Fever After Bypass Bad for Brain
Study finds high temperatures linked to cognitive decline later
FRIDAY, Feb. 1, 2002 (HealthDayNews) -- A high fever in the 24 hours after coronary bypass surgery may predict which patients are most at risk of suffering cognitive problems in the aftermath, a new study says.
Bypass patients have long been known to experience fever immediately after the operation, and they've been shown to be vulnerable to brain injury in the wake of the procedure. The latest work, by scientists at Duke University, is the first to connect these two threads.
The study, which appears in the February issue of the journal Stroke, merely associates the highest fevers with the greatest mental deficits. It can't say whether high temperatures cause this decline, or if some other factor -- perhaps trauma in the brain -- explains the link. However, it does suggest the current practice of merely waiting for the fever to subside might be wrongheaded.
"Maybe we shouldn't just be observing them or ignoring them," says Dr. Hilary Grocott, a Duke anesthesiologist and lead author of the study.
Grocott's group followed 300 patients undergoing coronary bypass graft surgery, in which doctors try to improve obstructed blood flow by stitching leg or chest vessels around blockages in the patients' coronary arteries. The patients were monitored for temperature in the 24 hours after the operation. In addition, 227 took a battery of standardized cognitive tests both before and six weeks after the procedure.
Of the 227 who completed the study, 89, or 39 percent, performed worse on the second set of tests than on the first, the researchers say.
A statistical measure of average temperature during the 24-hour period didn't seem to predict which patients had the greatest cognitive deficits six weeks later, Grocott says. However, the patients whose fever spiked the highest above normal after surgery -- the maximum reaching 102.7 degrees Fahrenheit -- were most likely to have evidence of mental decline.
While the researchers aren't sure how fever worsens cognitive function, they propose it may exacerbate another damaging effect of bypass. During the operation, small chunks of arterial plaque may find their way to the brain and choke off the blood supply to clusters of neurons. In addition, brain cells release excitatory chemicals in response to fever, compounding the problem.
"It's very well known that temperature is a tremendous modulator of brain injury. The lower it is the better, and the more it goes up the worse," Grocott says. "We also know that brain injuries themselves may actually lead to increases in body temperature, through damage to the hypothalamus (the brain's thermostat)."
Grocott says he believes post-operative fever reflects inflammatory reactions to the heart-lung machines that keep patients alive during surgery. Smaller responses to the cutting of the breastbone to get at the arteries and to the overall stress of surgery may also play a part in the elevated temperature. Although no evidence exists on whether the same reactions occur in people who have less-invasive heart surgery, Grocott says he has seen fever in at least some of these patients.
An obvious strategy for protecting surgery patients from fever would be to give them so-called antipyretic drugs, like Tylenol, that reduce temperature. However, Grocott says such medications must be absorbed by the gastric tract, and after an operation these organs absorb poorly.
A better approach, he says, would be to use special cooling blankets, which are available in every hospital that performs bypass, or even an implanted catheter that cools blood. Grocott is planning a follow-up study to test that theory.
Ola A. Selnes, a neurologist at Johns Hopkins University who has studied brain injury after surgery, calls the latest study "a curious finding, and I don't know exactly what to make of it."
The problem, Selnes says, "is that you can't really quite tell if it is the elevated temperature itself or is it whatever it was that caused the temperature to go up that is causing" the neurological complications.
On the other hand, Selnes adds, the results comport with previous research showing a link between fever and poor outcomes from heart attack and stroke.
"People have been noticing that after cardiac arrest, if the patient has hyperthermia [fever], the outcome is generally pretty terrible. And with ischemic stroke, fever predicts worse outcomes," he says.
At least one heart surgeon isn't convinced bypass surgery in fact threatens the brain.
"The reports are real, real disturbing," says Dr. Wayne Isom, chairman of cardiothoracic surgery at Weill Medical College of Cornell University in New York City. "But our data have not found that at all."
Isom says the Duke team's findings may be the result of technique, specifically the low level at which they kept their patients' blood pressure during the surgery.
"Our blood pressures were all in a conventionally acceptable range," he says. "There is absolutely no data to suggest that lower blood pressure during cardiopulmonary bypass is responsible for this cognitive loss. We have studied more than 600 patients, and have never found such a relationship. Many other investigators have also looked at this relationship, and found no effect of lower blood pressure."
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