AkI is associated with several complications as multiple organ dysfunction, systemic inflammation, and increased mortality. Based on a study on 50,314 adult surgical patients undergoing major inpatient surgery, the risk-adjusted average cost of care for patients undergoing surgery was $42,600 for patients with AKI compared to $26,700 for patients without AKI [8, 9].
Depending on the diagnostic criteria used to define AKI, the incidence of AKI following cardiovascular surgery ranges between 0.7–31%. This holds also true with respect to mortality rates which have been reported to vary between 30 and 90% [10]. The present study has been initiated to compare two different AKI-classification systems. The clinical SOP-criteria designated patients who were subjected to RRT following postoperative impairment of renal function as AKI positive. Patient data were then re-evaluated following the RIFLE-system, which includes serum creatinine and GFR, and AKI defined by this protocol.
Based on the SOP-definition, 10% of the patients developed AKI. Of these 48% died. The incidence of RRT was 10%, which is within the range reported by Kiers et al. (9.3%) [11]. Kristovic noted an incidence of 3.5% in a retrospective study, which is much lower than the values assessed in the present study [12]. Finally, from 31,677 patients who underwent open-heart surgery, 555 (1.7%) patients developed severe AKI requiring dialysis [13]. Presumably, the number of patients requiring RRT may at least partially depend on the patient cohort and kind of surgery.
Independent on this, 7% of the patients developed AKI according to the RIFLE classification with a mortality rate of 22%. The differences are remarkable. Obviously, integration of creatinine and GFR as risk parameters may reduce the number of patients classified into the AKI-group. Therefore, it is not surprising that mortality rates are also diminished compared to the one of patients who were subjected to hemofiltration (SOP-criteria). Similar to the present study, high AKI rates and mortality have been documented by others when AKI has been directly correlated to RRT. Based on Bahar et al., mortality of patients who developed postoperative AKI mandating hemodialysis was 79.9%, and Chertow et al. calculated the mortality of AKI patients sufficient to require dialysis to be 63.7% [14].
Since patients may develop AKI without requiring RRT, the AKI criteria has been widened by including elevation of serum creatinine levels or the reduction of urine output. Still, incidence and mortality of AKI strongly depends on the patient population and the final criteria included. The own AKI-evaluation was based on the RIFLE criteria with a threefold elevation of serum creatinine, or a maximum creatinine level of 4.0 mg/dl, or a decrease of the GFR by 75%.
Evaluation of patients after transcatheter aortic valve implantation by the RIFLE score revealed an AKI incidence of 17.9%, which is similar to the data given by Jorge-Monas on patients with cardiac surgery–associated AKI [15]. Incidence of AKI of patients undergoing elective, urgent or emergency cardiac surgery using the same scoring system was 25.1% [16].
In fact, 49.9% adult patients undergoing cardiac surgery, with or without cardiopulmonary bypass, developed postoperative AKI according to RIFLE, however, elevated serum creatinine was observed in only 9.7%, whereas oliguria was noted in 40.2% [17].
The different pathophysiologic significance of creatinine level and urine output might partially explain the heterogeneity of clinical studies. Indeed, the development of AKI is rather complex and depends on several parameters. AKI risk factors may closely depend on the patient cohort included in the study, the medical history and treatment practice. Based on the present work, a significant correlation has been found between the patient’s sex and AKI development, independent on whether the DOI or the RIFLE criteria has been applied. Gender associated AKI-incidence has also been reported by others with women being more likely than men to develop cardiac surgery-associated AKI postoperatively [18].
An observational study of 15,221 nondialysis-dependent patients undergoing cardiac surgery demonstrated an increased AKI-risk of males as well. The same was true with respect to AKI incidence after aortic valve replacement. In contrast, a retrospective single-centre cohort study of 565 consecutive patients who underwent isolated coronary artery bypass grafting with the use of cardiopulmonary bypass did not reveal any correlation between sex and AKI development [19], and Neugarten et al., analyzing sex differences in acute kidney injury requiring dialysis, even showed that male sex might be associated with an increased incidence of hospital-associated AKI [20]. Independent on the fact, that the references cited are based on different patient populations which may contribute to the contradictory statements, there is no general consensus that being a woman is an independent risk factor for the developing AKI. To optimize gender-associated risk assessment, a uniform scoring system is required which should additionally taken care on the type and severity of the illness and the type of surgical intervention. Although the own investigation did not reveal any relationship between gender and RRT, other reports did with nondialysis-dependent females but dialysis-dependent males being associated with AKI. This finding requires further evaluation.
