We observed that AKI occurred in over 30% of the patients undergoing SAVR. This was higher than the rate of 20–25% documented in a series involving three other cardiac centers [
13] and much higher compared with our prior observation of 5% [
6]. However, the current results were comparable to those found in another recent series [
15] and could be explained by an increase in age and comorbid burden. This increase was already apparent before 2007 and continued to the end of the inclusion in 2017 [
7,
8]. The currently used diagnosis of AKI is based on changes in the GFR. We observed a significant effect of age, chronic renal and pulmonary dysfunction, and cardiovascular factors on postoperative AKI. Furthermore, almost all recorded adverse postoperative events were associated with AKI. An increased need for postoperative resources was also documented in patients suffering from AKI. The ICU stay was 5 days longer, while the postoperative hospital stay was 6 days longer. The need for blood products and PPM implantation was also significantly higher. The duration of mechanical ventilation was prolonged by 24 h. All observed laboratory values were significantly worse in patients with AKI. CKD was identified as the dominant factor in a univariate analysis, but preoperative endocarditis and a need for reintervention were the most relevant independent predictors of AKI in a multivariate logistic regression analysis. The observed mortality was almost eight times higher in patients with AKI. Except in one patient, AKI was never the sole cause of death in the first 30 days.
4.1. Risk Factors and Predictors for AKI
In a univariate analysis, a reduced preoperative GFR was the strongest factor in the development of postoperative AKI. In a prior published series, a preoperative GFR below 30 mL/min resulted in a 4.5 times increase in AKI [
13] compared with patients with a higher GFR, and the estimated GFR was identified as the most significant factor [
16]. A comparable effect was also seen after cardiac surgery in general [
12]. CKD is more common in patients of 80 years and older [
17], which makes age an important factor in the development of AKI. This was confirmed in the current and prior series [
4,
5,
6,
12,
18]. One could assume that an age-related decrease in the GFR and renal blood flow within the glomerular capillaries plays a major role, but these effects vary widely between subjects. Associated structural changes include loss of renal mass, hyalinization of afferent arterioles, an increase in sclerotic glomeruli, and tubulointerstitial fibrosis [
19]. Responses to vasoconstrictor stimuli such as the renin–angiotensin–aldosterone system (RAAS) are enhanced in elderly people. Sex seems to play a modulating role [
19]. Male patients seem more vulnerable to AKI, since testosterone seems to activate the RAAS, while 17-beta-estradiol seems to lower it. The effect of male sex on the development of AKI was observed in some [
11,
13] but not all series [
3,
4]. Chronic pulmonary disease was also identified as a factor in the development of AKI in the current and past series [
3,
5,
12]. A higher body mass index (BMI) has also been identified as a factor for postoperative AKI after cardiac surgery [
3,
4,
15], but not in the current series, where a threshold of 30 kg/m
2 was used. Obese individuals suffer from a higher degree of hypertension and diabetes. The latter conditions contribute to renal damage and glomerulopathy [
15], which has been identified as a potent factor for AKI [
3,
4,
5,
12,
13,
18].
The need for concomitant CABG also increased the likelihood of postoperative AKI. This was also documented in earlier series and reviews, where a combined operation increased the risk compared with valve replacement alone or CABG alone [
12,
13,
20]. However, prolonged ACC and CPB times, which are associated with concomitant procedures, were also implicated in the development of postoperative AKI [
15,
16,
20]. A possible explanation is the increased rates of diabetes, hypertension, vascular disease, and CKD, which are well-known risk factors for coronary artery disease and, thus, for the need for CABG and longer CPB times [
20]. Other cardiovascular factors that can increase the risk for postoperative SAVR include severe symptoms [
13], prior cardiac surgery [
13], prior congestive heart failure [
18], preoperative myocardial infarction [
6,
18], preoperative atrial fibrillation [
6,
18], a need for non-elective surgery [
11,
12,
13,
18], and a low ejection fraction [
5,
13,
18]. In current and past series, a need for prolonged ventilation [
4,
5], reintervention [
5,
18], and transfusion [
3,
5,
11,
12,
18,
21,
22] was also associated with the development of AKI. A prolonged storage of red blood cells could cause structural cellular changes, which release pro-inflammatory molecules and lipids, promoting coagulation. Hypotensive states, which result from blood loss, should be avoided, since such states could promote the development of AKI [
3]. The issue of a prolonged CPB time can be important for other reasons. The ratio between the actual lowest pump flow and the target pump flow could serve as a surrogate for low oxygen delivery, which could have an effect on the development of AKI [
11]. Hypoperfusion of an oxygen-demanding medullary area of the kidney reduces oxygen-carrying delivery because of hemodilution, especially if the hematocrit level is below 25% [
12]. A strategy of maintaining a hemoglobin concentration level at 7.5 g/dL seems non-inferior to the level of 9.5 g/dL during the CPB run and to the level of 8.5 g/dL in the postoperative period with respect to adverse outcomes, including AKI [
23]. Since a prolonged need for mechanical ventilation and a need for reintervention because of bleeding could be considered early postoperative events, we reasonably expect these events to precede AKI. This allows us to enter these events into a multivariate analysis and identify them as predictors. Other factors include inflammation resulting from contact of blood with foreign material; manipulation and clamping of the aorta with consequent thromboembolism; ischemia–reperfusion damage; reduced cardiac output; hemolysis with release of free hemoglobin and free iron, promoting oxidative stress [
3]; contrast nephropathy in the case of recent medical imaging; downregulation of vasodilatory mediators, such as nitric oxide; and upregulation of vasoconstrictive mediators, such as endothelin, catecholamines, and angiotensin II [
3].
