Incidence, risk factors, and mortality of acute kidney injury within 1-month after lung transplantation

After lung transplantation (LT), some patients are at risk of acute kidney injury (AKI), which is associated with worse outcomes and increased mortality. Method 59 (39.8%) developed AKI within 1-month after LT. Stage I or II, and stage III AKI were recorded in 26 (17.5%) and 33 (22.2%), respectively. AKI III usually occurred within 7 days after transplantation (early vs. late AKI III, 72.5% vs 21.1%). Pre-operative anemia, units of red blood cells transfused during surgery, colistin intravenous infusion for treating multi drug resistant pathogens were independent risk factors for AKI development. Post-operative bleeding, grade 3 PGD within 72 hrs, and sepsis were more common in the AKI group. Patients with AKI III ([24/33] 72.7%) had signicantly higher 1-year mortality than the no-AKI ([18/89] 20.2%), and AKI I or II group ([9/26] 34.6%), log-rank test, P < 0.001). BMI = body mass index; IPF = idiopathic pulmonary brosis; CTD = connective tissue disease; ILD = interstitial lung disease; AIP = acute interstitial pneumonia; BO after PBSCT = bronchiolitis obliterans after peripheral blood stem cell transplantation; eGFR = estimated glomerular ltration rate; Hb = hemoglobin; Cr = creatinine; ECMO = extra corporeal membrane oxygenation; CPB = cardiopulmonary MV = RBC = red blood cell; APACHE = and = extra membrane oxygenation; MV mechanical = red blood cell; APACHE Acute and


Introduction
Lung transplantation (LT) has become the standard treatment for patients with terminal lung diseases, and over 69,000 LTs have been performed worldwide. The median survival of LT has improved recently to 6.7 years, compared to 5.6 years in the previous decade, as techniques and experiences with transplantation have grown. 1 Acute kidney injury (AKI) arises as a consequence of different pathological conditions, such as renal hypoperfusion, nephrotoxic exposure, sepsis, or major surgery. 2 In LT, AKI is a common complication, shown to be associated with higher mortality and morbidity, primary graft dysfunction (PGD), and longer intensive care unit stay. [3][4][5] The incidence of post-operative AKI after LT has been described literature, ranging from 9.4-68.8%. [3][4][5][6][7][8][9] This wide range could be explained by the use of different AKI de nitions, and recently, Ploypin et al. reported that the overall estimated incidence rate of AKI after LT is 52.5% via a systematic review adjusting for different AKI de nitions such as the Risk, Injury, Failure, Loss of function; and End-stage kidney disease (RIFLE) criteria and the Acute Kidney Injury Network (AKIN) criteria. 10 Another reason for the wide range of incidence rates could be the timing at which AKI is determined. Some studies have de ned AKI as that within 72 hours after LT, as a concept called early post-operative AKI, 7 others have de ned AKI as that occurring over a post-operative period of 3-14 days. 4,11−13 In the present study, AKI was de ned as that within 1 month after LT, longer than previous studies. We did this to include factors that have rarely been documented, but may pose greater susceptibility to AKI, such as infections with antimicrobial resistance species, and increasing exposure to nephrotoxic agents. We also compared risk factors, post-operative outcomes, including 3-month and 1-year mortality, according to AKI severity and timing of AKI development.

