Blood pressure and stroke pathological types in China: an analysis of 500,000 men and women in the China Kadoorie Biobank study
Keywords: prospective cohort
stroke
epidemiology
blood pressure
china

Background: Stroke is a leading cause of disability and premature death in China and blood pressure is widely considered to be a major cause. Despite this, substantial uncertainty remains about the shape and strength of the association between blood pressure and stroke pathological types in China.
Methods: Information from the China Kadoorie Biobank study (a prospective cohort study of 0.5 million men and women in China recruited during 2004-8) was used to relate usual blood pressure to risk of stroke, by stroke pathological type (cerebral infarction [ischaemic stroke], intracerebral haemorrhage and subarachnoid haemorrhage). Prospective analyses excluded participants with a history of vascular disease recorded at baseline; involved correction for regression dilution bias; used incident stroke events for which the diagnosis involved a head CT or MRI scan; and, assessed for confounding and effect modification by major vascular risk factors. These prospective analyses were informed by a set of prior analyses, including: a description of baseline associations between blood pressure and other vascular risk factors, to identify potential confounders; analyses of resurvey blood pressure data from ~20_000 participants, to assess regression dilution bias; and analyses of stroke follow-up data, involving an adjudication ‘sub-study’ performed specifically as part of this thesis, to evaluate the diagnostic accuracy of incident stroke events (~1000 events were adjudicated).
Results: During 2.1 million person-years at risk, there were 5783 incident stroke events. At ages 40-79 years, the proportional difference in risk of both cerebral infarction and intracerebral haemorrhage associated with a given absolute difference in usual blood pressure was constant throughout the range of blood pressures examined (SBP 120-170 mm Hg, DBP 70-100 mm Hg). Overall, the strength of association was approximately 1.5-times greater for intracerebral haemorrhage than for the other stroke pathological types: 10 mm Hg higher usual SBP was associated with 82% (95% CI: 76%-89%) higher risk of intracerebral haemorrhage, 47% (44%- 50%) higher risk of cerebral infarction and 52% (35%-71%) higher risk of subarachnoid haemorrhage (the overall mean age at event for each stroke pathological type was ~60 years). For both cerebral infarction and intracerebral haemorrhage, there was strong evidence of major effect modification by age and to a lesser extent by a number of other vascular risk factors. The associations by age were around a third as extreme at age 70-79 years than at 40-49 years. The annual absolute differences in risk associated with a given absolute increase in usual blood pressure, however, were greater at older age.
Conclusions: In Chinese adults, usual blood pressure was strongly and positively related to risk of all stroke pathological types. The strength of association was greater for intracerebral haemorrhage than other stroke pathological types. For both cerebral infarction and intracerebral haemorrhage, there was evidence of major effect modification by age. The overall effect of blood pressure on stroke risk was much greater than estimated by previous prospective studies in China, particularly for intracerebral haemorrhage.