasis differ between guidelines7 we used data from hypertensive patients in primary care to investigate the agreement between five sets of guidelines2 3 4 5 6 on the definition of controlled hypertension. We also analysed how well this definition reflected absolute risk, as set out in the New Zealand guidelines.2 14

Patients and methods

We collected data on patient morbidity from 18 of the 20 Oxfordshire practices who subscribe to the VAMP (Value Added Medical Products) computer system. A systematic sample of 50 patients was obtained by taking every 10th person from the practices' hypertension register. To be defined as hypertensive for the purposes of this study patients had to have hypertension clearly marked in their record in the computer database and be taking drugs to lower blood pressure at the time of the study.

We recorded the patients' age, sex, blood pressure (averaged over three most recent readings, when present), and the presence of major and minor risk factors (box 2).2 Patients were defined as having “controlled” or “uncontrolled” hypertension in accordance with the recommendations of each set of hypertension guidelines. Although the New Zealand guidelines contain no explicit statement about the reduction in absolute risk to prevent a cardiovascular event over the next 10 years, they suggest that treatment should be started when absolute risk levels exceed 20%.2 Thus we took an absolute risk of 20% or above as uncontrolled hypertension and below 20% as controlled hypertension for the purposes of this study. This level of risk equates to 150 people requiring treatment for one year to prevent a single cardiovascular event.2 For the other guidelines uncontrolled hypertension was defined as the blood pressure above which the guideline recommends starting treatment (box 2).

The proportions of patients classified as having controlled hypertension by each of the five guidelines were estimated with and without taking into account the different sampling fractions across the practices.15 In addition, 95% confidence intervals were calculated before and after adjustment for unequal sampling fractions and the clustering effects of sampling individuals through the practices—the second of these corrections requires the calculation of intrapractice correlation.16 17

Although the New Zealand guidelines are more time consuming and difficult to use,7 they give a clearer absolute risk than the others. We therefore took these guidelines as the standard for analysis of sensitivity, specificity, and positive and negative predictive values of the other guidelines. All calculations were performed with Microsoft Excel, Minitab for Windows, and SPSS for Windows.

Results

We collected data on 895 patients selected from the 18 (90%) practices that gave us permission and provided a hypertension register. Nineteen (2.1%) of these patients had no blood pressure record on the database and were excluded from further analysis. Table 1 shows the proportions of the remaining 876 patients with controlled hypertension according to the five sets of guidelines. The complex adjustments necessitated by the sampling method had little effect on the results. We therefore did not use these adjustments for the more detailed analyses which follow. The intrapractice correlation coefficients, which represent the inefficiency of the cluster sampling method compared with the (logistically much more difficult) simple random sampling approach, were similar for all the guidelines (table 1).

The guidelines agreed on the diagnosis of controlled hypertension for 18% (159) of patients and on uncontrolled hypertension for 13% (118) (table 2). The agreement on uncontrolled hypertension would be raised by another 19% (169) if the Canadian guidelines were excluded.

The potential for overtreatment and undertreatment becomes explicit when the other four guidelines are compared with those from New Zealand (table 3). The Canadian guidelines have a high threshold for uncontrolled hypertension and so a high proportion of patients are classified as controlled with this guideline (table 1). However, table 3 shows that 31% (238) of those with controlled hypertension by the Canadian standard have an absolute risk above 20% over 10 years (uncontrolled by the New Zealand standard). Conversely, with the United States guidelines half of the patients defined as uncontrolled (359) have an absolute risk under 20% over 10 years by the New Zealand guidelines. Guidelines from Britain and the World Health Organisation lie between these two extremes. The overall misclassification of patients (combination of those inappropriately classified as having uncontrolled and controlled hypertension compared with the New Zealand standard) was 41% for the United States guidelines, 35% for the WHO guidelines, 27% for the Canadian, and 25% for the British.

Discussion

Our study confirms that the emphasis and consistency of the hypertension guidelines differ.8 Furthermore, we have quantified control of hypertension and shown that the definition varies widely. This variation has a large effect when using the hypertension guidelines to manage patients.

Our study shows that the definition of controlled hypertension and thus the threshold for treatment varies considerably among the guidelines. The New Zealand guidelines allow the absolute level of risk to be estimated because they give the baseline risk and the consequent benefits of treating hypertension in different categories of patients. These guidelines do, however, require a great deal of information for each patient, which may prove to be a barrier to implementation.7 18 The other consequence of using the New Zealand guidelines is that, given their sensitivity to age, a greater proportion of elderly patients are likely to become candidates for treatment.

Nevertheless, if risk factors are not quantified and taken into account many people will have their hypertension misclassified and the risks and benefits of treatment cannot be clearly outlined for each patient. Patients with a low absolute risk may be exposed to the potential side effects of treatment for little or no therapeutic benefit.19 Conversely, those with a high absolute risk who are misclassified as having controlled hypertension will have a higher chance of a cardiovascular event than is necessary.