Study Population and Data Collection
We retrospectively reviewed the medical records of 161 LT recipients' treated at Severance Hospital, in Seoul, South Korea from October 2012 to September 2017. Patients were excluded if they were under the age of 18 years, underwent both lung and kidney transplantation, underwent LT with off-pump coronary artery bypass, had already undergone renal replacement therapy(RRT), or had undergone retransplantation. Thirteen patients were excluded (Fig. 1).
Data on variables related to baseline patient characteristics, pre-operative and post-operative outcomes, and survival status were obtained from the hospital medical records. Donor data were collected from the Korean Network of Organ Sharing database.
The primary outcome was the incidence of AKI within 1 month in patients that underwent LT. The AKIN de nition was used to grade the severity of AKI. Secondary outcomes were days of mechanical ventilator usage, length of stay (LOS) in the intensive care unit (ICU), and 3-month and 1-year mortality.
This retrospective study was approved by the Institutional Review Board of Severance Hospital (4-2020-0228) and adhered to the principles of the 2000 Declaration of Helsinki (2000) and the Declaration of Istanbul (2008). And informed consent was obtained from all subjects.
Immunosuppression Protocol and Post-operative Fluid Balance All patients received intravenous methylprednisolone (500 mg) intraoperatively just before the reperfusion of each graft. Post-operatively, patients received a 7-day tapered course of prednisolone, followed by oral prednisolone 0.5 mg/kg/day tapered to 0.15 mg/kg/day over 6 months. Triple immunosuppression therapy, such as prednisolone, tacrolimus, and mycophenolate mofetil, were used to maintain immunosuppression after transplantation. Cefepime and teicoplanin were used as routine prophylactic antibiotics after LT. Ganciclovir, itraconazole, and sulfamethoxazole/trimethoprim were used as routine universal prophylactic therapy.
In the immediate post-operative period, most patients were titrated a daily average of 500 ml of negative uid balance, based on the amount of body weight gain after surgery. Input and output control was slowly modulated in case of AKI or shock status using inotropics.

AKI De nition
AKI was evaluated in accordance with AKIN classi cation. AKI stage I was de ned as an increased serum creatinine (sCr) level ≥ 0.3 mg/dl (≥ 26.5 µmol/l) or an increase in sCr of 150-200% (1.5-to 2-fold) above baseline, along with a urine output of < 0.5 ml/kg/h for more than 6 hours. AKI stage II was de ned as an increase in sCr of 200-300% (> 2-to 3-fold) above baseline, with a urine output of < 0.5 ml/kg per hour for more than 12 hours. AKI stage III was de ned as an increase in sCr of more than 300% (> 3-fold) above baseline, or (sCr) level ≥ 4.0 mg/dl (≥ 35.4 µmol/l), with an acute increase of at least 0.5 mg/dl (44 µmol/l) or placement on RRT and urine output < 0.3 mg/kg/h for 24 hours or anuria for 12 hours. 14 Baseline sCr levels were evaluated before LT. If AKI developed in a patient, we tracked the data daily until the self-remission of AKI. We obtained sCr levels immediately after LT up until 1 month after surgery. The modi cation of diet in renal disease equation was utilized to obtain the baseline estimated glomerular ltration rate (eGFR). Pulmonary hypertension was de ned as the mean pulmonary artery pressure ≥ 25 mm Hg at rest, measured during right heart catheterization. 15 Early AKI was de ned as AKI that developed within 1week after LT. Late AKI was de ned as AKI that developed after 1 week, but within 1 month, after LT.

Post-operative Complications
Primary graft dysfunction was graded according to the criteria of the International Society of Heart and Lung Transplant (ISHLT) Working Group. The proposed standardized de nition of PGD was based on diffuse pulmonary edema in an allograft on a chest radiograph and a PaO /FiO (P/F) ratio. 16 Pneumonia was de ned as the presence of in ltration on chest X-ray and positive sputum cultures on bronchioalveolar lavage cultures requiring antibiotic treatment within 1 month. Sepsis was de ned as any positive blood culture within 1 month after LT, with evidence of organ dysfunction. 17 Operation time was de ned as duration between induction time to end of skin suture time. Ischemia time was de ned as the time interval between the application of the aortic cross-clamp during harvesting and reperfusion of the graft in the recipient.

