Respiratory Electrodialysis. A Novel, Highly Efficient Extracorporeal CO2Removal Technique

Enhanced extracorporeal carbon dioxide removal by acidification and metabolic control

BACKGROUND

Extracorporeal carbon dioxide removal (ECCO2R) promotes protective ventilation in patients with acute respiratory failure, but devices with high CO2 extraction capacity are required for clinically relevant impact. This study evaluates three novel low-flow techniques based on dialysate acidification, also combined with renal replacement therapy, and metabolic control.

METHODS

Eight swine were connected to a low-flow (350 mL/min) extracorporeal circuit including a dialyzer with a closed-loop dialysate circuit, and two membrane lungs on blood (MLb) and dialysate (MLd), respectively. The following 2-hour steps were performed: 1) MLb-start (MLb ventilated); 2) MLbd-start (MLb and MLd ventilated); 3) HLac (lactic acid infusion before MLd); 4) HCl-NaLac (hydrochloric acid infusion before MLd combined with renal replacement therapy and reinfusion of sodium lactate); 5) HCl-βHB-NaLac (hydrochloric acid infusion before MLd combined with renal replacement therapy and reinfusion of sodium lactate and sodium 3-hydroxybutyrate). Caloric and fluid inputs, temperature, blood glucose and arterial carbon dioxide pressure were kept constant.

RESULTS

The total MLs CO2 removal in HLac (130±25 mL/min), HCl-NaLac (130±21 mL/min) and HCl-βHB-NaLac (124±18 mL/min) were higher compared with MLbd-start (81±15 mL/min, P<0.05) and MLb-start (55±7 mL/min, P<0.05). Minute ventilation in HLac (4.3±0.9 L/min), HCl-NaLac (3.6±0.8 L/min) and HCl-βHB-NaLac (3.6±0.8 L/min) were lower compared to MLb-start (6.2±1.1 L/min, P<0.05) and MLbd-start (5.8±2.1 L/min, P<0.05). Arterial pH was 7.40±0.03 at MLb-start and decreased only during HCl-βHB-NaLac (7.35±0.03, P<0.05). No relevant changes in electrolyte concentrations, hemodynamics and significant adverse events were detected.

CONCLUSIONS

The three techniques achieved a significant extracorporeal CO2 removal allowing a relevant reduction in minute ventilation with a sufficient safety profile.

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.

Hyperbaric phototherapy augments blood carbon monoxide removal

BACKGROUND AND OBJECTIVES

Carbon monoxide (CO) poisoning is responsible for nearly 50,000 emergency department visits and 1200 deaths per year. Compared to oxygen, CO has a 250-fold higher affinity for hemoglobin (Hb), resulting in the displacement of oxygen from Hb and impaired oxygen delivery to tissues. Optimal treatment of CO-poisoned patients involves the administration of hyperbaric 100% oxygen to remove CO from Hb and to restore oxygen delivery. However, hyperbaric chambers are not widely available and this treatment requires transporting a CO-poisoned patient to a specialized center, which can result in delayed treatment. Visible light is known to dissociate CO from carboxyhemoglobin (COHb). In a previous study, we showed that a system composed of six photo-extracorporeal membrane oxygenation (ECMO) devices efficiently removes CO from a large animal with CO poisoning. In this study, we tested the hypothesis that the application of hyperbaric oxygen to the photo-ECMO device would further increase the rate of CO elimination.

STUDY DESIGN/MATERIAL AND METHODS

We developed a hyperbaric photo-ECMO device and assessed the ability of the device to remove CO from CO-poisoned human blood. We combined four devices into a "hyperbaric photo-ECMO system" and compared its ability to remove CO to our previously described photo-ECMO system, which was composed of six devices ventilated with normobaric oxygen.

RESULTS

Under normobaric conditions, an increase in oxygen concentration from 21% to 100% significantly increased CO elimination from CO-poisoned blood after a single pass through the device. Increased oxygen pressure within the photo-ECMO device was associated with higher exiting blood PO₂ levels and increased CO elimination. The system of four hyperbaric photo-ECMO devices removed CO from 1 L of CO-poisoned blood as quickly as the original, normobaric photo-ECMO system composed of six devices.

CONCLUSION

This study demonstrates the feasibility and efficacy of using a hyperbaric photo-ECMO system to increase the rate of CO elimination from CO-poisoned blood. This technology could provide a simple portable emergency device and facilitate immediate treatment of CO-poisoned patients at or near the site of injury.

