A total of 329 patients who underwent OPCAB were analyzed, and the incidence of AKI was 20.4%. In this study, the Kidney Disease: Improving Global Outcome (KDIGO) criteria were used to define AKI. One of the differences between the KDIGO criteria and other AKI guidelines is that the KDIGO criteria define AKI from 48 hours to 1 week. In the present study, AKI that occurred more than 48 hours postoperatively was defined as late AKI. Late AKI accounted for 34.3% of all AKI. The preoperative albumin and the NL ratio were found to be predictors of AKI and early AKI, and the sensitivity of the NL ratio was higher than that of preoperative albumin. However, these two factors were not significant predictors of late AKI.
AKI is a common complication after CABG that is associated with the postoperative mortality14,15. Most cases of AKI after cardiac surgery are temporary and reversible. Patients with mild AKI are usually responsive to prompt medical interventions, such as avoiding additional renal insults and optimizing volume status. Medical therapy can prevent the progression of perioperative AKI and improve postoperative outcomes such as mortality16–18. Early detection of AKI may contribute to improving patients’ outcomes19. Therefore, the reliable predictors of AKI that show changes earlier than decreased urine volume and elevated serum creatinine are needed.
The pathophysiology of AKI after cardiac surgery is multifactorial, including inflammation, ischemia-reperfusion injury, operative trauma, neurohormonal activation, metabolic changes, and oxidative stress20–22. Neutrophils play an important role in inflammation, and a reduced lymphocyte count reflects physiological stress23,24. Dynamic change in the NL ratio is attributed to systemic inflammation. The relationship between AKI and the NL ratio in patients undergoing isolated OPCAB has not been studied sufficiently. One meta-analysis of the NL ratio for the prediction of AKI estimated an area under the curve of 0.65 in a cardiac surgery subgroup25. In the present study, ROC curve analysis yielded a similar result for the NL ratio (AUC: 0.693) with a cutoff value of 7.3 (63% sensitivity, 70% specificity) (Figure 1). In addition, the NL ratio can also predict early AKI with a cutoff value of 7.8 (AUC: 0.755, sensitivity 68%, specificity 74%) (Figure 2).
Hypoalbuminemia (often defined as <3.5-4.0 g/dL) is a well-established risk factor for morbidity and mortality26,27. An association between hypoalbuminemia and AKI has been reported in contrast-induced nephropathy11 and postoperative AKI following cardiac surgery12,28. In patients with hypoalbuminemia, the glycocalyx might be compromised, leading to the loss of oncotic pressure gradients and barrier function, the fluid leakage into the tissue, and the microvascular flow alterations29,30. The serum albumin protects renal function by maintaining the oncotic pressure, which augments the intravascular volume31, maintains renal perfusion, and improves glomerular filtration32. Albumin also limits tubular cell apoptosis as a scavenger of radical oxygen species and roles as an anti-inflammation effector33,34. The reported cutoff values of albumin vary between previous studies due to the study populations and types of surgery. In a study with non-cardiac surgery patients, the cutoff value of 3.75 g/dL had a sensitivity of 54% and specificity of 67%; meanwhile, among patients undergoing brain tumor surgery, the cutoff value of 3.8 g/dL had a similar sensitivity of 54%, but lower specificity of 27%35,36. Our ROC curve analysis indicated that, albumin with a cutoff value of 3.8 (AUC 0.635, sensitivity 67%, specificity 58%) was a valuable predictor of AKI in isolated OPCAB. Our analysis also showed that albumin could predict early AKI with a cutoff value of 3.8, not for late AKI.
The present study hypothesized that CK-MB and CRP could be the predictors of AKI in isolated OPCAB patients. CK-MB is a specific myocardial marker that increased by myocardial ischemia and hypotension. Therefore, postoperative CK-MB elevation suggests intraoperative myocardial damage. Perioperative cardiac biomarkers including CK-MB are associated with an increased mortality risk after CABG with CPB10. Moreover, cardio-renal syndrome is a well-known interdependency of cardiac and renal dysfunction in cardiac disease. Preoperative CK-MB was a strong and independent predictor of postoperative AKI in cardiac surgery9. CRP is a representative marker of acute inflammation and predicts the severity of acute infection and long-term morbidity or mortality5,37. A high CRP level is a biomarker of AKI or mortality in cardiac disease6,37,38 and post-CABG patients with CPB7. However, they did not predict postoperative AKI in the present study. They are associated with AKI indirectly through myocardial damage or inflammation. Although these factors were measured on POD1, no significant increases were observed. For predictors to be useful, they need to increase early, before AKI is established.
The present study had limitations as a single-center, retrospective, observational study. A prospective, multicenter study is needed to substantiate its broad applicability.
In conclusion, this study showed that preoperative albumin and the POD1 NL ratio are robust and independent predictors of postoperative AKI in isolated OPCAB. Both markers can be easily measured in general ward and detect patients at high risk of AKI and early AKI with high sensitivity. The further investigation of the late AKI predictors should be needed in OPCAB patients.