Anemia either before or after surgery is associated with an incremental risk for postoperative acute kidney injury, and perhaps long-term mortality, a database analysis has concluded. Although prior research identified both preoperative and postoperative anemia as potential risk factors for complications after coronary artery bypass graft surgery, their relationship with AKI had not been defined with any degree of clarity.
“Over the last four decades, despite identifying multiple risk factors and preventive measures, the incidence of acute kidney injury after cardiac surgery hasn’t changed,” said Miklos Kertai, MD, PhD, associate professor of anesthesiology at Duke University Medical Center, in Durham, N.C. “On the contrary, it has actually increased. And as we all know, once patients develop postoperative AKI, they will require substantial mobilization of health care resources, and their chances of survival are reduced.”
Previous research identified several risk factors as predictors of postoperative AKI, including advanced age, preexisting renal dysfunction, diabetes mellitus, ischemia/reperfusion injury, inflammation and anemia. “But these studies did not really examine modifiable factors, which can improve the outcome of these patients undergoing cardiac surgery,” Dr. Kertai said. The incremental influences of preoperative and postoperative anemia on the risk for postoperative AKI and mortality have not been addressed in CABG patients, he added.
Post-Op AKI Rates High
To help address that research gap, the researchers evaluated the records of 6,131 adults who underwent CABG surgery at Duke. For purposes of the analysis, preoperative anemia was defined as hemoglobin less than 13 g/dL in men and less than 12 g/dL in women. Postoperative anemia, on the other hand, was defined as the median of the lowest in-hospital values measured for the first 10 postoperative days.
As Dr. Kertai reported at the 2016 annual meeting of the American Society of Anesthesiologists (abstract BOC04), 1,197 patients in the cohort (19.54%) had preoperative anemia, 1,596 (26%) developed postoperative anemia, and 1,253 (20.4%) had combined preoperative and postoperative anemia. AKI was observed in 3,544 (58%) individuals; 1,881 patients (30.7%) died, an average 6.84 years after surgery. The median nadir postoperative hemoglobin concentration was 8.8 g/dL.
“The incidence of postoperative AKI was really striking to us,” Dr. Kertai discussed. “This likely came from the fact that the definition we used was somewhat different than the one used in prior studies. Nevertheless, that definition allows us to detect early changes in creatinine, signaling that a kidney injury is occurring in these patients.”
Based on a risk-adjusted model for postoperative AKI, the investigators identified a number of factors that were associated with an incremental risk for the disorder. These factors included preoperative anemia (odds ratio [OR], 1.49; 95% CI, 1.27-1.75; P<0.0001), postoperative anemia (OR, 1.37; 95% CI, 1.19-1.57; P<0.0001), and the combination of preoperative and postoperative anemia (OR, 2.06; 95% CI, 1.75-2.43; P<0.0001).
“Clearly identifying preoperative anemia, or the combination of preoperative and postoperative anemia, as predictors of the severity of postoperative AKI could add more understanding of what is truly going on with these patients,” Dr. Kertai said. “This may also help us identify the different risk groups within the postoperative AKI cohort that will ultimately consume a significant proportion of health care resources.”
A similar analysis was performed with respect to long-term mortality. It was found that preoperative anemia (hazard ratio [HR], 1.27; 95% CI, 1.11-1.45; P=0.0004), as well as the combination of preoperative and postoperative anemia (HR, 1.31; 95% CI, 1.14-1.51; P=0.0001), were significantly associated with long-term mortality.
“To conclude, we observed that preoperative and postoperative anemia—and in particular the combination of the two—were significant predictors of postoperative acute kidney injury,” Dr. Kertai said. “And clearly, focusing on preventing anemia in the preoperative period in patients undergoing cardiac surgery stands to improve the outcome of these patients, both in the short term and also the long term, and can also prevent unnecessary deaths in individuals undergoing cardiac surgery.”
Keyvan Karkouti, MD, professor of anesthesia at the University of Toronto, told Anesthesiology News that it has been shown that perioperative anemia is a risk factor for AKI. “This study certainly adds more information to existing knowledge. However, the clinical relevance of this relationship is not known and merits further investigations. Specifically, we do not know if anemia is a marker of severity of illness or if it actually causes AKI. The impact of modifying arrhythmia on outcomes will depend on the answer to this question, and therefore is a very interesting and important topic of study.”