HEMATOCRIT, HYPERTENSION AND RISK OF STROKE

Objectives, To assess the relationship between haematocrit and risk of stroke. Design. Prospective study of a cohort of men followed up for 9.5 years. Setting. General practices in 24 towns in England, Scotland and Wales (British Regional Heart Study). Subjects. A total of 7735 men aged 40-59 years at screening, selected at random from one general practice in each of 24 towns. Main outcome measures. Fatal and non-fatal strokes. Results. During a follow-up period of 9.5 years for all men there were 123 stroke events (33 fatal) in the 7346 men in whom the haematocrit level had been determined. In the cohort as a whole, risk of stroke was significantly raised at haematocrit levels greater than or equal to 51% (relative risk [RR] = 2.5; 95% confidence intervals [CI] 1.2-5.0) after adjustment for age, social class, smelting, body mass index, physical activity, presence of diabetes and pre-existing ischaemic heart disease. Further adjustment for systolic blood pressure did not attenuate this association (RR = 2.4; 95% CI 1.2-4.9). A raised haematocrit was associated with an increase of stroke only in men with hypertension (systolic blood pressure greater than or equal to 160 mmHg or diastolic blood pressure greater than or equal to 90 mmHg or on regular antihypertensive treatment). No increased risk of stroke was seen at the higher haematocrit level (greater than or equal to 51%) in normotensive men. At haematocrit levels below 51%, hypertension was associated with a three-fold increase in risk of stroke compared with normotension (RR = 3.4, 95% CI 2.3, 5.1). At haematocrit levels greater than or equal to 51%, hypertension was associated with a nine-fold increase in risk of stroke compared with normotension (RR = 9.3; 95% CI 4.2, 21.0). Exclusion of men receiving regular antihypertensive therapy did not alter the strong associations seen. Conclusion. The data suggest that an elevated haematocrit is an independent risk factor for stroke and that it interacts synergistically with elevated blood pressure.