They keep lowering the bar for blood pressure. Normal is now 120/80 or below, revised from the previous mark of 130/85. But the "lower is better" agenda has been questioned, and some findings lend support to doctors who worry that going too far with diastolic blood pressure (the second number) might be bad for the heart, particularly for someone who's had heart problems in the past.
Blood pressure is a measurement of the force of your blood pushing against your arteries. The systolic (think "s" for squeeze) reading measures the pressure at the moment when your heart is contracting to pump blood out. Naturally, it's going to be a higher number than the diastolic reading, which is the pressure between heartbeats when the heart's refilling with blood.
High blood pressure — doctors call it hypertension — is bad for you because the extra pressure injures cells lining the inside of your arteries. Inflammation and atherosclerosis may ensue, setting the stage for heart attack and stroke.
But some pressure must be brought to bear; otherwise, our blood would just slosh around in our arteries and veins. When the heart contracts during the systolic phase, it pushes a reinvigorating supply of blood into most organs and tissues of the body.
We say most, not all, because the heart itself is an important exception: Fresh blood flows through its supply lines, the coronary arteries, during the diastolic phase, when the heart is relaxing for a fraction of a second.
Some experts describe the problem like this: Crank down blood pressure too low, and suppressed diastolic pressure may reduce blood flow to the heart to a trickle, especially if circulation to the heart is already compromised. Therefore, for some people, aggressive treatment of blood pressure may be an example of the cure being worse than the disease.
A recent study — the International Verapamil-Trandolapril Study (INVEST) — involved over 22,000 people with coronary artery disease and high blood pressure. The main finding was the similar effectiveness of two blood pressure–lowering medications: verapamil (Calan, Isoptin, other brands), a calcium-channel blocker; and atenolol (Tenormin), a beta blocker.
But data from the study were then used in a secondary analysis that grouped the patients by their average blood pressure readings. The researchers checked to see how many in each 10 mm Hg stratum had one of the "primary outcomes" — that is, they died or had a nonfatal heart attack or stroke.
We know about the perils of high blood pressure, so it's not surprising that people with high systolic and diastolic blood pressure — even after taking medication — were more likely to suffer cardiovascular misfortune. But the INVEST researchers also found unexpectedly high percentages of primary outcomes among study participants with lower systolic and diastolic readings — and the trend was especially pronounced for diastolic pressure. The researchers said this fit the J-curve theories about blood pressure: Lower is better, but when it gets very low, the risk curves back up.
When they investigated a bit further, they found, however, that stroke risk did not increase in the diastolic lower regions.
One study does not a treatment guideline make. But at the very least, some caution is warranted when it comes to lowering blood pressure, especially in people with coronary artery disease. Existing drugs do have different effects on systolic and diastolic pressure, but it's not always clear-cut.
Researchers are working on smarter drugs that will more selectively lower systolic blood pressure. Ideally, they'll find a medication that will be just right, lowering systolic blood pressure, but keeping diastolic pressure high enough so the heart is sure to get enough blood.
November 2006 Update