General practice

Case-control study of stroke and the quality of hypertension control in north west England

^a School of Epidemiology and Health Sciences, University of Manchester Medical School, Manchester M13 9PT, ^b Department of Computation, University of Manchester, Institute of Sciences and Technology, Manchester M60 1QD, ^c Department of Public Health, East Lancashire Health Authority, Nelson, Lancashire BB9 5SZ, ^d Department of Medicine for the Elderly, Queen's Park Hospital, Blackburn, Lancashire BB2 3HH, ^e The Surgery, Earby, Lancashire BB8 6QT

Correspondence to: Dr Du or Dr Cruickshank clinep@man.ac.uk

Objective: To examine the risk of stroke in relation to quality of hypertension control in routine general practice across an entire health district.
Design: Population based matched case-control study.
Setting: East Lancashire Health District with a participating population of 388 821 aged 80.
Subjects: Cases were patients under 80 with their first stroke identified from a population based stroke register between 1 July 1994 and 30 June 1995. For each case two controls matched with the case for age and sex were selected from the same practice register. Hypertension was defined as systolic blood pressure 160 mm Hg or diastolic blood pressure 95 mm Hg, or both, on at least two occasions within any three month period or any history of treatment with antihypertensive drugs.
Main outcome measures: Prevalence of hypertension and quality of control of hypertension (assessed by using the mean blood pressure recorded before stroke) and odds ratios of stroke (derived from conditional logistic regression).
Results: Records of 267 cases and 534 controls were examined; 61% and 42% of these subjects respectively were hypertensive. Compared with non-hypertensive subjects hypertensive patients receiving treatment whose average pre-event systolic blood pressure was controlled to <140 mm Hg had an adjusted odds ratio for stroke of 1.3 (95% confidence interval 0.6 to 2.7). Those fairly well controlled (140-149 mm Hg), moderately controlled (150-159 mm Hg), or poorly controlled (160 mm Hg) or untreated had progressively raised odds ratios of 1.6, 2.2, 3.2, and 3.5 respectively. Results for diastolic pressure were similar; both were independent of initial pressures before treatment. Around 21% of strokes were thus attributable to inadequate control with treatment, or 46 first events yearly per 100 000 population aged 40-79.
Conclusions: Risk of stroke was clearly related to quality of control of blood pressure with treatment. In routine practice consistent control of blood pressure to below 150/90 mm Hg seems to be required for optimal stroke prevention.

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