A new study suggests that patients who receive transfusions during heart bypass surgery have a higher risk of developing potentially dangerous infections, and dying, after their operation.
In fact, this increased risk may help explain a longstanding medical mystery: why women bypass patients are more likely than men to die in the first few months after surgery. Women are more likely to receive blood during heart bypass operations, which are performed on more than 465,000 Americans each year.
The findings, from the Patient Safety Enhancement Program (PSEP) at the University of Michigan Health System, are based on data from 9,218 Michigan bypass patients. After adjusting for factors such as the urgency of the operation, those who received blood transfusions from donors were five times more likely to die within 100 days of their operation than those who did not.
The paper is published in the December issue of the American Heart Journal. It builds on a previous U-M analysis that found that a difference in infection rates accounted for the difference in death risk between men and women bypass patients.
The U-M team, with the help of Neil Blumberg, M.D., of the University of Rochester Medical Center, focused on blood transfusions as a contributing factor. Prior research has shown that recipients of stored donor blood have more post-surgical infections, and that women receive more transfusions because they tend to have lower hemoglobin concentrations.
This new study connects the dots. "To the best of our knowledge, this is the first study to state that allogeneic transfusions may be the reason why women have a greater post-bypass surgery mortality risk than men," says author Mary A.M. Rogers, Ph.D., M.S., PSEP, research director and research assistant professor of internal medicine. Allogeneic is the term for blood from another person.
The authors strongly note that blood transfusions can be life-saving, and that the infections observed in this study are not likely due to contamination of the blood. Rather, they may be due to other factors, including the patient's immune response to substances such as white blood cells that are present in stored donor blood. These findings may help guide hospitals and blood banks in deciding whether to filter donated blood to reduce the levels of white blood cells. This practice is increasingly common, but not yet universal, in the United States.
The study is based on analysis of data from all Medicare beneficiaries ages 65 and older who had coronary artery bypass operations in Michigan in a single year.
The researchers performed statistical analyses that took into account the patients' blood transfusion status, their co-existing diseases, age, race, sex, and whether the bypass operation was done on an elective, urgent or emergency basis. They looked at infections and deaths that were reported during the 100 days after surgery.
In all, about 88 percent of women received an allogeneic blood transfusion during bypass surgery, compared with nearly 67 percent of men. When the researchers adjusted for other factors, women were 3.4 times as likely as men to receive blood. This gender difference was evident regardless of whether the operation was elective, urgent or emergency.
The odds of having an infection of any kind were about three times greater in patients who received allogeneic blood than in patients who did not. The more blood they received, the higher their infection risk. This "dose dependent" relationship strengthens the evidence that transfusions may be related to infections.
No single type of infection stood out as more common among blood recipients, which suggests a body-wide immune response issue rather than a problem, for example, at the site of the incision.
The analyses revealed that women were more likely to experience an infection than men after bypass surgery, which appeared to be due to the increased number of transfusions in women. This resulted in an increased mortality rate in women. Overall, 9 percent of women and 6 percent of the men died within 100 days of their operation.
For patients who had banked their own blood ahead of the operation and who received only their own blood, the infection risk was similar to that of patients who received no blood transfusions. Rogers notes that patients should ask their doctors regarding banking their own blood if possible, when scheduled for a bypass operation or other kind of surgery.
In addition, physicians are increasing their use of transfusion alternatives such as blood "expanders," blood substitutes and blood-conserving procedures during bypass surgeries.
The results also highlight the importance of the proper use of antibiotics and infection control practices in patients hospitalized for a surgical procedure, says Rogers.
The U-M team is investigating the issue further, including a new study funded by Blue Cross Blue Shield of Michigan Foundation to extend the research into elderly patients who recently underwent bypass surgeries in Michigan.