Thinning blood in bypass surgery problematic

Duke University Medical Center researchers have called into question the routine practice of "thinning" the blood of patients undergoing coronary artery bypass surgery in a study published by the Annals of Thoracic Surgery.

Although the procedure was introduced specifically in order to reduce kidney damage sustained as a result of the surgery, thinning the blood may be causing more damage than was previously thought.

Duke researchers studied more than 1,400 bypass patients and found that measurable kidney damage was occurring.

"We don't want to minimize the importance of cardiac surgery but we want to bring to light that these complications occur," said Dr. Mark Stafford-Smith, associate professor of anesthesiology in the cardiac division.

Nonetheless, oxygen-rich blood still needs to circulate throughout the body while the surgery is taking place. A heart-lung machine takes over this responsibility for the arrested heart and pumps the blood. To prime the pump, however, a balanced saline solution is used to fill the tubing and pumping chambers of the machine. This, in turn, "thins" the blood by reducing the percentage of oxygen-carrying red blood cells, also known as the measurement of hematocrit.

The results of the Duke study found that lower hematocrit levels are associated with high rates of acute kidney damage, said Dr. Madhav Swaminathan, assistant clinical professor of anesthesiology in the cardiac division. Additional preexisting health problems increased the likelihood of complications related to the surgery. For example, patients with diabetes, kidney damage or obesity are predisposed to the possibility of hurting the kidneys that much more, as a result of undergoing this surgery.

Stafford-Smith, however, was also quick to add that the level of kidney damage that the average coronary artery bypass surgery patient sustains is not all that severe.

"Only 2 percent of the patients studied went on kidney dialysis," Stafford-Smith said. "But 60 percent of those who end up with dialysis are associated with severe complications."

Newman also noted that while this research clearly showed that there is room for improvement in coronary artery bypass surgery procedure, the science has gone a long way since the 1960s, when the heart-lung machine was first introduced.

"When we first started, we were just happy that patients survived," he said. "We didn't have any other treatment option and you have to accept a certain amount of morbidity if you know that the patients would otherwise die."

In spite of the remarkable gains in the field, "there are still other things that need to be looked at; more things can improve," Newman said.

Researchers are currently in the process of investigating alterations to the traditional heart-lung machine that may reduce the chances of acute kidney damage. Such modifications involve reducing the diameter or length of the tubes in the heart-lung machine so that less blood dilution occurs.

Duke has received a National Institutes of Health grant to look at the impact of diluting the blood on the function of other organs in the body such as the brain. "We're finishing [the research] right now and looking at the long-term outcome of these [coronary artery bypass surgery] patients," Newman said. "And while it's too early to say completely, it seems that extreme levels of hemodilution may also not be positive."

Swaminathan is currently involved in taking a look at coronary artery bypass surgery patients in a prospective manner in order to identify individuals who may be at a higher risk for complications than others. "We're doing some work now on determining what the genetic factors that predispose certain people with this type of risk are," he said.

"Really, our best strategy is to try to prevent the problem in the first place."