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**Figure 6.** Meta-regression analyses showed that year of the study did not significantly affect hospital mortality among patients receiving ECMO with AKI requiring RRT (*p* = 0.86). The solid black line depicts the weighted regression line based on variance-weighted least squares. The inner and outer lines represent the 95%CI and prediction interval encompassing the regression line. The circles indicate log event rates in an individual study.

#### *3.3. Evaluation for Publication Bias*

Funnel plots (Figure 7) and Egger's regression asymmetry tests were utilized to assess for publication bias in our meta-analyses evaluating the incidence of AKI and severe AKI requiring RRT while on ECMO. There was no publication bias as determined by the funnel plot and Egger's regression asymmetry test with *p* = 0.62 and *p* = 0.17 for the incidence of AKI and severe AKI requiring RRT, respectively.

**Figure 7.** Funnel plot demonstrated no publication bias in analyses evaluating (**A**) incidence of AKI in patients requiring ECMO and (**B**) severe AKI requiring RRT.

## **4. Discussion**

The findings of our meta-analysis demonstrate that patients who required ECMO had incidence rates of AKI (using standard AKI definitions) and severe AKI requiring RRT of 62.8% and 44.9%, respectively. Moreover, patients with AKI and severe AKI requiring RRT had high associated mortality rates of 62.0% and 68.4%, respectively.

Although the mechanisms underlying ECMO associated-AKI remains unclear, it is likely complex and multifactorial, including contributing factors such as primary disease progression, altered hemodynamics, low cardiac output syndrome, exposure to nephrotoxic agents (for managemen<sup>t</sup> of underlying diseases), new-onset sepsis, high intrathoracic pressures, fluid overload, ischemia-reperfusion injury, release of proinflammatory mediators and oxidative stress, hemolysis and iron-mediated (hemoglobin-induced) renal injury, and hypercoagulable state resulting in renal microembolisms [4,8,68,78,79]. Studies have demonstrated the activation of proinflammatory mediators such as tumor necrosis factor-alpha (TNF-α), interleukins (e.g., IL-1β, IL-6, IL-8) and other cytokine signaling cascades due to the continuous exposure of blood to non-biological and non-endothelialized ECMO interface [68,80,81]. Activation of the inflammatory cascades can result in hyperdynamic vasodilated hypotensive states, leading to AKI [68,78].

Following the initiation of ECMO treatment, there are improvements in oxygenation and oxygen consumption as well as hemodynamics [3,5–9]. However, ischemia-reperfusion injury can also occur after the restoration of circulation to previously hypoxic cells and hypoperfused organs, leading to the production of reactive oxygen species (ROS) and oxidative stress-mediated injury [68,78]. In addition, ECMO-associated complications or adverse effects such as hemolysis, hemorrhage or thrombosis also can play important roles in the development of AKI [29,68,82–84]. Despite the advance of a new miniaturized ECMO system, hemolysis due to shear stress from the ECMO circuit has been reported among ECMO patients with incidences between 5% and 18% [17,85–87]. This can contribute to heme pigment-induced AKI [83,84]. Although improvements in the ECMO technology have led to less thrombus development in its circuit with an improved capacity of the circuit to remove large emboli [68,82], smaller thrombi can still develop and result in renal microembolism [68,82], particularly with VA-ECMO [82].

The type of ECMO may also differently affect AKI risk. Our study demonstrated a higher incidence of AKI among patients requiring VA-ECMO (60.8%) than those requiring VV-ECMO (45.7%). While VV-ECMO is typically utilized for patients with isolated respiratory failure, VA-ECMO is used for combined severe cardiac and respiratory failure [4]. In VA-ECMO, there is a mixture of pulsatile arterial flow from the native heart and non-pulsatile arterial flow from the ECMO pump. Conversely, VV-ECMO maintains pulsatile cardiac output, and alterations in renal perfusion may conceivably be smaller [4]. Recent studies have shown that pulsatile flow may provide beneficial effects over non-pulsatile flow, especially protective effects on microcirculation and renal perfusion [88–90]. The differences in patient population and pulsatility between the two types of ECMO are likely explanations underlying the higher AKI incidence among patients requiring VA-ECMO.

