The aim of this study was to find out the relationship between perioperative anaemia and postoperative AKI after PN surgery. In present findings, we found perioperative anaemia occurred in 35.29% of the patients undergoing PN for a single nonmetastatic renal tumor, and postoperative AKI developed in 32.4% of the patients when KDIGO criteria was applicated. In addition, we found that the patients with perioperative anaemia had a higher risk of AKI than those without anaemia (58.3% (14/24) vs. 18.2% (8/44)). Moreover, univariate logistic regression analyses revealed that anaemia was a risk factor of postoperative AKI.
Anaemia is a common complication within the surgical populations for various reasons. Numbers of studies suggested an association of anaemia in the perioperative period with postoperative adverse outcomes, like AKI, stroke and myocardial infarction[7, 8]. Our data exhibited the high prevalence of postoperative anaemia in patients undergoing PN. Multifactorial in origin, preoperative anemia, perioperative blood loss (surgical bleeding, coagulopathy, phlebotomy, etc.) and progressive blunted erythropoiesis are the main factors that cause postoperative anaemia after clinical surgery[7]. Likewise, our univariate analysis results showed that the occurrence of postoperative anemia was related to the ischemia time, blood loss and surgical approaches. Furthermore, haemodilution caused by excessive infusion may lead to "dilutional" anemia or aggravate previous anaemia.
The incidence of postoperative AKI in our research was high. An important cause of AKI in PN surgery is cellular ischemia, which results in tubular epithelial and vascular endothelial injury and activation[15, 16]. The higher NGAL level in the anaemia group confirmed this in our research. As reported, the decline in renal function after PN averages about 20% in the operative kidney[3]. In fact, the patients enrolled in our study underwent more complicated surgery (R.E.N.A.L. score was 6.7 ± 1.5 for patients in perioperative anaemia group and 6.1 ± 1.1 in non-anaemia group, which meant middle complexity and is concerned with outcome of PN[17]) and suffered more blood loss and longer ischemia time that culminated in renal hypoxaemia [1, 8], which might increase the risk of postoperative anaemia and AKI. In the past, postoperative AKI was thought to be a transient injury, which had no effect on prognosis after recovery. However, studies have demonstrated that postoperative AKI, even short-term AKI (lasting 48–72 hours), could increase the risk of long-term functional impairment[18].
However, it is not clear whether the presence of anaemia is a direct contributor to AKI or simply a co-morbid disease that indirectly leads to AKI through potential factors. In previous study, Hales[19] found that AKI could contribute to the development of anaemia as a result of reduced EPO production, an increased risk of bleeding and reduced red cell life span. It has also been demonstrated that anaemia was a risk factor of AKI in patients underwent major surgery, leading to increased postoperative adverse outcomes[7, 20, 21]. Postoperative anemia may affect renal function via either a higher incidence of hypotension or a decrease in oxygen-carrying capacity to enhance renal oxidative stress[22]. Although a blood transfusion might improve oxygen delivery, less transfusion is consistently related to a decreased risk of morbidity and mortality[23–25]. Moreover, red cell viability is impaired during storage, resulting in excess hemolysis after transfusion. As a result, transfusions may induce iron release from macrophages that results in oxidative renal injury[26, 27]. Therefore, transfusion itself may have affected the incidence of AKI [28, 29]. In our study, 3 patients had blood loss and adequate transfusion to correct their hematocrit to > 30%,of which 1 and 2 developed AKI when KDIGO and NGAL criteria was used respectively. The adverse consequences of anaemia are likely to be severer during PN surgery, during which, for reasons outlined earlier, postoperative AKI is more prone to occurrence.
Our results suggest that postoperative anaemia is a risk factor for the development of AKI in patients undergoing PN, though the correlation is weak, which is probably compromised by other stronger predictors of AKI, such as ischemia time and blood loss. Nevertheless, our study still has a few limitations: Firstly, it was a single-center retrospective study, which might undergo selection bias. Secondly, the sample size was small, so we cannot perform a multivariable analysis to determine the independent predictive ability of perioperative anemia. Thirdly, we did not evaluate the long-term effect on patient’s kidney function and could not draw any conclusion if perioperative AKI can be reversed. As a result, further large-scale, long-term and prospective studies should be conducted in multi-centers to verify the detailed correlation between anaemia and postoperative AKI.