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Q: I take blood pressure medicine. My top number is usually around 125, but my bottom number is often in the 60s. Is my bottom number too low?

A: First, a couple definitions. Systolic blood pressure (the top number) reflects the force produced by the heart when it pumps blood out to the body. The diastolic blood pressure (the bottom number) is the pressure in your blood vessels when the heart relaxes between beats.

The good news is your systolic blood pressure is excellent. It's the systolic blood pressure that is most closely associated with risk of stroke and heart disease. That's especially true in people ages 50 and older, which is why doctors tend to monitor the top number more closely. In general, the lower the better is the rule for systolic blood pressure.

Risk related to the bottom number (diastolic blood pressure) is a bit more complicated. A diastolic blood pressure can be as low as 50 mm Hg and still be normal for a young person who is healthy and feels fine. As we move from our twenties toward middle age, diastolic blood pressure tends to rise into the 70s and even higher.

For people taking medicine for high blood pressure, doctors adjust the dose based primarily on the level of systolic pressure. To achieve good control of the top number it often means the diastolic pressure falls into the 60s.

Some observational studies suggest a low diastolic blood pressure may pose increased health risks. One study, which analyzed the medical records of more than 11,000 adults over a period of three decades, found that people who had low diastolic blood pressure (60 to 69 mm Hg) were twice as likely to have subtle evidence of heart damage compared with people whose diastolic blood pressure was 80 to 89 mm Hg.

The findings make sense because diastolic pressure is measured during the point in the heart cycle where blood flows into the coronary arteries that feed the heart. If those arteries are clogged with fatty deposits (as in a person with coronary artery disease), blood pressure beyond the narrowed areas will drop as blood flows through that narrow channel. As a result, part of the heart muscle may not get enough blood. Starved for oxygen and nutrients, the heart may become weak and prone to damage.

So a low diastolic pressure may be a potential concern if a person has coronary artery disease. But for people without heart disease, a bottom number in the 60s may be fine. If you have no side effects such as lightheadedness from your blood pressure medicine, there is no rush to make any changes. But I would suggest bringing it up with your doctor at your next appointment.

(Howard LeWine, M.D., is an internist at Brigham and Women's Hospital in Boston and assistant professor at Harvard Medical School. For additional consumer health information, please visit www.health.harvard.edu.)

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