Results
Characteristics and Incidence of post-operative AKI A total of 148 lung transplants recipients were analyzed. Among them, 59 patients (39.9%) developed AKI within 1 month; 89 patients (60.1%) maintained normal renal function. Twenty-six patients (17.5%) were classi ed with AKI stage I or II and 33 recipients (22.2%) were classi ed with AKI stage III. Among patients with AKI stage III, 88% (29/33) developed AKI within 1 week after LT during the early post-operative period. Meanwhile, the majority of patients with AKI stage I or II had increased sCr levels after 1 week and within 1 month after LT ( Fig. 1).
There were no signi cant differences in demographic characteristics, such as age or sex, between the AKI and the no-AKI groups (Table 1). However, pre-operative comorbidities, including bridging mechanical ventilation (MV) and Extra corporeal membrane oxygenation (ECMO) for respiratory failure prior to surgery, were more severe in the AKI group. Pre-operative anemia and hypoalbuminemia was observed in the post-operative AKI group.

Intra-operative variables and Donor Characteristics
During LT, operation times were signi cantly longer in the AKI group than in the non-AKI group, and a higher number of packed red blood cell (RBC) units was transfused during surgery. Larger uid intake and output was observed in the AKI group during surgery. Ischemia time and size mismatch between the recipient and donor lungs were similar between the groups. There was no signi cant difference in hemodynamic instability (systolic blood pressure under 90 mm Hg or vasopressor use) during transplantation. Over 90% of the patients in the AKI and non-AKI groups underwent bilateral lung transplantation. Most recipients received ECMO support for intraoperative circulatory support than cardiopulmonary bypass (CPB), Donor variables, such as age, sex, and smoking pack years, were similar in the two groups.

Post-operative Variables
In the post-operative period, treatment with intravenous colistin or by inhalation was signi cantly higher in the AKI group. Use of other nephrotoxic agents such as amphotericin B and amikacin, were similar between the two groups. Among patients with AKI, 35.6% received renal replacement therapy due to kidney failure, which was signi cantly higher than that in the non-AKI group.
Univariable and Multivariable Analysis for AKI Risk factors associated with AKI after LT are shown in Table 2. Univariate analysis revealed that preoperative anemia, number of RBC transfusion units, usage of intravenous colistin or by inhalation, and bridging MV or ECMO were risk factors for AKI. After multivariable analysis, anemia, number of RBC transfusion units during surgery, and intravenous colistin infusion were independent risk factors for AKI.
Application of bridging ECMO or MV before LT was not associated with post-operative AKI

Association Between AKI and Other Post-operative Complications
We compared the occurrence of other post-operative complications between AKI and the no-AKI group.
Post-operative bleeding, grade 3 PGD, and sepsis developed more commonly in the AKI group. AKI was associated with longer length of stay in the ICU and prolonged ventilator support during weaning (Table 3). Data are presented as number (%) or medians (interquartile ranges).
Strati ed by AKI stage, grade 3 PGD and sepsis occurred more commonly in patients with AKI stage III.
Also, prolonged ventilator support was more common in the AKI III group (Table 4).  Table 1) Also, there were no signi cant changes in body weight during the post-operative period, indicating that similar management of uid balance was applied.
We further evaluated AKI according to the timing of AKI occurrence. Early AKI was de ned as that within 1 week after LT, late AKI development was classi ed as that from 1 week to 1 month after LT(Supple Table 2). The incidence of early AKI was 67.7% (n = 40), while that for late AKI was 32.2% (n = 19). The reason for limiting late AKI to just 1-month was because, after one month, the incidence of AKI decreased to 7.4%, and after 1 month, there were no cases of HD or RRT (Supple Table 3). There were no signi cant differences in age, primary lung diseases, comorbidities, and bridging MV or ECMO between the two AKI timing groups. Also, there were no differences in intra-operative variables, such as intra-operative bleeding, intra-operative uid intake, and RBC transfusion during surgery between the two groups. Interestingly, development of AKI stage III was more common in the early post-operative period (72.5% vs 21.1%) (Supple Table 2). AKI I or II tended to developed in late post-operative period (78.9% vs. 27.5%).
However, Colistin usage, which was a risk factor for AKI development, was similar and reached over 40% in both groups. Overall, body-weight during surgery increased nearly 6.0 kg in the AKI group, and, slowly decreased to near pre-op body -weight within 2 weeks. However, in the early AKI group, the mean body-  Table 2). In the early AKI group, the proportion of patients who received RRT due to renal failure was higher than that in the late AKI group (65.0% vs 21.1%, P = 0.002) (Supple Table 2).

