Q: I'm 72 and my systolic blood pressure has been between 115 and 125 and pretty steady. The diastolic number is low (55-65) and seems to be falling. Is the difference between these two numbers important, and is the falling diastolic number something to worry about?

A: The systolic pressure represents the blood pressure against the vessel walls when the heart is pumping blood. Starting at about age 60, it becomes the most important predictor of cardiovascular problems like stroke and heart disease. You should feel very good about your systolic blood pressure, which is in a fine range.

Diastolic pressure measures the force exerted by blood against vessel walls when the heart is relaxing between beats -- that's why it's the lower of the two blood pressure numbers. With age, our arteries get a little stiff. As a result, they don't stretch out as much every time the heart beats, so more blood gets pushed right on through the larger vessels into smaller ones. Because there's less blood in the large arteries between heartbeats, diastolic pressure tends to decrease.

The difference between these two readings is called pulse pressure. So, in your case, 120 systolic minus 60 diastolic equals a pulse pressure of 60. You can think of pulse pressure as the gap between the peaks (systole) and the valleys (diastole) of your blood pressure.

Older people with stiffer arteries tend to have a wider gap, and thus a higher pulse pressure, usually because the systolic pressure goes up while diastolic pressure stays the same or declines a little. So, in many cases, an increase in pulse pressure amounts to the same thing as an increase in systolic pressure, so doctors tend to focus just on the systolic number.

But you've asked about a gap caused by a stable systolic pressure and a falling diastolic one. That's a little out of the ordinary. It's worth asking your doctor to listen very carefully to your heart to see if your aortic valve is leaking. A leaky aortic valve can cause diastolic blood pressure to fall sharply. Also, it's important to ask that your blood pressure be checked in both arms occasionally. Sometimes blood pressure readings differ between arms because an artery in one arm is blocked.

If your aortic valve is in good shape and you feel fine, I wouldn't be concerned about your current blood pressure readings. I would recommend, though, that you get into a good exercise routine, if you aren't already. Physical activity helps arteries stay nice and supple -- and that will keep your blood pressure readings in the no-worries range.

Thomas H. Lee, M.D. is a professor of medicine at Harvard Medical School and an internist and cardiologist at Brigham and Women's Hospital, Boston, Mass.)

(c) 2010 Harvard Health Watch