Systolic blood pressure and mortality

Background The current systolic blood-pressure threshold for hypertension treatment is 140 mm Hg for all adults. WHO and the International Society of Hypertension have proposed that normal pressure be lower than 130 mm Hg, with an optimum pressure of less than 120 mm Hg. These recommendations are based largely on the assumption that cardiovascular and overall mortality depend in a strictly increasing manner on systolic blood pressure. The Framingham study was instrumental in establishing this viewpoint. We reassessed data from that study to find out whether the relation is strictly increasing or whether there is a threshold in this relation. Methods We used logistic splines to model the relation of risk of cardiovascular and all-cause death with systolic blood pressure, using age-specific and sex-specific rates. We tested for the independence of the slope parameters from age and sex, and the reduced model with common slopes was used to produce a model different from the conventional linear logistic model. Findings Against the predictions of the linear logistic model, neither all-cause nor cardiovascular deaths depended on systolic blood pressure in a strictly increasing manner. The linear logistic model was rejected by the Framingham data. Instead, risk was independent of systolic blood pressure for all pressures lower than a threshold at the 70th percentile for a person of a given age and sex. Risk sharply increased with pressure higher than the 80th percentile. Since systolic blood pressure steadily increases with age, the threshold increases with age, but more rapidly in women than in men. Interpretation The Framingham data contradict the concept that lower pressures imply lower risk and the idea that 140 mm Hg is a useful cut-off value for hypertension for all adults. There is an age-dependent and sex-dependent threshold for hypertension. A substantial proportion of the population who would currently be thought to be at increased risk are, therefore, at no increased risk.