Optimizing CO 2 clearance in combined extracorporeal carbon dioxide removal and continuous renal replacement therapy: The role of bicarbonate and circuit configuration modality

To the Editor,

The concurrent need for extracorporeal carbon dioxide removal (ECCO₂R) and continuous renal replacement therapy (CRRT) frequently arises in managing critically ill patients with acute respiratory distress syndrome (ARDS) complicated by acute kidney injury. While technically feasible, emerging evidence suggests that this combination may paradoxically impair the efficiency of CO₂ removal compared to ECCO₂R alone, highlighting an urgent need for optimized integration strategies focused on bicarbonate management and circuit design. ¹ While direct evidence of improved clinical outcomes such as reduced ventilatory requirements is still lacking, the following strategies represent testable hypotheses.

CO₂ transfer in a membrane lung depends on inlet PCO₂, sweep gas flow, blood flow (Qb), membrane surface area, and the balance between delivered CO₂ load and device clearance capacity. ² At low Qb (e.g., 200 mL/min), the membrane’s capacity is limited; additional CO₂ load from bicarbonate rich CRRT fluids can reduce net CO₂ removal rate (VCO₂). At higher Qb (≥350 mL/min), greater clearance capacity compensates for this extra load. ³ Hematocrit also influences CO₂ carriage and may affect performance in anemic patients. ³

A key modifiable factor is the bicarbonate load from CRRT replacement fluids, which represents a “hidden” CO₂ burden. A recent physiological crossover study showed that at a lower blood flow rate (200 mL/min), standard bicarbonate (25 mmol/L) in pre-dilution mode significantly reduced the VCO₂ versus ECCO₂R alone. In contrast, a low-bicarbonate solution (16 mmol/L), at the same total fluid volume preserved VCO₂, ⁴ of note, this study did not measure patient level outcomes such as PaCO2 reduction or ventilator free days; thus, the findings primarily highlight a biomechanical interaction requiring clinical validation. This effect was attenuated at a higher blood flow (350 mL/min), where the gas exchanger compensated for the added load. This suggests a dual optimization principle that warrants testing: using lower-bicarbonate fluids (with vigilant monitoring of acid-base and renal status and secondly maximizing feasible blood flow to enhance the clearance capacity.

Circuit configuration also influences systemic CO₂ balance. Positioning the gas exchanger downstream of the hemofilter may offer advantages. While an upstream placement might yield a higher device-specific VCO₂ (as the gas exchanger receives undiluted, high PCO₂ blood), this configuration allows bicarbonate-rich effluent to return a direct CO₂ load to the patient’s circulation. Conversely, a downstream high efficacy gas exchanger acts as a final “clearance stage,” removing both native CO₂ and the CO₂ generated from the infused bicarbonate buffer before blood returns to the patient. Thus, even if the instantaneously measured device specific VCO₂ appears similar or slightly lower (due to inlet PCO₂ dilution), the net systematic CO₂ reduction may be superior.^1,5 An additional theoretical advantage of this downstream configuration is improved circuit longevity. Placing the hemofilter upstream may reduce clotting in the subsequent gas exchanger, possibly via a lower transmembrane pressure (reducing shear-induced coagulation) and a local anti-platelet effect of bicarbonate.^5–7

Recent innovations support circuit engineering. “Respiratory dialysis” using low bicarbonate dialysate achieved meaningful CO₂ removal in animals. ⁸ Double parallel oxygenators have been used to overcome inadequate CO₂ clearance during ECMO. ⁹ Dialysate acidification ¹⁰ and novel membrane “ventilation” techniques ¹¹ show promise. A highly effective combined ECCO₂R CRRT device has also been reported. ⁷ The CICERO study further demonstrated the clinical feasibility of combined low-flow ECCO₂R-CRRT in a real-world setting. ¹² Future research should focus on: (i) whether low bicarbonate fluids reduce ventilatory support; (ii) optimal blood flow balancing clearance and hemolysis; (iii) patient selection based on hematocrit and acid base status; and (iv) closed loop control of sweep gas or bicarbonate delivery.

In summary, optimizing combined ECCO₂R-CRRT may benefit from three strategies: managing bicarbonate load, pursuing high blood flow rates, and placing a high efficacy gas exchanger downstream. These proposals should be viewed as working hypothesis requiring clinical validation. Integrating these considerations could maximize the efficacy and safety of multi-organ support system.