May 7 (HealthDay News) -- While too-high blood pressure is a clear hazard for most people, too-low pressure can apparently be a threat in some cases as well.
A new study of 10,001 people with coronary artery disease found what statisticians call a J-shaped curve of mortality, meaning a higher death rate for people with the lowest blood pressure. Dr. Franz H. Messerli, a professor of clinical medicine at Columbia University Medical Center and director of the hypertension program at St. Luke's-Roosevelt Hospital in New York City, reported on the finding Thursday at the American Society of Hypertension meeting in San Francisco.
"It stands to reason that there has to be a J-shaped curve," Messerli said. "If your blood pressure were zero, you would be dead."
He acknowledged that the people in the study were a bit out of the ordinary, with already-diagnosed coronary artery disease. The study was aimed at determining the effect of treatment with different amounts of a cholesterol-lowering statin drug, with blood pressure measured as a matter of routine.
When the results were in, the lowest rate of deaths and major coronary problems such as heart disease was seen not in the participants with the lowest blood pressure but in those slightly to the right on the curve, with a reading of 139.9 for systolic pressure (the reading when the heart contracts) and 79.2 for diastolic pressure.
Though Messerli stressed that this was "a unique population, with coronary artery disease, where the coronary arteries are compromised," he noted that "there has to be a point where lowering blood pressure is counterproductive."
That point can be seen on the curve of systolic pressure in this group, he said. "When you go from 120 to 130, even from 110 to 130, there is very little difference," Messerli said. "When it goes below 110, then all of a sudden it becomes very obvious."
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."
SOURCES: Franz H. Messerli, M.D., professor, clinical medicine, Columbia University Medical Center, and director, hypertension program, St. Luke's-Roosevelt Hospital, New York City; Alan H. Gradman, M.D., professor, medicine, Temple University, Philadelphia; May 7, 2009, meeting, American Society of Hypertension, San Francisco