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

Original source found here: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3138604/
J Gen Intern Med. 2011 Jul; 26(7): 685–690.
Published online 2011 Mar 15. doi: 10.1007/s11606-011-1660-6
PMCID: PMC3138604
Brent C. Taylor, PhD, MPH, 1,2,3 Timothy J. Wilt, MD, MPH,1,2 and H. Gilbert Welch, MD, MPH4,5

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.

COMMENT
Our examination of the independent effects of diastolic and systolic blood pressure on mortality confirms a central tenet of the Seventh Report of the Joint National Committee on Prevention, Detection, Evaluation and Treatment of High Blood Pressure (JNC 7): systolic blood pressure elevations are more important than diastolic blood pressure elevations in persons over age 50. In fact, in our data, diastolic blood pressures are largely irrelevant in this age group. The situation was reversed in persons age 50 and younger: in whom diastolic blood pressure was the more important predictor of mortality.
Our analysis was also directed at a broader question that we hope JNC 8 will consider in its ongoing deliberations: What is the impact of various definitions of normal blood pressure? The current definition of normal is less than 120/80. Our analysis offers one possible alternative definition: a blood pressure that does not confer an increased mortality risk in a cohort of over 10,000 individuals followed for nearly 20 years. From our data this would mean that abnormal for individuals over age 50 would be a SBP of ≥140 (independent of DBP), and for individuals less than 50, a DBP ≥100 or a SBP ≥200. While it should not be viewed as the final word on this topic, we hope it serves as an example of an alternative approach. If nothing else, our findings highlight that the choice about the approach used to define normal blood pressure will impact literally millions of Americans.