Alkaline Liquid Ventilation of the Membrane Lung for Extracorporeal Carbon Dioxide Removal (ECCO2R): In Vitro Study

Extracorporeal carbon dioxide removal (ECCO₂R) is a promising strategy to manage acute respiratory failure. We hypothesized that ECCO₂R could be enhanced by ventilating the membrane lung with a sodium hydroxide (NaOH) solution with high CO₂ absorbing capacity. A computed mathematical model was implemented to assess NaOH–CO₂ interactions. Subsequently, we compared NaOH infusion, named “alkaline liquid ventilation”, to conventional oxygen sweeping flows. We built an extracorporeal circuit with two polypropylene membrane lungs, one to remove CO₂ and the other to maintain a constant PCO₂ (60 ± 2 mmHg). The circuit was primed with swine blood. Blood flow was 500 mL × min⁻¹. After testing the safety and feasibility of increasing concentrations of aqueous NaOH (up to 100 mmol × L⁻¹), the CO₂ removal capacity of sweeping oxygen was compared to that of 100 mmol × L⁻¹ NaOH. We performed six experiments to randomly test four sweep flows (100, 250, 500, 1000 mL × min⁻¹) for each fluid plus 10 L × min⁻¹ oxygen. Alkaline liquid ventilation proved to be feasible and safe. No damages or hemolysis were detected. NaOH showed higher CO₂ removal capacity compared to oxygen for flows up to 1 L × min⁻¹. However, the highest CO₂ extraction power exerted by NaOH was comparable to that of 10 L × min⁻¹ oxygen. Further studies with dedicated devices are required to exploit potential clinical applications of alkaline liquid ventilation.

Respiratory Dialysis-A Novel Low Bicarbonate Dialysate to Provide Extracorporeal CO2 Removal

OBJECTIVES

We designed a novel respiratory dialysis system to remove CO2 from blood in the form of bicarbonate. We aimed to determine if our respiratory dialysis system removes CO2 at rates comparable to low-flow extracorporeal CO2 removal devices (blood flow < 500 mL/min) in a large animal model.

DESIGN

Experimental study.

SETTING

Animal research laboratory.

SUBJECTS

Female Yorkshire pigs.

INTERVENTIONS

Five bicarbonate dialysis experiments were performed. Hypercapnia (PCO2 90-100 mm Hg) was established in mechanically ventilated swine by adjusting the tidal volume. Dialysis was then performed with a novel low bicarbonate dialysate.

MEASUREMENTS AND MAIN RESULTS

We measured electrolytes, blood gases, and plasma-free hemoglobin in arterial blood, as well as blood entering and exiting the dialyzer. We used a physical-chemical acid-base model to understand the factors influencing blood pH after bicarbonate removal. During dialysis, we removed 101 (±13) mL/min of CO2 (59 mL/min when normalized to venous PCO2 of 45 mm Hg), corresponding to a 29% reduction in PaCO2 (104.0 ± 8.1 vs 74.2 ± 8.4 mm Hg; p < 0.001). Minute ventilation and body temperature were unchanged during dialysis (1.2 ± 0.4 vs 1.1 ± 0.4 L/min; p = 1.0 and 35.3°C ± 0.9 vs 35.2°C ± 0.6; p = 1.0). Arterial pH increased after bicarbonate removal (7.13 ± 0.04 vs 7.21 ± 0.05; p < 0.001) despite no attempt to realkalinize the blood. Our modeling showed that dialysate electrolyte composition, plasma albumin, and plasma total CO2 accurately predict the measured pH of blood exiting the dialyser. However, the final effluent dose exceeded conventional doses, depleting plasma glucose and electrolytes, such as potassium and phosphate.

CONCLUSIONS

Bicarbonate dialysis results in CO2 removal at rates comparable with existing low-flow extracorporeal CO2 removal in a large animal model, but the final dialysis dose delivered needs to be reduced before the technique can be used for prolonged periods.

Physiological Basis of Extracorporeal Membrane Oxygenation and Extracorporeal Carbon Dioxide Removal in Respiratory Failure

Extracorporeal life support (ECLS) for severe respiratory failure has seen an exponential growth in recent years. Extracorporeal membrane oxygenation (ECMO) and extracorporeal CO2 removal (ECCO2R) represent two modalities that can provide full or partial support of the native lung function, when mechanical ventilation is either unable to achieve sufficient gas exchange to meet metabolic demands, or when its intensity is considered injurious. While the use of ECMO has defined indications in clinical practice, ECCO2R remains a promising technique, whose safety and efficacy are still being investigated. Understanding the physiological principles of gas exchange during respiratory ECLS and the interactions with native gas exchange and haemodynamics are essential for the safe applications of these techniques in clinical practice. In this review, we will present the physiological basis of gas exchange in ECMO and ECCO2R, and the implications of their interaction with native lung function. We will also discuss the rationale for their use in clinical practice, their current advances, and future directions.

