The principal findings of this single-centre, retrospective propensity score matching study can be summarized as follows. Comparing with normal preoperative renal function, mild preoperative RD: (1) did not influence the in-hosptial mortality and most of the postoperative in-hosptial morbidities. (2) increased postoperative in-hosptial rates and severity of AKI. (3) decreased long-term survival and increased long-term new onset of dialysis.
Propensity matched pairs analysis provided the opportunity to rule out confusion by providing balanced baseline and procedural characteristics except the baseline eGFR. In this study, patients with mild preoperative RD or with normal preoperative renal function had similar rates of in-hospital surgical mortality. The univariate factor analysis manifested that the 2 propensity score-matched groups had similar in-hospital outcomes, including surgical mortality, myocardial infarction, stroke, respiratory failure, pneumonia, redo for bleeding, RBC transfusion, DSWI, low cardiac output syndrome, IABP application, except the rates and severity of AKI. The same result was obtained by Weitie Wang et al., they reported older patients with mild preoperative RD had a higher mortality rate than normal patients in long-term survival, whereas no evidence of worse in-hospital mortality rate was found . Jyrala et al. who analyzed a cohort of 885 patients with or without mild preoperative RD received on-pump cardiac surgery, with respect to short- and long-term outcomes . They found mild preoperative RD was a marker for patients with increased cardiac risk factors and the risk for poor outcomes. Their conclusion was in line with our study about postoperative late survival but was different from in-hospital mortality. Reasons of this difference lay in the study population, because our cohort was primary isolated CCABG, and we chose the eGFR as the indicator for the evaluation of renal function.
CPB still contributes to RD due to multiple perturbations in renal physiology and function as mentioned before. We speculated the reasons why there was no difference of mortality between the 2 groups were as follows: (1) average CPB time of primary isolated CABG was relatively short, most of patients with mild RD had enough functional reserve and had the ability to withstand acute insults. (2) group B had an increased postoperative in-hosptial rates and severity of AKI, but AKIN stage I accounted for most cases. (3) relative high perfusion pressure was kept to guarantee the renal perfusion . (4) There are no active treatments for AKI, and therefore, perioperative preventative strategies seem particularly promising. Keep adequate hydration and avoid the use of diuretics, except for specified medical indications; minimize the use of medications with adverse effects on renal function, such as contrast agents, α-adrenergic agents, colloids and exogenous blood products; keep an optimal hemodynamic status and correct the acid-base or metabolic imbalance.
It is reported that age, DM, preoperative RD, blood transfusion, low cardiac output syndrome, low LVEF are independent risk factors of AKI [13, 14]. There was no significant difference of these independent risk factors except the baseline eGFR between the 2 propensity score-matched groups. In this study, compared with patients with normal preoperative renal function, patients with mild preoperative RD were associated with increased postoperative in-hosptial rates and severity of AKI. The severity of AKI was defined by the AKIN , AKIN stage I accounted for most patients in both groups in this study. AKI defines as a clinical syndrome characterized by a sudden decline of the excretory kidney function, with accumulation of urea and creatinine and decreased urinary output [16, 17]. Depending on the specific definition and methodological concerns, AKI occurs in up to one third of patients undergoing CABG, and approximately 2% require temporary dialysis . The AKIN criteria may be quite sensitive for detecting subtle changes in kidney function, especially the creatinine increase of 0.3 mg/dL within 2 days. Some previous studies demonstrated that development of AKI was associated with high short-term and long-term morbidity and mortality . The demonstration was consistent with our study about the long-term mortality but was different from postoperative short-term mortality. Reason of this difference was supposed that we only recruited first isolated CCABG with normal or mild RD, and AKIN stage I accounted for most patients postoperatively, and higher incidence of complete revascularization achieved by CCABG may also be a possible explanation. Another hypothesis was that the increased hemodynamic instability that occured with off-pump CABG negate the benefits of avoiding CPB .
As expected, the follow-up demonstrated that patients with mild preoperative RD showed a trend to lower long-term survival compared with normal preoperative renal function patients (96.2% vs 93.1%, p = 0.037). Cox regression manifested that grouping (mild vs. normal preoperative renal function) was a significant variable related to the long-term survival, and the HR was 1.67 (95%CI 1.09–2.90, p = 0.035). Furthermore, in our study patients with mild RD were associated with a higher rate of new onset of dialysis compared with normal renal function patients during the follow-up (0 vs 6, χ2 = 4.432, p = 0.039). Cox regression also showed the HR for new onset of dialysis was 1.52(mild vs normal preoperative renal function, 95%CI 1.14–2.49, p = 0.027). The exact causes of these differences are unknown and further research is needed. Preexisting kidney disease has been repeatedly identified as a strong predictor of AKI after cardiac operations , moreover, AKI has been associated with progression to chronic kidney disease (CKD) and dialysis in many reports . CKD and dialysis might exert negative efferts on the long-term survival inevitably. Günday and colleagues  conducted a study focused on the CABG patients with normal renal function or mild RD, they concluded that mild preoperative RD reduced coronary flow reserve after CABG surgery due to deterioration of the micro-vascular bed. Unfortunately, there are no pharmacologic agents known to reduce the risk of AKI or treat established AKI . Therefore, CABG patients with mild RD need to strengthen the follow-up of nephropathy, more strictly management of the risk factors of coronary artery disease postoperatively.
Firstly, a retrospective, non-randomized single-centre analysis over a long period of time and with different surgeon’s procedures on patients undergoing CABG is subjected to the effects of selection bias. Although propensity score matching is implemented, a prospective, multi-centre study involving larger sample size is needed. Secondly, the GFR was derived by using the MDRD formula, which was not designed for determining GFR. Finally, the initial RIFLE (risk of RD; injury to the kidney; failure of kidney function; loss of kidney function; and end-stage kidney disease) criteria do not include the creatinine increase within 48 h for defining the mildest form of AKI, whereas the RIFLE categories injury and loss are similar to AKIN stage 2 and 3, respectively . AKI defined according to the RIFLE criteria may therefore avoid an iatrogenic hemodilution but may miss more subtle changes of kidney function [19, 25].