A comparison between SAVR and TAVI might be instructive with respect to AKI. A significant decrease in postprocedural AKI was observed over time, but its occurrence was associated with an increase in one-year major adverse events and mortality rates [
24]. The occurrence of AKI had no major effect on transvalvular gradients after TAVT [
25], which was also observed in the current series. AKI does not seem to be more common in patients of the ‘grey zone’ after TAVI compared with SAVR with a sutureless valve, according to a recently published meta-analysis [
26]. The use of sodium–glucose cotransporter inhibitor-2 (SGLT2i) therapy seemed to protect against or at least mitigate AKI in diabetic patients with chronic kidney disease undergoing TAVI. Its effect could be mediated via improvement of the tubulo-glomerular feedback and the reduction in glomerular hyperfiltration via the vasoconstriction of the afferent arterioles. SGLT2i also decreased oxidative stress, the degree of inflammation, and overactivity of the sympathetic nervous system and the renin–angiotensin–aldosterone system [
27]. AKI was associated with early mortality in the current and prior series [
13,
17]. A need for RRT also increased early death [
3]. However, age, ACC duration, CPB, and age, which affected AKI in the current series, were also predictors for mortality [
16]. In the current series, postoperative LOS in the intensive care unit significantly increased by 5 days, while the postoperative LOS increased by 6 days. Other series showed an increase in LOS of 2–4 days in grade-1 AKI, 4–10 days in grade 2, and 9–16 days in grade 3 [
13]. In addition, patients suffering from AKI were significantly more likely to be readmitted to the ICU during their hospital stay [
3]. In the current series, patients suffering from AKI had reduced long-term survival. After an initial divergence, both survival curves ran parallel. Eleven predictors for long-term mortality were identified using Cox’s proportional hazard analysis. Age was the strongest predictor for reduced survival, but seven other preoperative predictors were identified. A CPB time of over 120 min reflects a more complex operation and, hence, a more complex disease. The two postoperative predictors included delirium and AKI. The effect of AKI on survival was documented in an earlier series of patients undergoing cardiac surgery and was most evident within the first three months after surgery [
2]. AKI of grade 2 or more affected survival to a large degree [
14]. The effect of AKI on survival was confirmed in a meta-analysis in a non-cardiosurgical setting [
28].
An important issue is a limited postoperative increase in plasma creatinine levels of between 0.06 and 0.30 mg%, which is below those corresponding to stage-1 AKI. In a nationwide observational series, this event was associated with an increased risk for 30-day all-cause mortality and a higher risk for long-term CKD and heart failure, but the incidence of these adverse events was lower compared with patients with a clinical AKI of grade 1 [
29]. This so-called “subclinical AKI” could include some form of tubular kidney damage mediated by hemodynamic alterations and inflammatory responses [
29]. The association between small creatinine increases and the development of heart failure may be attributed to a type-3 cardiorenal syndrome [
29]. In patients suffering from a clinically overt AKI, these mechanisms would act more strongly. Readmission within 30 days because of heart failure after cardiac surgery was significantly higher in patients who suffered from overt postoperative AKI [
30]. A meta-analysis showed that in patients from the general population who suffered an episode of AKI, the risk for long-term heart failure, acute coronary syndrome, and other unspecified major adverse cardiac events significantly increased. The rate of heart failure increased with the severity of AKI. Inflammation, activation of neuro-endocrine systems, mitochondrial dysfunction, metabolic acidosis, and high serum potassium levels could play a role in the development of these conditions. On the one hand, the renin–angiotensin system could contribute to renal vasoconstriction; on the other hand, this system could promote endothelial dysfunction, cardiac fibrosis, ventricular dysfunction, and heart failure [
28].
4.2. Prevention of AKI
Maintaining adequate circulation during CPB and avoiding nephrotoxic agents are preventive measures against the development of AKI. Renal autoregulation is a mechanism to maintain renal blood flow and the GFR, even with oscillating pressures during the use of CPB. The interaction between maintenance of the systemic circulation and renal autoregulation determines the risk for AKI. A reasonable target for mean arterial pressure during CPB would be between 65 and 75 mm Hg, with a flow rate between 2.2 and 2.4 L/min/m
2, which would ensure adequate oxygenation. The use of CPB should be kept as short as possible [
12,
31]. In patients with left-ventricular hypertrophy, which is often the case in aortic valve disease, an adequate preload can be maintained via judicious administration of intravenous fluids, but large volumes of isotonic saline should be avoided [
12]. An intensive control of plasma glucose by keeping it between 80 and 110 mg% reduced the need for RRT [
32]. However, strict control of plasma glucose in critically ill patients carries the risk of hypoglycemia [
33]. Once AKI is established, an early initiation of RRT could prevent the development of metabolic acidosis, symptomatic uremia, hyperkalemia, and volume overload unresponsive to diuretics. The optimal timing to start RRT in severe AKI is uncertain, but a meta-analysis suggested that starting within 24 h resulted in a favorable outcome [
34]. A routine use of prophylactic RRT after cardiac surgery in patients at risk for AKI could not be supported [
12].
The limitations of this study are its retrospective nature. Selection bias was limited by the consecutive inclusion of patients undergoing SAVR. Postoperative GFR was estimated at its lowest level and not at a fixed time after surgery. However, almost all values of GFR were determined within the first 24 postoperative hours. The robustness of the model predicting AKI as an outcome was improved by sensitivity analysis. Cox’s proportional hazard analysis for long-term survival was used as an alternative to a propensity score-matching analysis. Access to digitalized medical files allowed a more detailed description of the patients compared with a nationwide series. Many patients resided in a nursing facility during the long-term follow-up because of the high mean age at inclusion, which limited access to the data needed with respect to adverse cardiac events.