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Departments of Surgery, Epidemiology, and Cellular and Molecular Medicine, University of Ottawa Heart Institute, 40 Ruskin St, Suite 3403, Ottawa, Ontario, Canada K1Y 4W7
(Email: mruel{at}ottawaheart.ca).
Cardiopulmonary bypass (CPB) has been germane to the development of cardiac surgery. Although its use is often necessary, CPB is associated with the development of a well-described systemic inflammatory response. Benedetto and colleagues [1] examined the hypothesis that the Medtronic Resting Heart System mini-CPB circuit (Medtronic Inc, Minneapolis, MN) reduces acute kidney injury (AKI) by mitigating systemic inflammatory pathways by minimization of the blood-artificial surface interface.
In an observational study of 705 coronary artery bypass grafting (CABG) patients, Benedetto and colleagues [1] found that the use of a mini-CPB circuit resulted in a 20% absolute reduction in the incidence of AKI. However, two previous randomized controlled trials have observed otherwise [2, 3]. As the authors have acknowledged, the definition of renal dysfunction differed in these studies. The stringent definition of AKI used by Benedetto and colleagues [1] is more sensitive than other established criteria for detecting renal dysfunction; however, it has been previously shown by another group as not being better in predicting clinical outcomes [4]. Defining outcomes according to a stringent definition of renal dysfunction in the Benedetto and colleagues' [1] study may have been biased in favor of a significant result, without necessarily translating it into a clinical impact.
In the present study, the authors have utilized AKI as a dichotomous response variable. The AKI is arguably an ordinal variable, and in our opinion, the use of ordinal logistic regression models would have better reflected the relationship between the type of CPB circuit used and the development of renal dysfunction. As previously noted, early stage AKI as used by authors does not significantly correlate with clinical outcomes. Furthermore, another recent analysis found that only the most severe level of renal dysfunction within the AKI classification (ie, level 3) was predictive of increased intensive care unit mortality [5]. In the Benedetto and colleagues' [1] study, only 28 of 705 patients (4%) had an AKI level of 3 [5], thereby making the cohort size insufficiently powered to allow the detection of clinically relevant outcomes.
Finally, AKI may merely be the result of operation complexity. Perioperative factors that might have had an impact on outcomes were not exhaustively described. For instance, although the incidence of red blood cell transfusion was reported, the amount of transfusion was not quantified.
The findings by Benedetto and colleagues [1] are intriguing; however, we believe that further research and development are needed to better determine the impact of the mini-CPB on renal and other clinical outcomes.
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