The use of extracorporeal CO2 removal in acute respiratory failure

Background

Chronic obstructive pulmonary disease (COPD) exacerbation and protective mechanical ventilation of acute respiratory distress syndrome (ARDS) patients induce hypercapnic respiratory acidosis.

Main text

Extracorporeal carbon dioxide removal (ECCO₂R) aims to eliminate blood CO₂ to fight against the adverse effects of hypercapnia and related acidosis. Hypercapnia has deleterious extrapulmonary consequences, particularly for the brain. In addition, in the lung, hypercapnia leads to: lower pH, pulmonary vasoconstriction, increases in right ventricular afterload, acute cor pulmonale. Moreover, hypercapnic acidosis may further damage the lungs by increasing both nitric oxide production and inflammation and altering alveolar epithelial cells. During an exacerbation of COPD, relieving the native lungs of at least a portion of the CO₂ could potentially reduce the patient's respiratory work, Instead of mechanically increasing alveolar ventilation with MV in an already hyperinflated lung to increase CO₂ removal, the use of ECCO₂R may allow a decrease in respiratory volume and respiratory rate, resulting in improvement of lung mechanic. Thus, the use of ECCO₂R may prevent noninvasive ventilation failure and allow intubated patients to be weaned off mechanical ventilation. In ARDS patients, ECCO₂R may be used to promote an ultraprotective ventilation in allowing to lower tidal volume, plateau (Pplat) and driving pressures, parameters that have identified as a major risk factors for mortality. However, although ECCO₂R appears to be effective in improving gas exchange and possibly in reducing the rate of endotracheal intubation and allowing more protective ventilation, its use may have pulmonary and hemodynamic consequences and may be associated with complications.

Conclusion

In selected patients, ECCO₂R may be a promising adjunctive therapeutic strategy for the management of patients with severe COPD exacerbation and for the establishment of protective or ultraprotective ventilation in patients with ARDS without prognosis-threatening hypoxemia.

Extracorporeal Gas Exchange for Acute Respiratory Distress Syndrome: Open Questions, Controversies and Future Directions

Veno-venous extracorporeal membrane oxygenation (V-V ECMO) in acute respiratory distress syndrome (ARDS) improves gas exchange and allows lung rest, thus minimizing ventilation-induced lung injury. In the last forty years, a major technological and clinical improvement allowed to dramatically improve the outcome of patients treated with V-V ECMO. However, many aspects of the care of patients on V-V ECMO remain debated. In this review, we will focus on main issues and controversies on caring of ARDS patients on V-V ECMO support. Particularly, the indications to V-V ECMO and the feasibility of a less invasive extracorporeal carbon dioxide removal will be discussed. Moreover, the controversies on management of mechanical ventilation, prone position and sedation will be explored. In conclusion, we will discuss evidences on transfusions and management of anticoagulation, also focusing on patients who undergo simultaneous treatment with ECMO and renal replacement therapy. This review aims to discuss all these clinical aspects with an eye on future directions and perspectives.

Evaluation of a New Extracorporeal CO2 Removal Device in an Experimental Setting

Background: Ultra-protective lung ventilation in acute respiratory distress syndrome or early weaning and/or avoidance of mechanical ventilation in decompensated chronic obstructive pulmonary disease may be facilitated by the use of extracorporeal CO₂ removal (ECCO₂R). We tested the CO₂ removal performance of a new ECCO₂R (CO₂RESET) device in an experimental animal model. Methods: Three healthy pigs were mechanically ventilated and connected to the CO₂RESET device (surface area = 1.8 m², EUROSETS S.r.l., Medolla, Italy). Respiratory settings were adjusted to induce respiratory acidosis with the adjunct of an external source of pure CO₂ (target pre membrane lung venous PCO₂ (P_preCO₂): 80–120 mmHg). The amount of CO₂ removed (VCO₂, mL/min) by the membrane lung was assessed directly by the ECCO₂R device. Results: Before the initiation of ECCO₂R, the median P_preCO₂ was 102.50 (95.30–118.20) mmHg. Using fixed incremental steps of the sweep gas flow and maintaining a fixed blood flow of 600 mL/min, VCO₂ progressively increased from 0 mL/min (gas flow of 0 mL/min) to 170.00 (160.00–200.00) mL/min at a gas flow of 10 L/min. In particular, a high increase of VCO₂ was observed increasing the gas flow from 0 to 2 L/min, then, VCO₂ tended to progressively achieve a steady-state for higher gas flows. No animal or pump complications were observed. Conclusions: Medium-flow ECCO₂R devices with a blood flow of 600 mL/min and a high surface membrane lung (1.8 m²) provided a high VCO₂ using moderate sweep gas flows (i.e., >2 L/min) in an experimental swine models with healthy lungs.

