Combined Respiratory and Renal Support in a Novel Device - Influence of Utilizing Dialysis Fibers Outside-In on Full Blood Clearance and Filtration Efficiency

Background: Up to 70% of extracorporeal membrane oxygenation patients suffer from acute kidney injury. For these patients, our group is developing a novel membrane oxygenator with integrated kidney support. In this device, blood flows outside gas exchange fiber mats and dialysis fiber mats for simultaneous lung support and continuous renal replacement therapy (CRRT). In our previous study, we indicated that 25% of the gas exchange fibers in our oxygenator could be replaced by dialysis fibers maintaining sufficient lung support. However, the effect of outside-in blood flow in the efficiency of dialysis fibers to deliver CRRT is still unknown. Thus, this study aims to evaluate the efficiency of commercial dialyzer membranes utilized outside-in (blood flow outside the fibers) compared to traditional inside-out mode regarding clearance, and ultrafiltration coefficient.

Methods: Commercial dialyzers (1.2 m2, asymmetric membranes) were compared during in-vitro blood tests with porcine blood adapting the ISO 8637:2016 during experiments under CRRT conditions. Dialyzers utilized outside-in and in conventional inside-out mode were compared in terms of clearance of urea and creatinine during continuous hemodialysis with a blood flow rate of 150 mL/min, and a dialysate flow rate of 30 mL/min and 50 mL/min. In addition, clearance during continuous hemofiltration with an ultrafiltration rate of 20 mL/min was measured. Fluid removal capacity was evaluated in terms of the dialyzer’s ultrafiltration coefficient.

Results: Dialyzers utilized outside-in provided similar levels of urea and creatinine as dialyzers utilized in a traditional inside-out mode during continuous hemodialysis and hemofiltration (p > 0.2), except for experiments with a dialysate flow of 50 mL/min where outside-in dialyzers provided higher urea clearance (p = 0.003). Dialyzers utilized outside-in had an ultrafiltration coefficient of 11.7 mL/h/mmHg, about 4 times lower than the ultrafiltration coefficient achieved in traditional dialyzers. Nevertheless, required fluid removal rates could be achieved if higher transmembrane pressures were applied in the outside-in system. With clinical application in mind, our results indicate that dialyzers utilized outside-in could achieve small solute clearance and fluid removal doses of 20 – 25 mL/kgpatient/h comparable to the doses required for kidney support during CRRT, assuming the treatment of a hypothetical 80 kg patient.

Conclusions: Dialyzers utilized outside-in achieved similar urea and creatinine clearance as traditional dialyzers in the continuous hemodialysis and hemofiltration of whole blood. Moreover, comparable fluid removal rates could be achieved by the outside-in fibers if pressures are controlled in the system. We consider this a step towards the development of novel artificial lung with integrated kidney support.