TUESDAY, Feb. 11, 2003 (HealthDayNews) -- Heart bypass patients often suffer a decline in mental skills after their surgery.
Now, Duke University researchers think they may know why.
Bacteria from the intestines released into the bloodstream during the surgery causes inflammation of the brain, the researchers say.
When surgeons operate on a stopped heart during a bypass operation, a heart-lung machine circulates blood throughout the body, resulting in less blood flow to the intestines, the researchers say in a study in the February issue of Stroke.
The reduced blood flow weakens a blood barrier in the lining of the large intestine, letting bacteria enter the bloodstream. This, in turn, causes inflammation of the brain in patients with lowered immune response to the bacteria, the researchers say.
Elderly patients seem particularly vulnerable because of lower immune response to endotoxins -- part of bacteria -- including those present in the intestinal tract, the study found.
The Duke team followed 343 patients who had elective bypass surgery, formally known as coronary artery bypass grafting, in which a blood vessel from another part of the body is used to reroute blood around a blocked artery leading to the heart.
Before surgery, researchers measured antibodies to the endotoxins in the patients and gave them cognitive tests measuring abilities such as short-term memory, attention, concentration and language comprehension.
Thirty-six percent of the patients suffered a decline in mental abilities, based on tests six weeks after surgery. Patients over 64 who had low immunity to endotoxins faced nearly twice the risk of cognitive decline after surgery, says Dr. Joseph P. Mathew, the study's principal investigator.
"Even though they recover from the physical symptoms, if we leave them with mental difficulties or memory problems, we haven't really cured the problem," says Mathew, a cardiac anesthesiologist at Duke. "We've given them a different problem, and that's the rationale for trying to address this problem."
Mathew says efforts to improve immunity to endotoxins might reduce the decline in mental ability among older patients. Duke researchers, for example, plan a trial in which they will give "antagonists" designed to block the effects of endotoxins before surgery. Vaccines may be another option, he says.
"We need to address the issue of cognitive decline because we are successfully operating on a progressively older and sicker group of patients," Mathew says. "While many of the complications of cardiac surgery have been minimized, cognitive decline is still one of the main areas where continued research can improve the quality of life for these patients."
Duke researchers reported in 2001 that many heart bypass patients who were on a heart-lung machine had suffered a loss of mental ability. Six months after bypass surgery, 24 percent had suffered declines in mental ability; after five years, 42 percent had, the 2001 study found.
However, that study did not determine why, and Mathew says the latest research was designed to provide some answers.
Dr. Timothy Gardner, a professor of heart surgery at the University of Pennsylvania, says the new study could shed light on the body's possible responses to heart-lung machine use.
However, Gardner, who is also a spokesman for the American Heart Association, says the study's findings do not prove bypass surgery leads to a long-term decline in mental ability.
"I'd put this study in the category of interesting, but preliminary," Gardner says. "It's interesting because they've explained a pathological condition that seems to occur, but I don't think this study, in particular, is important to proving that heart surgery is dangerous to the brain."
He points out that the study looked at patients' mental ability just six weeks after the operation, and did not assess whether they would recover later.
Patients, Gardner says, frequently suffer depression or confusion after surgery. And, he adds, the study failed to answer whether higher levels of endotoxins in the bloodstream are common only in heart surgery or also other major operations, such as removal of a tumor.
Gardner cites research presented last month by a team from Johns Hopkins University that looked at bypass patients and a control group of heart disease patients that did not have surgery. The Hopkins researchers found that whether the patients had surgery had no bearing on changes in mental ability measured a year later.
Most bypass surgery is performed on a stopped heart while a heart-lung machine keeps the patient alive, the American Heart Association says. However, the number of procedures performed without a pump, while the heart is beating, is increasing.
More information
For more on bypass surgery, visit the American Heart Association. Check the National Heart, Lung, and Blood Institute for more on
coronary heart disease.
