Blood pressure is typically recorded as two numbers, systolic and diastolic. Systolic, which is the top number, measures the pressure in the arteries when the heart beats (which is when the heart muscle contracts). Diastolic, the bottom number, measures the pressure in the arteries between heartbeats (when the heart muscle is resting).

The American Heart Association currently defines normal blood pressure as less than 120 (systolic) over 80 (diastolic) millimeters of mercury. People with blood pressure of 120-139 over 80-89 millimeters of mercury are considered pre-hypertensive and therefore at increased risk of mortality. However, a new study published in the Journal of Internal Medicine suggests that people labeled as pre-hypertensive may actually have normal blood pressure and therefore are not at increased risk of death.

The researchers reviewed 20 years of blood pressure data for 13,792 people who were enrolled in the National Health and Nutrition Examination Survey between 1971 to 1976. They also reviewed similar data for 6,682 adults between 1959 and 1962. The researchers focused on older data because blood pressure treatment was not as prevalent years during those years. Therefore, it was easier to study the health effects of changes in blood pressure as people age.

For people over the age of 50, the most meaningful predictor of poor health was the systolic blood pressure. In this older group, a systolic pressure of 140 or higher was highly related to increased mortality. For people under 50, it was the diastolic pressure that was most predictive of mortality. For these, a diastolic pressure of 100 or more was most predictive of poor health.

The current guidelines suggest that anyone with blood pressure over 120/80 is at increased risk of serious health problems like heart attack and stroke. This relates to over 100 million people in the US. However, Brent Taylor, investigator for the V.A. in Minneapolis, assistant professor of medicine at the University of Minnesota and study author, comments:

"If we can't identify an increased risk, is that where abnormal should be set? It should at least start a conversation about why normal is set where it is. If we make small changes to where we set normal blood pressure, it has huge implications in the numbers of people we identify as being at risk."

Dr. Taylor says it is very important for the guidelines to focus on the people who can most benefit from treatment. He also notes that guidelines can end up hurting people if they incorrectly label someone as being at risk. He adds:

"Maybe those people should be focusing on conditions other than their borderline blood pressure, If we intervene with these people who don't have a lot of risk, maybe we&'re going to cause more harm than benefit."

However, while he acknowledges that a single study is not likely to change the guidelines, he at least hopes the new research will be included in the discussion of blood pressure guidelines.