Effect of low bicarbonate substitution solution on CO2 removal rate in the combined system of extracorporeal CO2 removal and continuous renal replacement therapy.

BACKGROUND: The concurrent application of extracorporeal carbon dioxide removal (ECCO₂R) and continuous renal replacement therapy (CRRT) delivers essential respiratory and renal support. However, the use of bicarbonate (HCO₃⁻) in substitution solution increases the external HCO₃⁻ load and affect the carbon dioxide removal rate (VCO₂). This study aims to investigate the influence of low bicarbonate substitution solution on VCO₂ within the combined ECCO₂R-CRRT system. METHODS: This crossover study was conducted with hypercapnic pigs and patients with acute respiratory distress syndrome (ARDS). In pigs, we tested two extracorporeal blood flow rates (200 and 350 mL/min) alongside three continuous veno-venous hemofiltration (CVVH) strategies: a control group receiving ECCO₂R alone without CVVH, a low HCO₃⁻ group receiving ECCO₂R combined with CVVH (HCO₃⁻ concentration of 16 mmol/L at a substitution rate of 30 mL/kg/h), and a normal HCO₃⁻ group (HCO₃⁻ concentration of 25 mmol/L). Respiratory variables, hemodynamic parameters, and VCO₂ were measured 30 min after each intervention. In ARDS patients, we also assessed ECCO₂R combined with these CVVH strategies. The primary endpoint was the comparison of VCO₂ among the three groups in both the pig and patient. RESULTS: This study involved 12 hypercapnic pigs. At a blood flow rate of 200 mL/min, the VCO2 were significantly different among groups (P = 0.029). The VCO₂ in the low HCO₃⁻ group (51.7 ± 6.0 mL/min) was significantly higher than that in the normal HCO₃⁻ group (46.1 ± 2.9 mL/min) and comparable to the control group (50.3 ± 5.4 mL/min). However, at a blood flow rate of 350 mL/min, VCO₂ values were similar across all three groups. In 10 ARDS patients with a mean age of 64 ± 8 years, the PaCO₂ was 60.0 ± 4.7 mmHg prior to ECCO₂R. At a blood flow rate of 293 ± 59 mL/min, VCO₂ did not change significantly in the low HCO₃⁻ group (77.0 ± 16.2 mL/min) compared to the control group (75.2 ± 15.9 mL/min), a decrease was noted in the normal HCO₃⁻ group (69.9 ± 16.6 mL/min, P < 0.010). CONCLUSION: A low bicarbonate concentration of 16 mmol/L in the substitution solution may optimize CO₂ elimination in the ECCO₂R-CRRT system, especially at lower extracorporeal blood flow rates.