Acute kidney injury (AKI) remains a common and serious complication following cardiac surgery, affecting approximately 10% to 30% of patients. The challenge of predicting and preventing AKI lies in its multifactorial origins, including systemic inflammation, ischemia-reperfusion injury, and hemodynamic instability. One hypothesized contributor is perioperative bleeding, which has been associated with AKI in previous observational studies. However, the causality of this relationship remains under scrutiny.
In a robust post-hoc analysis of the ALBICS (ALBumin In Cardiac Surgery) trial, researchers investigated whether perioperative bleeding was independently responsible for AKI in patients undergoing on-pump cardiac surgery. The ALBICS trial itself randomized 1,386 adult patients to receive either 4% albumin or Ringer’s acetate for cardiopulmonary bypass (CPB) priming and perioperative volume replacement. The study cohort was drawn from Helsinki University Hospital in Finland and followed rigorous ethical and methodological standards.
The key outcome of interest was AKI, defined using the KDIGO (Kidney Disease: Improving Global Outcomes) serum creatinine criteria within the first four postoperative days. Bleeding was categorized according to the Universal Definition of Perioperative Bleeding (UDPB) classification system. This five-tiered scale accounts for blood loss, transfusion needs, and surgical interventions like resternotomy.
Initial univariable analyses showed that patients who developed AKI were older, had higher body mass index, more comorbidities (like hypertension and reduced ejection fraction), and experienced more severe bleeding events as defined by the UDPB. These patients also received higher volumes of red blood cells (RBCs), fresh frozen plasma (FFP), platelets, and coagulation factor concentrates such as fibrinogen and FVIII/VWF.
However, the multivariable regression models painted a more nuanced picture. In Model 1, which included risk scores and UDPB class, severe and massive bleeding (UDPB classes 3 and 4) were statistically associated with AKI. Yet, when more detailed components of bleeding (e.g., individual transfusions and interventions) were analyzed in Model 2, only the AKI Risk Score and FFP transfusion remained significantly associated with AKI.
Crucially, the mediation analysis (Model 3) revealed that UDPB-high bleeding did not have a direct effect on AKI. Instead, bleeding’s impact on AKI was mediated through hemodynamic instability—specifically, lower mean arterial pressure (MAP) and higher fluid balance. These results suggest that perioperative bleeding contributes to AKI not directly, but by inducing hypotension and requiring large-volume fluid resuscitation.
This insight is supported by a key finding: AKI incidence increased sharply when MAP fell below 75 mmHg. Similarly, both overly restrictive and overly liberal fluid strategies were associated with higher AKI rates, highlighting a U-shaped relationship between fluid balance and kidney injury.
The only blood product independently associated with AKI in this study was FFP. The authors speculate this may not be due to FFP itself but rather its use as a surrogate for more significant bleeding and fluid replacement needs. These findings diverge from some earlier studies which linked RBC transfusion and platelet use with AKI, indicating that the etiology may be more complex and context-dependent.
Importantly, the study has limitations. AKI was monitored only for four days postoperatively due to early patient discharges, possibly underestimating its true incidence. Moreover, as a single-center trial with relatively low AKI and mortality rates, generalizability to higher-risk populations may be limited.
Nonetheless, the study’s strengths include the use of a validated bleeding classification system (UDPB), comprehensive perioperative data, and sophisticated statistical modeling including mediation analysis. Together, these elements bolster the conclusion that while bleeding is not an independent cause of AKI, it sets the stage for hemodynamic conditions that increase renal risk.
From a clinical standpoint, the findings highlight the importance of managing not just bleeding itself but also its downstream effects—particularly maintaining stable blood pressure and optimizing fluid therapy. Strategies aimed at preventing significant bleeding may, in turn, help reduce AKI incidence, not by removing a direct cause, but by preventing the cascade of instability that follows.
As AKI continues to pose a threat to postoperative recovery and long-term outcomes, this research provides valuable guidance. By reframing the role of bleeding in AKI pathogenesis, it helps clinicians focus on modifiable targets like MAP and fluid balance to better protect kidney function in cardiac surgical patients.
Study ranking = 4 (high quality) This was a large-scale, well-designed post-hoc analysis of a randomized clinical trial with robust statistical modeling, including mediation analysis. While not double-blinded with AKI as the primary outcome, the data quality and methodology are strong.
