Rapid rewarming can lead to an overshoot in brain temperature, the researchers said. By allowing patients to return to normal temperatures at a slower rate, physicians can also reduce the potential of temperatures spiking high enough to harm the brain. Over a seven-year period, the maximum temperatures of patients being rewarmed after undergoing coronary artery bypass surgery at Duke University Hospital dropped an average of more than 1.5 degrees Celsius.
In order to protect the brain and other organs from damage while the heart is stopped during surgery, physicians cool a patient's blood as it passes through a heart-lung machine. However, toward the end of the operation, this blood needs to be rewarmed. While the current study demonstrates that the results of past studies have been incorporated into clinical practice at Duke and other centers, the researchers said that slower rewarming has not yet become the standard of care nationally.
"There are still many surgical teams that are unaware of the importance of temperature in effecting outcomes after bypass surgery," said Duke cardiothoracic anesthesiologist Hilary Grocott, M.D., who presented the results of the Duke analysis April 26, 2004, at the annual scientific sessions of the Society of Cardiovascular Anesthesiologists.
"The reasons why slower rewarming has not become more widespread are numerous, ranging from the inherent resistance to changing long-standing medical practices, to not wanting to spend the additional time in the operating room," Grocott continued. "However, these new data show that it is indeed possible to change practice, even at a very large institution like Duke. The small amount of extra time it takes to rewarm slowly is definitely worth the benefits to our patients."
In a typical bypass procedure, a heart-lung machine takes over from the heart and pumps blood throughout the body, allowing surgeons to operate on a temporarily stopped heart. Additionally, the heart-lung machine cools the circulating blood in order to reduce the metabolic needs of the brain and other vital organs.
In earlier studies of the effect of slower rewarming, Duke researchers found that patients who were allowed an additional 10 to 12 minutes to return to normal body temperature scored almost one-third better on standard tests of cognition six weeks after surgery.
During surgery, patients are typically cooled to temperatures between 30 and 32 degrees Celsius; normal body temperature is 37 degrees Celsius. Duke researchers found patients who were returned to normal temperature an average of 0.49 degrees Celsius per minute fared better that those rewarmed at the typical rate of 0.56 degrees Celsius per minute.
During the standard, faster rewarming process, the temperature of the brain may temporarily rise above 38 degrees Celsius, as the temperature at the core of the brain comes into equilibrium with the temperatures of the brain's outer shell. A slower rewarming allows the temperature to be transferred from the core to the shell and prevents overheating.
It is known that brain temperatures approaching 38 degrees Celsius can be detrimental to the brain, so a slower rewarming ensures that fewer patients became hyperthermic, Grocott said.
To see how readily physicians accepted the results of the earlier studies, the Duke researchers analyzed the data of the 6,334 patients who underwent bypass surgery at Duke from 1993 to 2000. The analysis was possible because Duke's department of anesthesiology maintains an extensive clinical database of all patients undergoing heart surgery.
From 1993 to 1996, the maximum temperatures recorded during rewarming decreased 0.34 degrees Celsius each year. From 1997 to 2000, the rate decreased a further 0.10 degree Celsius each year.
"In the beginning before we started our original study (1993), just about all patients were hyperthermic at some point, while now hyperthermia occurs very infrequently," Grocott explained.
Specifically, 82.9 percent of the earliest studied patients in 1993 had maximum temperatures greater than 38 degrees Celsius, while only 2.9 percent of the most recent patients in 2000 had a maximum temperature greater than 38 degrees Celsius.
While it is not known exactly why temperature plays a role in cognitive impairment, the researchers think that a slower rewarming allows for less discrepancy between the brain's oxygen supply and its growing demand for oxygen as it warms up. When patients are rewarmed quickly, the brain's need for oxygen and its supply is most out of balance -- the higher demands for oxygen are not matched by increased blood flow. Irrespective of its oxygen supply and demand, hyperthermia can also directly injure brain tissue by itself, Grocott said.
It is during this period of time of inadequate oxygen supply that may help explain why up to one-third of heart patients suffer from significant declines in cognitive abilities after undergoing major open heart surgery, Grocott said.
"We are very close to putting this issue of the role of temperature on cognitive decline to rest," Grocott continued. "With this knowledge, we can now focus on the other issues that can adversely affect outcomes after coronary artery bypass surgery."
The current analysis was supported by Duke's department of anesthesiology. Other Duke members of the research team were W.B. Corkey, M.D., Barbara Phillips-Bute, Ph.D., B. Baudet, Joseph Mathew, M.D., and Mark Newman, M.D.