Abstract

Methods: Between January 2007 and July 2018, a total of 119 patients (38 males, 81 females; mean age 54.4±13.0 years; range, 24 to 74 years) who received extracorporeal membrane oxygenation support for postcardiotomy shock and survived at least 48 hours in our center were retrospectively analyzed. Pre- and postoperative neutrophil-to-lymphocyte ratios, demographic characteristics, renal function parameters, and extracorporeal membrane oxygenationrelated data of the patients were recorded. Occurrence and stages of renal injury was determined using the revised Acute Kidney Injury Network and Kidney Disease Improving Global Outcomes criteria.

Results: No significant relationship was found between preoperative neutrophil-to-lymphocyte ratio and renal injury (p>0.05). Postoperative neutrophil-to-lymphocyte ratio was higher in patients who developed renal injury, compared to patients without renal injury (8.68 [0.84-42.00] vs. 4.02 [1.04-21.21], respectively, p=0.004). When patients were grouped for renal injury stage according to the revised Acute Kidney Injury Network (p=0.015) and Kidney Disease Improving Global Outcomes (p=0.006) criteria, the patients with more severe renal injury had higher neutrophil-to-lymphocyte ratio. The receiver operating characteristics analysis revealed a cut-off value of 6.71 for the neutrophil-to-lymphocyte ratio for the detection of renal injury. Patients with a value above 6.71 had an odds ratio of 5.941 for occurrence of renal injury.

Conclusion: Postoperative, but not preoperative neutrophil-tolymphocyte ratio is associated with presence and severity of renal injury in patients under extracorporeal membrane oxygenation support for postcardiotomy shock. Neutrophil-to-lymphocyte ratio is a simple and inexpensive marker of inflammation in this patient population.

An appreciable inf lammatory response is initiated with the placement of ECMO. The release of proinflammatory cytokines, activation of the coagulative cascades, the complement system, and lymphocytes result in endothelial injury and impaired microcirculation, leading to end-organ damage.[6] The inflammatory milieu triggers the endothelium, lymphocytes, and platelets, causing neutrophil infiltration of tissues and inflammatory insult into organs.[6] This and other factors including preexisting multisystemic disease, ischemiareperfusion damage, hemolysis are involved in the pathogenesis of EAKI.[3]

The neutrophil-to-leukocyte ratio (NLR) has been implicated in oncology, neurology, and cardiovascular diseases as a surrogate of inflammatory state associated with responsiveness to treatment, disease severity, and adverse events.[7-13] Recently, the NLR has been shown to have a predictive value for mortality in patients supported with ECMO.[14]

In the present study, we aimed to evaluate NLR in patients under ECMO support for postcardiotomy shock and to investigate the possible relationship between the NLR and renal injury.

Methods

Pre- and postoperative NLR for each patient was recorded along with parameters of renal function before and after ECMO treatment. Difference in pre- and postoperative NLR denoted an arithmetic difference of the calculated values. Demographic characteristics, preexisting chronic diseases, method of ECMO placement, and postoperative renal function parameters for all patients were recorded. In addition, ECMOrelated data were recorded including the site of insertion (central or peripheral) and duration of ECMO support.

The patients were initiated on veno-arterial (VA) ECMO via heparin-coated arterial and venous cannulas either through the ascending aorta and right atrium or peripherally through the femoral artery and vein. A centrifugal blood pump sustained flow at 3 to 4 L/min. The ECMO circuit was primed with 600 mL of balanced Ringer"s solution. The target mean arterial pressure was >60 mmHg and the target hemoglobin level was >7 g/dL.

Definitions
The revised Acute Kidney Injury Network (AKIN)[15] and Kidney Disease Improving Global Outcomes KDIGO[16] criteria were used to classify patients into respective AKI and KDIGO stages of renal failure. Early renal failure was defined as renal failure within 72 h of ECMO initiation. The highest serum creatinine level within the first 72 h, urine output during this period, and renal replacement therapy (RRT) requirement were recorded.

Statistics
Statistical analysis was performed using the (Number Cruncher Statistical System) (NCSS) version 2007 software (NCSS LLC, Kaysville, UT, USA). Descriptive data were expressed in mean ± standard deviation (SD), median (min-max), or number and frequency. The Mann-Whitney U and Kruskal-Wallis H tests were used for the comparison of groups without normal distribution. The Kruskal-Wallis H test with Dunn"s test was applied for post-hoc analysis of the groups. The Pearson"s chi-square test was used to compare the groups with categorical data. To estimate a cut-off value for variables, diagnostic tests and Receiver Operating Characteristics (ROC) curve were used. A p value of <0.05 was considered statistically significant.

Results

Table 1. Demographic and clinical characteristics of patients

There was no statistically significant difference in the preoperative NLR with respect to the presence of AKI according to either AKIN or KDIGO criteria, while a significant relationship between postoperative NLR and presence of early kidney injury was observed (Table 2). The patients who developed early kidney injury according to either AKIN or KDIGO criteria had higher NLR compared to the patients without early kidney injury. The difference between the preand postoperative NLR was also higher in the patients with AKI.

Table 2. Pre- and postoperative NLR according to presence of early kidney injury

Postoperative NLR was also significantly different according to the AKIN and KDIGO stages, when the patients were staged according to these respective criteria (Table 3). The number of the patients with AKI was identical using both criteria. However, the stage of AKI varied in three patients using both criteria and a statistically significant difference was found between the groups for different disease stages. Post-hoc analysis with the Dunn"s test also revealed a significant difference.

