Extracorporeal CO2 removal by hemodialysis: in vitro model and feasibility

Alexandra G. May, Ayan Sen, Matthew E. Cove, John A. Kellum, William J. Federspiel

Critically ill patients with acute respiratory distress syndrome and acute exacerbations of chronic obstructive pulmonary disease often develop hypercapnia and require mechanical ventilation. Extracorporeal carbon dioxide removal can manage hypercarbia by removing carbon dioxide directly from the bloodstream. Respiratory hemodialysis uses traditional hemodialysis to remove CO2 from the blood, mainly as bicarbonate. In this study, Stewart's approach to acid-base chemistry was used to create a dialysate that would maintain blood pH while removing CO2 as well as determine the blood and dialysate flow rates necessary to remove clinically relevant CO2 volumes. Bench studies were performed using a scaled down respiratory hemodialyzer in bovine or porcine blood. The scaling factor for the bench top experiments was 22.5. In vitro dialysate flow rates ranged from 2.2 to 24 mL/min (49.5-540 mL/min scaled up) and blood flow rates were set at 11 and 18.7 mL/min (248-421 mL/min scaled up). Blood inlet CO2 concentrations were set at 50 and 100 mmHg. Results are reported as scaled up values. The CO2 removal rate was highest at intermittent hemodialysis blood and dialysate flow rates. At an inlet pCO2 of 50 mmHg, the CO2 removal rate increased from 62.6 ± 4.8 to 77.7 ± 3 mL/min when the blood flow rate increased from 248 to 421 mL/min. At an inlet pCO2 of 100 mmHg, the device was able to remove up to 117.8 ± 3.8 mL/min of CO2. None of the test conditions caused the blood pH to decrease, and increases were ≤0.08. When the bench top data is scaled up, the system removes a therapeutic amount of CO2 standard intermittent hemodialysis flow rates. The zero bicarbonate dialysate did not cause acidosis in the post-dialyzer blood. These results demonstrate that, with further development, respiratory hemodialysis can be a minimally invasive extracorporeal carbon dioxide removal treatment option.