Systolic blood pressure and 6-year mortality in South Africa: a country-wide, population-based cohort study

Abstract

Summary: Background: Improving hypertension control is an important global health priority, yet, to our knowledge, there is no direct evidence on the relationship between blood pressure and mortality in sub-Saharan Africa. We aimed to investigate the relationship between systolic blood pressure and mortality in South Africa and to assess the comparative effectiveness of different systolic blood pressure targets for clinical care and population-wide hypertension management efforts. Methods: In this country-wide, population-based cohort study, we used longitudinal data on adults aged 30 years and older from five waves (2008, 2010–11, 2012, 2014–15, and 2017) of the South African National Income Dynamics Study. We estimated the relationship between systolic blood pressure and 6-year all-cause mortality and compared the mortality reductions associated with lowering systolic blood pressure to different targets (120 mm Hg, 130 mm Hg, 140 mm Hg, 150 mm Hg). We also estimated the mean blood pressure reduction required to achieve each target, the share of the population in need of management, and the number needed to treat (NNT) to avert one death under different hypothetical population-wide scale-up scenarios. Findings: Of the 8338 age-eligible respondents in the 2010–11 survey, 4993 had all required data and were included in our study. We found a weak, non-linear relationship between systolic blood pressure and 6-year mortality, with larger incremental mortality benefits at higher systolic blood pressure values: reducing systolic blood pressure from 160 mm Hg to 150 mm Hg was associated with a relative risk of mortality of 0·95 (95% CI 0·90 to 0·99; p=0·033), reducing systolic blood pressure from 150 mm Hg to 140 mm Hg had a relative risk of 0·96 (0·91 to 1·01; p=0·12), with no evidence of incremental benefits of reducing systolic blood pressure below 140 mm Hg. At the population level, reducing systolic blood pressure to 150 mm Hg among all those with a starting systolic blood pressure of more than 150 mm Hg was associated with the lowest NNT (n=50), 3·3 deaths averted (95% CI −0·6 to 0·3) per 1000 population, blood pressure management for 16% (95% CI 15·2 to 17·3) of individuals, and a −2·7 mm Hg mean change in systolic blood pressure required to achieve the 150 mm Hg scale-up target (−3·0 to −2·5; p<0·0001). Interpretation: The relationship between systolic blood pressure and mortality is weaker in South Africa than in high-income and many low-income and middle-income countries. As such, we do not find compelling evidence in support of targets below 140 mm Hg and find that scaling up management based on a 150 mm Hg target is more efficient in terms of the NNT compared with strategies to reduce systolic blood pressure to lower values. Funding: None