Providing Continuous Renal Replacement Therapy in Patients on Extracorporeal Membrane Oxygenation

When to Initiate

Definitive studies establishing the ideal time to initiate KRT in critically ill AKI patients, especially ECMO patients, are lacking. In the absence of a validated indicator to initiate KRT, we have adopted the demand/capacity imbalance concept (5). Thus, we take into consideration the patients’ severity of illness, and the balance between the demands on the kidney (solute and fluid load) and function in deciding when to start. We most commonly initiate KRT for fluid management, which is similar to a published survey (6); fluid overload prompted KRT initiation in 60% of patients. Indeed, because fluid overload is an independent risk factor for mortality (7) and preventing it has been associated with better survival (8), we strive to prevent it, but we most often find ourselves treating it. Along these lines, a meta-analysis (21,624 patients) found an association between earlier initiation of KRT and survival (9). Whereas the authors did not study potential mechanisms behind this association, it is tempting to speculate that it was related to enhanced fluid management.

Modality of Kidney Replacement Therapy

KRT options during ECMO include peritoneal dialysis, intermittent hemodialysis, prolonged intermittent RRT, and continuous renal replacement therapy (CRRT). Whereas each has advantages and disadvantages, CRRT is most commonly used because it facilitates continuous volume management (with less hemodynamic compromise) and ample solute clearance (2,4).

CRRT in ECMO patients was originally provided via an in-line filter placed directly into the ECMO circuit by creating a shunt from a postpump to a prepump segment of the ECMO circuit (Figure 1) (10). The pressure gradient between these segments drives blood flow through a hemofilter placed along this shunt. Blood flow is controlled through the use of resistive tubing, whereas infusion pumps deliver replacement fluid and/or dialysate, and regulate effluent flow. Urometers are used to accurately measure effluent rates. Advantages of this system include its simple setup; specialized equipment is not needed. Disadvantages include limitations in accuracy and CRRT dosing (due to infusion pump constraints).

Modern CRRT machines provide precise dialysate/replacement fluid delivery and fluid removal rates; consequently, they have become the preferred method for providing KRT. They can be operated either entirely independent of the ECMO circuit via a standard dialysis catheter, as in any patient not on ECMO. This approach is less complex and many nephrologists may be more comfortable with this familiar setup. Moreover, interruptions in ECMO have fewer effects on the CRRT treatment. The main limitations are related to the extravascular access.

We prefer the alternative option of integrating a CRRT device into the ECMO circuit via a shunt (Figure 1). In this case, we hook up both ports to the positive pressure segments of the circuits before the oxygenator (to prevent embolic events). Depending on the CRRT device being used, the access alarm pressure limits will either have to be reset to a positive pressure range or disabled. Resistors may be placed at the inlet and outlet ports to maintain access pressures within favorable ranges. CRRT is then performed as usual. This approach has the advantage of not requiring another access point, drawing from a high-flow circuit, and better filter life. However, standardizing the setup and sustaining appropriate training and monitoring are essential.

Other Technical Considerations

As with initiation, dosing of CRRT is extrapolated from data in non-ECMO patients and ultimately depends on physician discretion. We target a minimum of 20 ml/kg per minute, but adjust the CRRT dose on the basis of the patients’ metabolic parameters. Nephrologists should be particularly attentive to changes in the patient’s volume status, because abrupt or excessive changes can affect ECMO flow/function. Electrolyte levels, medication dosing, and delivery of nutrition, vitamins, and essential elements should be carefully monitored. Finally, most patients on ECMO are anticoagulated, obviating the need for CRRT anticoagulation. However, either regional citrate anticoagulation or heparin are used when needed.