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

## TL;DR

Using a low bicarbonate solution in a combined system for CO2 and kidney support improves CO2 removal, especially at lower blood flow rates.

## Contribution

Demonstrates that low bicarbonate substitution enhances CO2 elimination in ECCO₂R-CRRT systems at lower flow rates.

## Key findings

- In pigs, low HCO₃⁻ group showed higher CO2 removal at 200 mL/min flow rate.
- In ARDS patients, normal HCO₃⁻ group had reduced CO2 removal compared to low HCO₃⁻ and control groups.
- Low bicarbonate substitution may optimize CO2 elimination at lower extracorporeal blood flow rates.

## Abstract

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.

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.

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).

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.

The online version contains supplementary material available at 10.1186/s40635-025-00827-8.