The effect is more pronounced for diastolic pressure readings. "If you go to 70 or below, say to 60, there is a fourfold higher risk in the primary outcomes," he said.

A too-low reading, he noted, could mean that the brain is not getting enough blood. "Obviously, if there is a lack of blood, there can be danger similar to that when there is too much blood," Messerli said.

The finding in this particular group certainly doesn't mean that most people should worry about blood pressure being too low, he said. "By and large, within reason, lower is better," Messerli said.

But that might not be true in special cases, he said. "You can have a funny situation where one organ in the body is demanding more blood than is good for the rest of the body," Messerli said. "What is OK for the kidney and OK for the brain may not be OK for the heart."

Controlling high blood pressure remains a major concern for physicians, Messerli said. "There are a lot of patients who are untreated and uncontrolled," he said. "We need to do a better job."

Dr. Alan H. Gradman, professor of medicine at Temple University, said that the study should be treated with caution because the number of people with very low blood pressure was small, but he said that "it does suggest that there may well be a J curve in people with coronary artery disease."

Though many other studies have not shown a J curve, "which is why the idea that you can't go too low is out there," the new study results might mean a slight revision of that rule in some cases, Gradman said.

"If you treat people with coronary artery disease for hypertension, you don't want to go too low, to diastolic pressure below about 70," he said. "That's the take-home message here."

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