Table 3. Postoperative NLR according to AKI stage using AKIN and KDIGO criteria

A cut-off value of 6.71 was calculated for an NLR value distinguishing the presence of early renal injury under ECMO. The ROC curve is shown in Figure 1. A cut-off value of 6.71 for NLR in distinguishing the presence of early renal injury was significant with p=0.001, with an area under curve of 0.693 with a sensitivity of 67.71% and a specificity of 73.91% (Table 4). An odds ratio of 5.941 was calculated for the risk of renal injury in the patients whose NLR was above 6.71.

Figure 1. R eceiver o perating c haracteristics c urve f or Neutrophil-to-lymphocyte ratio and presence of renal injury. AUC: Area under curve.

Table 4. ROC curve for a cut-off value of 6.71

Discussion

Renal injury under ECMO results from an interplay of several patient-related and ECMO-related mechanisms. Implicated factors include progression of preexisting multisystemic diseases, pre-ECMO management, alterations in the renal macro/microvasculature, ischemia-reperfusion, hemolysis and oxidative stress, disruption of the glycocalyx, impaired renal autoregulation, nephrotoxic medications, and iatrogenic coagulation abnormalities.[3] The period under cardiopulmonary bypass in the preceding operation through inflammatory and non-inflammatory mechanisms with the hemodynamically unstable period, before the initiation of ECMO threatens the renal function, and AKI is a common presence in postcardiotomy shock before ECMO implantation.[17] The double-edged sword of ECMO is that the patient hemodynamics and kidney perfusion are augmented in exchange for the loss of pulsatile flow, hemolysis, ischemia-reperfusion damage, and systemic inflammatory response, resulting in an impaired renal function.[3,17,18]

Proinflammatory cytokines are released with the activation of coagulative and complement cascades. Neutrophil activation is a key element of the increased inflammatory response to ECMO.[17] As a result of increased neutrophil infiltration and inflammatory insult from activated endothelium, lymphocytes, and platelets, the kidney suffers from endothelial injury and impaired microcirculation.[3,6]

Defining renal injury is a challenge in many studies. The revised AKIN criteria are accepted as a validated definition and used widely in the literature.[19] The AKIN criteria along with the Risk, Injury, Failure, Loss, and End-stage renal failure (RIFLE) and KDIGO have been used to examine prognostic parameters under ECMO and allows for reproducible classification which aids in studying AKI.[18,19] Furthermore, higher stages of renal injury were associated with worse survival under ECMO, and one study found the KDIGO criteria to have better outcome prediction than the RIFLE and AKIN classifications.[20]

The immune response to the initiation of ECMO has detrimental consequences. Endothelial dysfunction, end-organ damage, and increased mortality are the results of different pathways that are activated in response to the ECMO circuitry.[6] Accordingly, a higher NLR was shown to be a predictor of poor outcome in ECMO patients of different indications, presumably as it mirrors a maladaptive immune response to ECMO.[14]

On the other hand, we found no correlation between the preoperative NLR in postcardiotomy shock patients undergoing cardiac surgery and kidney injury within the first three days of ECMO initiation. We, therefore, cannot conclude that preoperative NLR is a predictor for postoperative AKI. This finding indicates that the preoperative NLR of an immunologically unprovoked patient undergoing elective cardiac surgery does not reflect their potential inflammatory response. Our study population was not comprised of heart failure patients whose distinctive inflammatory milieu might alter the postoperative course and end-organ status. Intraoperative variables and a new-onset inflammatory response might play a more prominent role in the development of AKI in postcardiotomy shock patients. This is reflected by the higher difference in NLR from the preoperative values in patients with AKI. A more profound change in the inflammatory status parallels its detrimental results on the kidney.

In the present study, the NLR cut-off value as calculated by the ROC analysis was 6.72, which is higher than cut-off values reported in the literature.[7-9,11,13] We believe this is due to our association of EAKI with postoperative NLR where these ratios are higher with the inflammatory response in place, as opposed to the preoperative period, when the NLR is lower before any inflammatory stimulus.

Postoperative NLR was associated with AKI developing within 72 h of ECMO initiation. This association is still valid, when AKI is defined with both the AKIN and KDIGO criteria. This relationship can be explained with a higher postoperative NLR, suggesting a more profound immune reaction after ECMO placement. Furthermore, NLR was higher with a more severe AKIN or KDIGO disease stage. A more profound immune response could, therefore, be cause a more severe damage to the renal function.

In general, NLR is a valuable and simple parameter which can provide an insight into the frequently emerging risk of kidney injury. This relationship may translate into an association with mortality in the longer term. In future studies, the relationship between the NLR and mortality and other adverse events under ECMO can be investigated. Further studies may also address into the correlation of other markers of inflammation and adverse events with ECMO.

The limitations of the study include its retrospective design and single-center involvement. Another point worth mentioning is that while this study focused on postcardiotomy shock cases and therefore involved a set of patients with a homogenous mechanism of inflammatory response, operation type and intraoperative variables that could influence kidney injury were not separately analyzed. Future research can be directed towards clarifying the effects of operative variables to the resulting NLR a nd A KI.

In conclusion, the neutrophil-to-lymphocyte ratio is a simple and inexpensive marker of inflammation and the increase after the initiation of extracorporeal membrane oxygenation treatment is correlated with early renal injury. A higher neutrophil-to-lymphocyte ratio is associated with a more severe renal injury. The severity of a new-onset inflammatory response in postcardiotomy shock may be responsible for early kidney injury after the initiation of extracorporeal membrane oxygenation support.

Declaration of conflicting interests
The authors declared no conflicts of interest with respect to the authorship and/or publication of this article.

Funding
The authors received no financial support for the research and/or authorship of this article.

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