Patients with an old age more often developed AKI than younger patients did. These alterations concern loss of renal mass, loss of tubules and sclerotic changes, which all may be accompanied by a decreased glomerular filtration rate and renal blood flow [21]. Screening healthy kidney donors have revealed a decline of GFR at a rate of 6.3 mL/min/1.73 m2 per decade. Consequently, due to the less kidney functional reserve, older people may be at higher risk for AKI. Not at least additional basic complaints which are more common in older people may account for the relationship between renal function and age. In good agreement with the own data, recent trials have identified old age as an independent factor associated with a higher risk for AKI and RRT failure [22].
Beside patient specific characteristics, several clinical risk factors have been identified to be associated with AKI. Patients requiring IABP more often received RRT than patients without IABP. The finding, which has also been shown by others, is interpreted in a way that IABP is used for critically ill patients with a low cardiac output. Since impairment of the cardiac output is directly attributed to hypoperfusion of the kidney and loss of kidney function, it might not be astonishing that patients with a pre-damaged kidney are highly predestinated for developing AKI. Interestingly, Zhang and coworkers provided evidence that preoperatively applied IABP may significantly reduce the incidence of AKI of high-risk patients [23]. A study on patients with acute myocardial infarction, severely impaired left ventricular ejection fraction or low output syndrome undergoing coronary surgery demonstrated a potent benefit of the preoperative compared to intraoperative IABP [24]. Nevertheless, whether preoperative IABP might provide any advantage for the patient cohort evaluated here is not clear. Further evaluation is necessary to investigate the risk-benefit balance of this strategy.
A high percentage of patients undergoing heart valve operations developed AKI. The relevance of valve surgery as a risk parameter became evident in both the SOP and the RIFLE cohort. This finding is notable, indicating that AKI in the context of valve replacement is not exclusively restricted to patients with RRT. Rather a high creatinine level may also linked to AKI in this matter. Indeed, data provided by others indicate that AKI, defined by either RIFLE, AKIN (Acute Kidney Injury Network) or KDIGO (Kidney Disease: Improving Global Outcomes), occurs in up to a quarter of patients following aortic valve replacement [25].
Haase et al. suggested that AKI may also reflect a form of pigment nephropathy with hemoglobin being the pathophysiologic key factor [26]. This postulate has recently been supported by others demonstrating distinct hemolysis and heme pigment deposition in the renal tubules following cardiopulmonary bypass. In good context, a case–control study of AKI patients provided evidence that patients who developed AKI had twice the plasma-free hemoglobin at the end of cardiopulmonary bypass than those who did not develop AKI, despite similar AKI risk profiles and identical cardiopulmonary bypass durations in each group. The authors reasoned that the pump of the CPB circuit along with the oxygenator, suction catheters, and filters damage erythrocytes and increase plasma-free hemoglobin which then may contribute to the development of AKI following cardiac surgery [27].
When interpreting the valve data, it must be considered that co-parameters may additionally influence the patients outcome. In this concern, a retrospective analysis of 7233 cardiac surgery patients showed that valve surgery combined with coronary artery bypass grafting might contribute to AKI more distinctly than each factor alone [18].
Further independent risk factors have been identified in the present evaluation: Previous coronary intervention, cross clamp time and duration of the surgical process. Mean value of surgical time was calculated to be 270 min. Yamauchi et al. recently pointed to an operation time longer than 8 h as an independent risk factor of postoperative AKI. However and in contrast to the own analysis, this study was enrolled on non-dialysis-dependent patients who underwent valvular operations using cardiopulmonary bypass. It is, therefore, concluded that AKI more often develops when the duration of cardiac surgery is prolonged. Still, there is no general rule, the critical time point rather depends on the kind of operation.
