Extracorporeal CO2 Removal Integrated within a Continuous Renal Replacement Circuit Offers Multiple Advantages

Extracorporeal carbon dioxide removal for acute respiratory failure: a review of potential indications, clinical practice and open research questions

Extracorporeal carbon dioxide removal (ECCO₂R) is a form of extracorporeal life support (ECLS) largely aimed at removing carbon dioxide in patients with acute hypoxemic or acute hypercapnic respiratory failure, so as to minimize respiratory acidosis, allowing more lung protective ventilatory settings which should decrease ventilator-induced lung injury. ECCO₂R is increasingly being used despite the lack of high-quality evidence, while complications associated with the technique remain an issue of concern. This review explains the physiological basis underlying the use of ECCO₂R, reviews the evidence regarding indications and contraindications, patient management and complications, and addresses organizational and ethical considerations. The indications and the risk-to-benefit ratio of this technique should now be carefully evaluated using structured national or international registries and large randomized trials.

A Minimally Invasive and Highly Effective Extracorporeal CO2 Removal Device Combined With a Continuous Renal Replacement Therapy

OBJECTIVES

Extracorporeal carbon dioxide removal is used to treat patients suffering from acute respiratory failure. However, the procedure is hampered by the high blood flow required to achieve a significant CO2 clearance. We aimed to develop an ultralow blood flow device to effectively remove CO2 combined with continuous renal replacement therapy (CRRT).

DESIGN

Preclinical, proof-of-concept study.

SETTING

An extracorporeal circuit where 200 mL/min of blood flowed through a hemofilter connected to a closed-loop dialysate circuit. An ion-exchange resin acidified the dialysate upstream, a membrane lung to increase Pco2 and promote CO2 removal.

PATIENTS

Six, 38.7 ± 2.0-kg female pigs.

INTERVENTIONS

Different levels of acidification were tested (from 0 to 5 mEq/min). Two l/hr of postdilution CRRT were performed continuously. The respiratory rate was modified at each step to maintain arterial Pco2 at 50 mm Hg.

MEASUREMENTS AND MAIN RESULTS

Increasing acidification enhanced CO2 removal efficiency of the membrane lung from 30 ± 5 (0 mEq/min) up to 145 ± 8 mL/min (5 mEq/min), with a 483% increase, representing the 73% ± 7% of the total body CO2 production. Minute ventilation decreased accordingly from 6.5 ± 0.7 to 1.7 ± 0.5 L/min. No major side effects occurred, except for transient tachycardia episodes. As expected from the alveolar gas equation, the natural lung Pao2 dropped at increasing acidification steps, given the high dissociation between the oxygenation and CO2 removal capability of the device, thus Pao2 decreased.

CONCLUSIONS

This new extracorporeal ion-exchange resin-based multiple-organ support device proved extremely high efficiency in CO2 removal and continuous renal support in a preclinical setting. Further studies are required before clinical implementation.

Modeling acid-base balance for in-series extracorporeal carbon dioxide removal and continuous venovenous hemofiltration devices

Patients with acute respiratory distress syndrome and acute kidney injury (AKI) treated by kidney replacement therapy may also require treatment with extracorporeal carbon dioxide removal (ECCO₂ R) devices to permit protective or ultraprotective mechanical ventilation. We developed a mathematical model of acid-base balance during extracorporeal therapy using ECCO₂ R and continuous venovenous hemofiltration (CVVH) devices applied in series for the treatment of mechanically ventilated AKI patients. Published data from clinical studies of mechanically ventilated AKI patients treated by CVVH at known infusion rates of substitution fluid without ECCO₂ R were used to adjust the model parameters to fit plasma levels of arterial partial pressure of carbon dioxide (PaCO₂ ), arterial plasma bicarbonate concentration ([HCO₃ ]), and plasma pH (as well as certain other unmeasured physiological variables). The effects of applying ECCO₂ R at an unchanged and a reduced tidal volume on PaCO₂ , [HCO₃ ] and plasma pH were then simulated assuming carbon dioxide removal rates from the ECCO₂ R device measured in the clinical studies. Agreement of such model predictions with clinical data was good whether the ECCO₂ R device was positioned proximal or distal to the CVVH device in the extracorporeal circuit. Although carbon dioxide removal rates from the ECCO₂ R device measured in one previous clinical study were higher when it was placed proximal to the CVVH device, suggesting that such in-series positioning was optimal, the current mathematical model demonstrates that proximal positioning of the ECCO₂ R device also results in lower bicarbonate (and, therefore, total carbon dioxide) removal from the distal CVVH device. Thus, the removal of total carbon dioxide by such extracorporeal circuits is relatively independent of the position of the in-series devices. It is concluded that the described mathematical model has quantitative accuracy; these results suggest that the overall acid-base balance when using ECCO₂ R and CVVH devices in a single extracorporeal circuit will be similar, independent of their in-series position.