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.