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Blood Dilution During Bypass Surgery Associated with Kidney Damage

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Duke Health News 919-660-1306

DURHAM, N.C. -- When physicians routinely "thin" the blood
of patients undergoing coronary artery bypass surgery in order
to place them on the heart-lung machine, they may be causing
more damage to the kidneys and other organs than previously
appreciated, according to a new study by Duke University
Medical Center researchers.

For years moderate dilution of the blood has been thought to
protect the kidneys from damage, but the Duke researchers found
in their study of more than 1,400 bypass patients that dilution
to the lower levels of accepted ranges is associated with
measurable kidney damage. The Duke team published the results
of its study in the September 2003 issue of the Annals of
Thoracic Surgery.

In order to safely operate on a non-beating heart,
physicians attach the body to a heart-lung machine, which takes
over for the stopped heart in circulating oxygen-rich blood
throughout the body. To prime the pump, physicians add fluid --
usually a balanced saline solution -- to the circuit to fill
the tubing and pumping chambers of the machine.

This additional fluid lowers the percentage of
oxygen-carrying red blood cells in the blood, a measurement
known as hematocrit. Normal hematocrit ranges from 36 to 40
percent. During bypass surgeries, the hematocrit can range from
22 to 26 percent, with even lower percentages being commonly
attained at different points during the operation.

"Using hematocrit as a tool to assess a patient's anemia, we
found that the lowest hematocrit achieved during the bypass
procedure was significantly associated with acute kidney
damage," said Duke anesthesiologist and study leader Mark
Stafford-Smith, M.D. "Furthermore, we found the risk to kidneys
increases as a patient's body weight increases.

"This is the first report highlighting the association of
hemodilution during bypass surgery with acute injury to the
kidneys," Stafford-Smith continued. "Our findings question the
wisdom of tolerating the lowest levels of hematocrit during
bypass surgery."

Transfusing additional blood is not considered an ideal
solution, Stafford-Smith said, since this and other studies
have shown that transfusions are also associated with kidney
damage. He recommended that more attention be paid to
shortening the bypass circuit or using smaller diameter tubing
to reduce the levels of hemodilution.

Every year, more than 750,000 patients worldwide undergo
bypass surgery, and researchers estimate that about one of
every 12 will suffer kidney damage as a result of the surgery.
While most cases of kidney injury are transient, up to 2
percent of bypass patients will require kidney dialysis, with
60 percent of those dying before hospital discharge,
Stafford-Smith said.

One commonly accepted benefit of hemodilution has been that
it makes the blood less viscous, Stafford-Smith said. Also, it
has been thought that since body temperature is lowered during
surgery, there were enough red blood cells in the diluted blood
to satisfy the tissue's reduced need for oxygen. Stafford-Smith
said that the physicians want to minimize the use of donated
human blood to prime the heart-lung machine pump.

Other studies have suggested that the lowest hematocrit
levels reached during surgeries may be linked with worse
outcomes, so Stafford-Smith and his colleagues consulted the
medical records of 1,404 patients receiving bypass surgery at
Duke University Hospital to answer the question.

He correlated hematocrit levels during surgery with levels
of creatinine -- a byproduct of normal metabolism -- in the
blood before and after surgery. Kidneys normally filter
creatinine out of the blood and excrete it in the urine, so
higher-than-normal levels in the blood indicate that the
kidneys' ability to filter blood has been impaired.

Interestingly, the researchers found a strong link between
the weight of patients and increases in the levels of
creatinine in the blood.

"For example, for a patient weighing 165 pounds, there is no
association between lowest hematocrit and increased
creatinine," Stafford-Smith said. "However, in the 330-pound
patient, there is a highly significant inverse association. The
significance of the association rises as weight increases.

"For this reason, it is important for physicians to pay
special attention to their patients who are overweight or who
have existing kidney damage," he said.

Stafford-Smith pointed out that up to 20 percent of patients
who are scheduled for bypass surgery have some degree of
existing kidney damage, further emphasizing the need for
physicians to consider these factors in the care of their
patients.

The researchers also looked at the role of transfused blood
during bypass. They came to the conclusion that if it seems
likely during the course of the procedure that the patient will
require a later transfusion, it is best to give that blood
before the hematocrit drops to the lowest levels.

"The level of a patient's hematocrit is a factor that we as
physicians have control over, and it is also a factor that is
amenable to proper management," Stafford-Smith.

In the course of their studies, the researchers also
followed minute-by-minute changes in blood pressure during the
period of support on the heart-lung machine and were surprised
to find that changes in blood pressure had little effect on
kidney damage.

Stafford-Smith's analysis of the data was supported by the
cardiothoracic division of the Duke's department of
anesthesiology.

Members of Stafford-Smith's Duke team were Madhav
Swaminatham, M.D., Barbara Phillips-Bute, Ph.D., Peter Conlon,
M.D., Peter Smith, M.D., and Mark Newman, M.D.

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