Heart surgery often has detrimental effects on patients' cognitive abilities, reducing their short-term memories, attention spans and concentration, and even interfering with their abilities to perform complex calculations.
But these deficiencies-which may arise when patients are rewarmed after surgery-can be reduced by slowing the rate of temperature increase, according to a study by Medical Center researchers.
"It appears that hypothermia will reduce cognition... but rewarming the body more slowly can help retain those cognitive functions," said Dr. Mark Newman, chief of cardiothoracic anesthesia at the Duke Heart Center.
During a major heart operation, the patient's body is cooled to between 30 and 32 degrees Celsius in order to reduce potential organ damage. This cooldown also slows the brain's metabolic rate.
The patient is then connected to a heart-lung machine which provides the body with oxygen, serves as a temporary replacement for the heart and cools down the blood that passes through it.
Once the operation is complete, the blood passing through the machine is gradually rewarmed to 37 degrees Celsius. Over the next 20 minutes, the rewarmed blood courses through the circulatory system, giving off heat and warming the body in the process.
The two-and-a-half-year study involved 165 patients, half of whom had diabetes. The patients were given cognitive tests shortly before the surgery and six weeks afterward.
Patients scored better on the second set of exams because they had seen the material before, said Dr. Alina Grigore, lead researcher of the study and associate in the department of anesthesiology. But those whose blood was rewarmed more slowly outperformed the control group by a statistically significant margin; among patients with diabetes, the improvement was even greater.
Although researchers are not certain why there is a decline in cognitive ability after heart surgery, they hypothesize that several factors could be involved. Among them are the possibility of small air bubbles escaping from the heart-lung machine into the bloodstream, an inflammatory response to the machine or an imbalance between the brain's metabolic rate and the blood flow supplying it with oxygen.
"Cerebral metabolic rate is temperature dependent," said Grigore. "Blood that is rewarmed more slowly is a better match between the brain's metabolism and cerebral blood flow."
In other words, when the blood is heated too quickly and then circulated through the body, the metabolic rate increases. The brain's demand for oxygen can then exceed the amount of oxygen carried by the blood.
Diabetics already have an altered balance between cerebral metabolic rate and blood flow, and Grigore speculated that the added mismatch caused by faster rewarming is more likely to cause cognitive dysfunction in these patients. Thus, diabetics benefit even more than other patients from a slower rewarming process.
The study was presented at the annual meeting of the Society of Cardiovascular Anesthesiologists on May 9.
"The presentation was very well received," said Grigore. Although there have been many studies on cognitive dysfunction after surgery, none of the previous studies focused on patients' rewarming rates.
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