As there is no treatment available for AKI, managemen<sup>t</sup> of AKI is limited to appropriate secondary preventive measures and supportive strategies [91–96]. RRT in the form of continuous renal replacement therapy (CRRT) is often required among patients requiring ECMO with severe AKI [42,55,97]. Our study demonstrated no significant correlation between the year of study and the incidence of AKI and/or severe AKI requiring RRT despite considerable changes in technology and practice of ECMO among adult patients. Furthermore, we showed a 3.7-fold increased risk of hospital mortality among ECMO patients with severe AKI requiring RRT. Thus, prevention and early identification of AKI among patients at-risk of ECMO-associated AKI could potentially play a crucial role in improved survival. Studies have shown several important AKI risk factors among patients requiring ECMO including older age, elevated lactate levels before ECMO initiation, high dose of inotropic drugs, severely reduced left ventricular ejection fraction, cirrhosis, postcardiotomy shock as an indication for ECMO, and finally ECMO pump speed and its duration [56,64,67]. Lee et al. recently observed a lower AKI association with a higher ECMO pump speed [56]. Although the underlying pathophysiology remains unclear, excessive ECMO pump speed has been shown to induce hemolysis and complement activation in vitro and animal model [98,99]. In pediatric patients receiving ECMO, Lou et al. also demonstrated higher pump speeds are associated with hemolysis and a number of other adverse clinical outcomes [100]. To prevent hemolysis-mediated kidney injury, it is suggested to limit pump revolutions/min (RPM) to safe levels (i.e., 3000 to 3500 RPM) in order to avoid excessive negative pressures generated within the pump [101]. Future prospective studies are required to assess the effects of ECMO pump speed on AKI risk in ECMO patients. In addition, future studies creating risk prediction models for ECMO-associated AKI are needed to assist with the prevention of AKI in a timely manner, which could potentially lead to an improvement in patient survival.

Our study has several limitations. Firstly, there are statistical heterogeneities in our meta-analysis. Potential sources for heterogeneities were the variations in patient characteristics among the included studies. However, we performed subgroup analysis to assess the AKI incidence based on types of ECMO and a separate meta-analysis that only included studies with confounder-adjusted analysis for mortality risk. Another limitation was that AKI diagnosis was mainly based on serum creatinine [102–104] while the data on urine output and novel biomarkers for AKI [105–108] were limited. Lastly, this systematic

review is primarily based on observational studies, as the data from clinical trials or population-based studies were limited. Therefore, it can at best, demonstrate an association but not a causal relationship.

## **5. Conclusions**

In conclusion, there is an overall high incidence of AKI and severe AKI requiring RRT in ECMO patients of 62.8% and 44.9%, respectively. The incidence of ECMO-associated AKI has not changed over time. AKI requiring RRT while on ECMO is associated with 3.7-fold increased risk of hospital mortality. Future studies should focus on strategies for prediction, detection, and prevention of AKI among patients who receive ECMO.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2077-0383/8/7/981/s1.

**Author Contributions:** Conceptualization, C.T., W.C., M.A.M. and K.K.; Data curation, C.T., W.C. and P.L.; Formal analysis, C.T., W.C.; Funding acquisition, K.K.; Investigation, C.T., W.C., P.L. and K.K.; Methodology, C.T., W.C. and K.K.; Project administration, P.L., K.W. and N.S.; Resources, P.L., T.B. and K.W.; Software, K.W.; Supervision, M.A.M. and K.K.; Validation, W.C. and P.L.; Visualization, W.C., T.B. and K.K.; Writing – original draft, C.T. and N.R.A; Writing – review & editing, C.T., W.C., P.L., N.R.A., T.B., K.W., N.S., M.A.M. and K.K.

**Conflicts of Interest:** The authors deny any conflict of interest.
