Is the Relation of Systolic Blood Pressure to Risk of Cardiovascular Disease Continuous and Graded, or Are There Critical Values?

Abstract

Because epidemiologic data indicate a continuous incremental risk of cardiovascular disease (CVD) in relation to blood pressure (BP), hypertension treatment guidelines now recommend treating high-normal and mild BP elevations. 1 Port and coworkers, 2 analyzing Framingham Study data, challenge this assertion. They claim that Framingham data in actuality "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." They suggest that there is an age-and sex-dependent threshold for "hypertension" and that a substantial proportion of the population currently deemed at increased risk are, in fact, not.Their conclusion derives from the use of "logistic splines" methodology to examine the relation of systolic blood pressure (SBP) to CVD and all-cause mortality. They contend that previous linear logistic analysis depicting a continuous, graded relation is misleading and that there is actually a threshold at the 70th percentile of SBP for a person at a given age and sex. They also suggest that because BP increases steadily with age, the threshold also increases with age. 2 By inference, Port et al seem to suggest that we to return to the discarded concept that a "normal" systolic pressure is, roughly, "100ϩ years of age mm Hg." They focus on all-cause mortality because "it is most free of misclassifications and, importantly the number of events is sufficiently high to allow accurate estimates of the shape of the relation with systolic blood pressure." However, the more relevant outcome is CVD mortality and the CVD events promoted by hypertension unconfounded by non-CVD mortality, which could be associated with low BP. Furthermore, even a focus on CVD mortality, excluding the large majority of nonfatal events, needlessly reduces the amount of data on which to determine the shape of the BP-CVD incidence curve. Normal BP for AgeA review of the evidence for a continuous, graded influence of BP on the incidence and mortality of CVD appears to be in order. The first consideration is the issue of what constitutes a "normal" BP. It is evident that in most developed countries, SBP rises with age. 3 Diastolic blood pressure (DBP) rises with age until age 55 years, after which it declines in both sexes. 4 However, in some less-developed countries where obesity and high salt intake are uncommon, adults experience little increase in BP with age. 3 It is well established that the disproportionate rise in SBP relative to DBP with age is a consequence of a pathologic reduction in arterial compliance: as elastic fibers fragment, the elastic lamellae disrupt, and the collagen-to-elastin ratio in the arterial wall is altered. 5 Usual Versus Optimal BPThe current concept of "desirable" BP is based not on what BP level is "usually" but rather on what is "optimal" (ie, associated with a low rate of hypertension-related CVD). It is clear from the Framingham Study and other prospective, epidemiologic data that at all ages, the risk of developing a CVD event i...

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