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While coronary artery bypass graft surgery has saved the lives of millions of Americans since its inception decades ago, physicians have long noticed a nagging problem -- many patients, while restored to good health, have noticed declines in their cognitive abilities. In the largest study of its kind, Duke University Medical Center researchers have now measured this loss and shown that five years after surgery, more than one-third of the patients will have measurable cognitive decline. It has been generally accepted that many bypass patients exhibit some cognitive defects shortly after surgery and improve over the ensuing months, and now the long-term effects of the surgery on cognition are becoming better understood.
The researchers gave 261 heart surgery patients the same battery of standardized tests of cognition at different intervals during a five-year period and discovered that 53 percent had measurable declines at discharge from the hospital, 36 percent had measurable declines six weeks after surgery, and 24 percent had measurable declines at six months. However, by five years after surgery, 42 percent had measurable declines.
In the study, the researchers tested such cognitive abilities as short-term memory, attention, concentration, language comprehension, abstraction and spatial orientation.
The results of the Duke study, which was supported by grants from the National Heart, Lung and Blood Institute and the American Heart Association, were published Feb. 8 in The New England Journal of Medicine. Dr. Mark Newman, chief of cardiothoracic anesthesia at Duke, led the multi-disciplinary study of heart surgery patients treated at Duke University Hospital.
"Little is more devastating to patient and family than for the patient to have a successful operation that prolongs life, but diminishes the quality of that prolonged life," Newman said. "Our results confirm long-term persistence of cognitive dysfunction and the importance preventing these deficits. They will also help us design strategies to make an already safe procedure even safer."
In addition to demonstrating the cognitive declines, the researchers also found that three factors were important determinants of the five-year decline: age, level of education and the level of cognitive decline at discharge.
"The older the patient, the higher the probability of suffering decline," Newman said. "Age is an important factor, especially since we can safely operate on an older population of patients. Also, the more educated a patient is, the smaller the decline. While we don't know the exact reasons for this, it seems that education confers more of a cognitive 'reserve,' so the loss is not noticed as much."
This study did not compare the study participants with similar people who did not undergo surgery. While studies have shown that cognitive function has been shown to decline gradually with age, the researchers pointed out that study participants who suffered cognitive declines showed a decline more than two times that demonstrated by 5,888 Medicare patients in a recent five-year study.
Coronary artery bypass grafting (CABG) surgery is a surgical procedure performed more than 600,000 times a year in the United States. Typically, surgeons use pieces of blood vessels from other parts of the body to "bypass" clogs in coronary arteries, and thereby restoring blood flow to the heart.
The researchers say it is likely that many factors are behind the cognitive declines, although they believe that the heart-lung bypass machine commonly used in CABG surgery is an important culprit. This machine essentially pumps and oxygenates the blood for the body while surgeons operate on the stopped heart.
"While we have known for some time that the heart-lung machine is probably a cause, we don't know for certain how it might affect cognition," Newman said. "It is likely that tiny emboli, or clots, are formed and go to the brain. Other factors, such as inflammation and the lowered blood pressures, could play a part as well. There needs to be further investigation into operative neuroprotection to allow us to reduce the short- and long-term consequences of cognitive decline after surgery."
The Duke team is investigating many strategies to better understand this phenomenon of cognitive decline with the hope of developing new strategies to protect the brain. These include:
-- Using minimally invasive techniques, surgeons are now operating on beating hearts, meaning that the heart-lung machine is not needed. Preliminary results are encouraging, although the long-term effects are not yet known.
-- Duke researchers have found that temperature has an effect on cognitive decline. During surgery, the heart-lung machine cools the blood to lessen the metabolic needs of the body during the procedure. Duke studies have shown that patients who are rewarmed more slowly after surgery tend to do better on cognitive tests.
-- Duke researchers also have found a genetic component to the decline. Patients with the E-4 variant of the APOE gene (which has also been linked to early onset Alzheimer's disease) tend to do worse than patients with other variants of the gene.
Other Duke members of the team are: Jerry Kirchner, Barbara Phillips-Bute, Vincent Gaver, Dr. Hilary Grocott, Dr. Robert Jones, Dr. Daniel Mark, Dr. Jerry Reves and James Blumenthal. All are members of the Duke Neurological Outcome Research Group and the Cardiothoracic Anesthesiology Research Endeavors group.
Note to editors: Dr. Mark Newman is available at 919-681-5458, or via email at newma005@mc.duke.edu.