We used data from the CHNS study, which was a nationally representative prospective cohort in nine provinces around China, enabling us to provide a general method to estimate the association of BP and risk of stroke for the Chinese population. In this study, increasing SBP was an independent risk factor and positively associated with overall stroke, ischemic stroke, and hemorrhagic stroke. The risks of ischemic and hemorrhagic stroke were higher when SBP was more than 140 mmHg and 160 mmHg(Table 2, Table 3 and Table 4). Multivariable adjusted restricted cubic spline analyses showed linear association of SBP with overall, ischemic and hemorrhagic stroke (P < 0.001, P < 0.001 and P = 0.022).
A previous study proved that rising SBP and DBP levels were associated with the development of CVD[13]. Higher SBP level was positively associated with increased risk of CVD with adjustment or stratification for DBP[14]. By contrast, another study demonstrated that DBP was not consistently associated with the risk of CVD, after adjusting or stratification for SBP [15][16]. Similar to previous results, our findings showed that SBP and DBP were positively associated with stroke in the multivariate model(model 1 and model 2). However, in a mutually adjusted model(model 3) including both SBP and DBP, the association with the risk of overall stroke was positive for SBP, but not significant for DBP(Table 2, Table 3 and Table 4).
In our study, the relation of SBP to the risk of overall stroke is continuous and graded and no critical value has been observed(Table 2, Fig. 1). Consistently, in another retrospective study with 1.25 million patients[17], a dose-response association for SBP with overall stroke had also been observed. Further, in a low-income population in China, significant linear associations of SBP to the risk of overall stroke had also been reported in a prospective cohort study[18]. In addition, a meta-analysis involved nearly 1 million participants and 56,000 vascular deaths from 61 prospective studies has also confirmed the continued and graded impact of BP on CVD risk[7]. This prospective research observed that BP was associated with vascular mortality, without any clear indication of a threshold. In the entire BP range, every 20 mmHg increase in SBP (or 10 mm Hg increase in DBP) will double the risk of stroke or coronary heart disease for people between 40 and 69 years of age. Furthermore, findings from the Framingham study further suggested that there is a continuous, graded association of SBP to the rate of development of CVD at all ages, without any indication of a threshold was observed in any group from age 35 to 84 years[19]. Other studies have also reported similar linear associations of BP to coronary heart disease (CHD) and all-cause mortality [20][21].
In this study, compared to the reference group (SBP < 130 mmHg ), the ischemic and hemorrhagic stroke risks were significantly increased among those with SBP values ≥ 140 mmHg and ≥ 160 mmHg, after adjusting other conventional CVD risk factors(model 2 in Table 3 and Table 4). The ideal BP cut-off values for decreasing stroke risk were SBP values ≤ 140 mmHg and ≤ 160 mmHg for ischemic stroke and hemorrhagic stroke. These results were similar to a prospective cohort study[18], showing that stroke risk significantly increased among individuals with SBP levels ≥ 140 mmHg (for ischemic stroke) and ≥ 160 mmHg (for hemorrhagic stroke), compared with those with SBP levels < 130 mmHg. Further, in another retrospective study with 1.25 million patients[17], for SBP levels ≥ 160 mmHg, SBP was more strongly associated with hemorrhagic stroke than ischaemic stroke. The stroke subtypes for the association of BP and stroke risk needs to be demonstrated in further studies.
We found no significant age, sex and BMI interaction for the associations of SBP with ischemic stroke, hemorrhagic stroke, and overall stroke(Fig. 2). In sensitivity analysis, the shape of BP-stroke risk relation after winsorization of SBP and DBP at the 1st/99th did not change substantially (Figure s3 in appendix).
Our study has several limitations. First, we only screen residents older than 40 and younger than 90 years old from nine provinces of China. Therefore, our current results cannot be generalized to all populations in China. Second, the long-term, > 6 years, incidence of stroke for BP is unknown. In future studies, we project to prospective follow-up participants, gather data about mortality and incidence of stroke in participants. Finally, we did not collect information on usage of BP-lowering medication or treatment at baseline for participants from CHNS data.