Acute kidney injury (AKI) following cardiothoracic surgery has been well reported in the nephrology literature with numerous studies published in the last decade, although the definition of AKI was variable in many of these studies. In a recent article published in AJKD, Pickering et al perform a systematic review and meta-analysis of the literature to assess the different definitions of AKI in these studies. Dr. John Pickering (JP), the first and corresponding author of the study, discusses this topic with Dr. Kenar Jhaveri (eAJKD), eAJKD Editor.
eAJKD: Can you explain why your team felt this topic was important to study?
JP: There is a long history of studies evaluating AKI after cardiopulmonary bypass surgery, but the information is heterogeneous and cannot be easily used to understand the extent of the problem. We thought it was important to quantify the association and its consistency across several global regions, and were surprised this had not already been done. In doing this study, we hoped to provide a benchmark which may be used in future epidemiology and clinical-decision making.
eAJKD: Please summarize the major findings of your study.
JP: Since 2004, following the first global consensus definition of AKI, studies show that kidney injury is associated with a four-fold increased risk of in-hospital mortality or death within 30-days after cardiac surgery (i.e., about one excess death for every 100 operations). Differences in estimates of mortality risk in individual studies were beyond that expected by chance, but importantly, the different methods for defining kidney injury did not influence estimated mortality risk. The need for kidney replacement therapy after cardiac surgery was associated with a five-fold increase in early mortality.
eAJKD: Your search included studies that looked at kidney outcomes in patients that went to cardiac surgery on cardiopulmonary bypass only. Why did you choose to exclude studies of off-pump CABG procedures?
JP: There were two reasons for doing this. First, on-pump procedures are still more common than off-pump. The second reason is because some studies have reported lower incidence of AKI following off-pump procedures. While we could have considered exploring this as a variable associated with heterogeneity, we chose to reduce the variability that may be introduced by on-pump versus off-pump procedures by concentrating on just one procedure. Obviously, we would like to see a similar analysis of off-pump procedures in the future.
eAJKD: Why was race not accounted for as a pre-specified variable? Do you think it matters in terms of AKI risk following cardiac surgery?
JP: Given this analysis was global and inclusive of several different world populations, we were particularly interested in the impact of ethnicity on the outcomes of AKI in this clinical setting. Unfortunately, this was not possible. As is common in this type of study, information on ethnicity was sparse and we had insufficient data to include this aspect in analyses. But, we are aware that people with different ethnic backgrounds may experience poorer outcomes following cardiac surgery in general. Individual participant data would be needed to explore ethnicity as an attribute that influences clinical outcomes following cardiac surgery.
eAJKD: Do you think that different geographic regions differ in their rates of AKI following CABG?
JP: In the final analysis, we did not show that geographic region influenced outcomes following cardiopulmonary bypass-associated AKI, but we did not analyze AKI rates per se. We did notice that published research did not reflect geographic populations very well (e.g., lack of studies from India and from low-middle income countries). These limitations make this question very difficult to answer. We think it is important that future studies of both AKI rates and associated outcomes include low-middle income countries. Such studies would provide the benchmark needed against which to measure changes in practice.
eAJKD: With many studies now suggesting the risk of CKD following CABG-associated AKI affects mortality, would a “renal” pre-op evaluation be beneficial in patients going to CABG for better follow-up and prevention strategies?
JP: In this study we did not have studies evaluating CKD outcomes. Future research will be needed to identify effective strategies to reduce the risk of AKI after cardiopulmonary bypass and to mitigate the adverse outcomes associated with AKI, including CKD, stroke, cardiac events, and mortality. The findings of our systematic review will help inform the design and evaluation of future interventional strategies that can be used to develop evidence-based policies for AKI risk assessment and management.