Viewpoint: How Low Should Blood Pressure Go?

Charles P. Vega, MD, FAAFP


October 07, 2005

Prehypertension and Cardiovascular Morbidity

Liszka HA, Mainous AG, King DE, Everett CJ, Egan BM
Ann Fam Med. 2005;3:294-299

The Seventh Report of the Joint National Commission of Prevention, Detection, Evaluation, and Treatment of High Blood Pressure (JNC 7) not only challenged physicians to control the blood pressure of patients with hypertension, but it also suggested that individuals with prehypertension (systolic blood pressure 120-139 mmHg or diastolic blood pressure 80-90 mmHg) should be identified in order to prevent progression to overt hypertension and, possibly, adverse cardiovascular outcomes.[1] In an analysis of patients from the Framingham study, rates of progression to hypertension over 4 years were 5.3% and 16% in adults over and under age 65, respectively, with blood pressure < 120/80. However, corresponding rates of progression to hypertension among subjects with prehypertension were 37.3% and 49.5%, respectively.[2]

Whether prehypertension in and of itself leads to worse cardiovascular outcomes is open to debate. One study examining data from the second National Health and Nutrition Examination Survey 1976 to 1980 (NHANES II) demonstrated that individuals with prehypertension had an elevated hazard ratio of 1.27 for all-cause mortality and 1.66 for cardiovascular disease compared with normotensive subjects.[3] However, after adjustment for other cardiovascular risk factors, the individual risk associated with prehypertension was not significant. This research stands in contrast to an analysis of patients without hypertension but with blood pressure 130-139/85-89 who participated in the Framingham study.[4] In this study, prehypertension was associated with a risk-factor-adjusted hazard ratio for cardiovascular disease of 2.5 in women and 1.6 in men when compared with normal blood pressure.

The current study uses data available from NHANES I and a follow-up period of 18 years, which represents a reasonable amount of time to diagnose trends in cardiovascular disease. The study also included many participants, and the time period examined (1971-1975) allowed researchers to gather data on the outcomes of prehypertension prior to any recommendations for treatment of these levels of blood pressure.

The study suffers from a few deficits in its data collection and methodology, however. Like all cohort studies, it is limited by possibly incomplete data and varying length of follow-up. Data were further limited by relying on only 1 blood pressure reading at baseline. Finally, although the authors attempted to adjust for other cardiovascular risk factors, their statistical adjustment was limited in 2 ways. First, only data on total cholesterol were available, meaning that fractionating the cholesterol values to give a more accurate picture of cardiovascular risk was not possible. Also, certain risk factors now established for cardiovascular disease, including chronic kidney disease, retinopathy, and family history of premature coronary artery disease, were not assessed during the initial patient evaluation.

Nonetheless, this study adds to the body of literature describing the risks of prehypertension. It is the first to differentiate between "low" and "high" prehypertension. Although all patients with prehypertension should receive advice on therapeutic lifestyle changes, patients with "high" prehypertension should understand that these changes might save them from significant cardiovascular morbidity.

How low should the blood pressure go? A meta-analysis of 61 trials of blood pressure control demonstrated that mortality appeared to decrease proportionally with blood pressure, all the way down to a pressure of 115/75.[5] Future research might find even lower healthy thresholds for blood pressure. Until then, physicians should encourage patients to reduce their blood pressure to < 120/80.