AKI and Mortality Rates after Lung Transplantation
In Kaplan-Meier curves, the post-operative 3-month mortality rate in patients with no AKI, AKI I or II, and AKI III were 4.5% (4/89), 3.8% (1/26), and 36.4% (12/33), respectively (log-rank test, p < 0.001, Fig. 2A). Patients with no AKI, and those with AKI I or II had no signi cant difference in 3-month mortality. However, the AKI stage III group had signi cantly higher mortality.
The 1-year mortality rates in patients with no AKI, AKI I or II, and AKI III were 20.2% (18/89), 34.6% (9/26), and 72.7% (24/33), respectively (log-rank test, P < 0.001, Fig. 2B). The more severe AKI became, the higher the 1-year morality rate was. We further compared mortality in the AKI III group according to RRT use, and there was no signi cant difference in mortality between those who did or did not receive RRT(Supple Table 4). In the univariate Cox proportional model used for analyzing 1-year mortality, age, albumin, number of RBC transfusion units, and sepsis were risk factors for mortality after LT. In multivariable analysis, age and AKI were independent risk factors for 1-year mortality (Table 5).

Discussion
In our study, we focused on the development of AKI within 1 month, a longer follow-up duration than those in previous studies, which primarily focused on AKI at post-operative 72 hrs or 1 week. [3][4][5][6][7][8][9]18 Re ecting the natural course of kidney injury after LT, AKI more often appears to be an epiphenomenon of the pre-, intra-, and post-operative clinical course after LT. A longer duration of follow up would be necessary to analyze post-operative factors, such as using nephrotoxic agents, sepsis, and other postoperative complications that affect AKI.
The incidence of AKI within 1 month after LT de ned by AKIN classi cation was 39% in our cohort, lower than that reported in other studies. This could be explained by the older age and different proportions of primary lung disease compared to other studies. Serum creatinine is usually underestimated in older adult patients due to reduced muscle mass 19 , and thus, changes in sCr levels could have been underestimated in our cohort. In the ISHLT registry report chronic obstructive pulmonary disease (COPD) was the most common indication in the last decade, while idiopathic interstitial pneumonia was the second. 20 However, in our cohort the proportions of IPF and connective-tissue disease related interstitial lung disease were over 60%, while that of COPD was lower than 10%. These differences may explain the The main nding of our study is that pre-operative anemia, number of RBC units transfused during surgery, and usage of colistin intravenous for multi drug resistant (MDR) pathogens independently are correlated with AKI development. Ho et al reported that in cases of cardiac surgery with cardiopulmonary bypass, nearly all patients are at the 'initiation phase' of ischemia-reperfusion kidney injury with proximal tubular dysfunction. They suggested that the 'extension phase' of kidney injury is related with the severity of in ammatory response, renal hypoxia, and oxidative stress. 22,23 As CPB, and ECMO are used during LT, similar principals would likely apply. Accordingly, one could be reason that those described risk factors would be variables preventing recovery from initial kidney injury, therby leading to, progressing to AKI.
Underlying anemia has been shown to be associated with prolonged hypoxemia and could result in AKI in the post LT period. 24 Recipients of LT already face respiratory failure due to primary lung disease, which could be further aggravated by anemia, inducing a prolonged hypoxemic status. This reduced oxygencarrying capacity elicits sympathetic nerve system increases in vascular resistance, causing vasoconstriction and leading to renal hypo-perfusion. 25 Thus, anemia prior to surgery risk for developing AKI in LT recipients.
An increased amount of packed RBC transfusion units can induce transfusion-related adverse effects. Intraoperative RBC requirements are most likely necessary due to intraoperative requirements for hemodynamics, although in our analysis, observed hemodynamic instability during surgery was not statically related to AKI development, despite RBC requirements being statistically higher in the AKI group.
