Impact of Diastolic and Systolic Blood Pressure on Mortality: Implications for the Definition of “Normal”

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Abstract
BACKGROUND The National Heart, Lung and Blood Institute currently defines a blood pressure under 120/80 as “normal.” OBJECTIVE To examine the independent effects of diastolic (DBP) and systolic blood pressure (SBP) on mortality and to estimate the number of Americans affected by accounting for these effects in the definition of “normal.” DESIGN, PARTICIPANTS AND MEASURES Data on adults (age 25–75) collected in the early 1970s in the first National Health and Nutrition Examination Survey were linked to vital status data through 1992 (N = 13,792) to model the relationship between blood pressure and mortality rate adjusting for age, sex, race, smoking status, BMI, cholesterol, education and income. To estimate the number of Americans in each blood pressure category, nationally representative data collected in the early 1960s (as a proxy for the underlying distribution of untreated blood pressure) were combined with 2008 population estimates from the US Census. RESULTS The mortality rate for individuals over age 50 began to increase in a stepwise fashion with increasing DBP levels of over 90. However, adjusting for SBP made the relationship disappear. For individuals over 50, the mortality rate began to significantly increase at a SBP ≥140 independent of DBP. In individuals ≤50 years of age, the situation was reversed; DBP was the more important predictor of mortality. Using these data to redefine a normal blood pressure as one that does not confer an increased mortality risk would reduce the number of American adults currently labeled as abnormal by about 100 million. CONCLUSIONS DBP provides relatively little independent mortality risk information in adults over 50, but is an important predictor of mortality in younger adults. Conversely, SBP is more important in older adults than in younger adults. Accounting for these relationships in the definition of normal would avoid unnecessarily labeling millions of Americans as abnormal.

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