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Lower Temperatures Improve Outcomes After Bypass Surgery

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Duke Health News 919-660-1306

HONOLULU -- Duke University Medical Center researchers are
urging their colleagues in the field to spend an extra 10 to 15
minutes slowly rewarming their patients after cardiac bypass
surgery, which they have found reduces the likelihood of
cognitive decline after surgery. In a new study, they present
data from Duke as an example of an institution that was able to
change long-standing practice, instituting slower rewarming
rates in such patients.

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.

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