Garg et al. reported in a randomized controlled trial, that restrictive transfusion of RBC was not inferior as an outcome of post-operative AKI in cardiac surgery with CPB. 23,26 A unit of RBC is known to be stored up to 42 days and can undergo erythrocyte membrane change, becoming more fragile and leading to progressive hemolysis. Accumulation of pro-in ammatory molecules, free hemoglobin, and iron can lead to the development of AKI. 27 Among nephrotoxic antibiotics, Intravenous colistin which was used to treat MDR pathogens, was an independent risk factor for developing AKI. Colistin has high intrinsic renal toxicity and accumulates within the proximal cortical tissue. Combined with anemia, age, liver disease, and baseline GFR, colistin has been reported as a risk factor for AKI. 28 Thus, these ndings support that LT recipients who already have anemia are prone to AKI after colistin use. In Southeast Asia, an increased incidence of MDR infections among ICU patients has been reported. 29 A similar situation was observed in our study. A total of 54% (n = 80) of the recipients experienced infections with MDR pathogens. The proportions of Acinetobacter, Pseudomonas, and Klebsiella were 30.4% (n = 45), 15.1% (n = 22), and 18.9% (n = 28), respectively.
In patients who developed AKI, post-operative complications of post-operative bleeding, grade 3 PGD, and sepsis occurrence were more common, compared to those without AKI. The cause and effect of AKI, as well as the complications, were unclear because patients with these complications could develop AKI both early and later after LT. However, in our study, it was clear that sepsis and grade 3 PGD were strongly associated with severe AKI, the majority of cases of which occurred within 7 post-operative days, resulting in increased mortality.
In this study, poor respiratory outcomes (prolonged ventilator days and longer ICU stay) were more common in the AKI group. The renal tubular epithelium is a major site of cell injury and death in AKI, where cytokines, oxidative stress, and leukocytes initiate local and systemic in ammation. The lungs, with thier large capillary networks, are capable of sequestering a large number of in ammatory/immune cells. 30 Moreover, AKI-induced derangement of nitric oxide synthase and heme oxygenase (key oxidative stress enzymes) may in uence lung function. 31 This could lead to post-LT respiratory failure, leading to di culty in weaning a patient off the ventilator.
Post-operative treatment of transplanted sick patients can be di cult and challenging. In our study, recipients who develop of AKI experience poor post-operative outcomes and mortality. Post-operative AKI could be a marker indicating that LT recipients are not going to do well. Pre-operative anemia, number of RBC transfused units during surgery, and usage of intravenous colistin were independent predictors of the development of AKI (Fig. 3).
Managing the di cult course involving AKI could be important in post-operative care of transplanted sick patients. Technical bleeding control and not having an excessive packed RBC transfusion would be considered in individuals susceptible to AKI. Also, in instances where an infection with MDR pathogens is documented within 7 days, waiting to determine whether it is a true infection or contamination may be advisable, and using colistin inhalation rather than colistin infusion could be another option for post op management. Also, if the candidates were in vulnerable for developing many complications such as AKI selection of recipients for lung transplantation would be a better management strategy, considering organ scarcity and predicted survival outcome.
Our study had several limitations. It was single-center, retrospective cohort study, thus generalizability of our ndings may be limited. Second, dosage and duration of administered nephrotoxic agents were not fully evaluated due to inadequate information.

Conclusion
AKI was associated with worse post-operative outcome and 3-month ,1-year mortality after LT. Severity of AKI was usually determined in early post op period after LT, so optimal post-operative management as well as recipients selection should be done in sick candidates with highly susceptible to become AKI.

Con ict of interest statement
Authors have nothing to disclose with regard to commercial support.

Funding
There is no funding to declare.