Prolonged Weaning: S2k Guideline Published by the German Respiratory Society

Mechanical ventilation (MV) is an essential part of modern intensive care medicine. MV is performed in patients with severe respiratory failure caused by respiratory muscle insufficiency and/or lung parenchymal disease; that is, when other treatments such as medication, oxygen administration, secretion management, continuous positive airway pressure (CPAP), or nasal high-flow therapy have failed. MV is required for maintaining gas exchange and allows more time to curatively treat the underlying cause of respiratory failure. In the majority of ventilated patients, liberation or "weaning" from MV is routine, without the occurrence of any major problems. However, approximately 20% of patients require ongoing MV, despite amelioration of the conditions that precipitated the need for it in the first place. Approximately 40-50% of the time spent on MV is required to liberate the patient from the ventilator, a process called "weaning". In addition to acute respiratory failure, numerous factors can influence the duration and success rate of the weaning process; these include age, comorbidities, and conditions and complications acquired during the ICU stay. According to international consensus, "prolonged weaning" is defined as the weaning process in patients who have failed at least 3 weaning attempts, or require more than 7 days of weaning after the first spontaneous breathing trial (SBT). Given that prolonged weaning is a complex process, an interdisciplinary approach is essential for it to be successful. In specialised weaning centres, approximately 50% of patients with initial weaning failure can be liberated from MV after prolonged weaning. However, the heterogeneity of patients undergoing prolonged weaning precludes the direct comparison of individual centres. Patients with persistent weaning failure either die during the weaning process, or are discharged back to their home or to a long-term care facility with ongoing MV. Urged by the growing importance of prolonged weaning, this Sk2 Guideline was first published in 2014 as an initiative of the German Respiratory Society (DGP), in conjunction with other scientific societies involved in prolonged weaning. The emergence of new research, clinical study findings and registry data, as well as the accumulation of experience in daily practice, have made the revision of this guideline necessary. The following topics are dealt with in the present guideline: Definitions, epidemiology, weaning categories, underlying pathophysiology, prevention of prolonged weaning, treatment strategies in prolonged weaning, the weaning unit, discharge from hospital on MV, and recommendations for end-of-life decisions. Special emphasis was placed on the following themes: (1) A new classification of patient sub-groups in prolonged weaning. (2) Important aspects of pulmonary rehabilitation and neurorehabilitation in prolonged weaning. (3) Infrastructure and process organisation in the care of patients in prolonged weaning based on a continuous treatment concept. (4) Changes in therapeutic goals and communication with relatives. Aspects of paediatric weaning are addressed separately within individual chapters. The main aim of the revised guideline was to summarize both current evidence and expert-based knowledge on the topic of "prolonged weaning", and to use this information as a foundation for formulating recommendations related to "prolonged weaning", not only in acute medicine but also in the field of chronic intensive care medicine. The following professionals served as important addressees for this guideline: intensivists, pulmonary medicine specialists, anaesthesiologists, internists, cardiologists, surgeons, neurologists, paediatricians, geriatricians, palliative care clinicians, rehabilitation physicians, intensive/chronic care nurses, physiotherapists, respiratory therapists, speech therapists, medical service of health insurance, and associated ventilator manufacturers.

Extracorporeal life support for adults with acute respiratory distress syndrome

Extracorporeal life support (ECLS) can support gas exchange in patients with the acute respiratory distress syndrome (ARDS). During ECLS, venous blood is drained from a central vein via a cannula, pumped through a semipermeable membrane that permits diffusion of oxygen and carbon dioxide, and returned via a cannula to a central vein. Two related forms of ECLS are used. Venovenous extracorporeal membrane oxygenation (ECMO), which uses high blood flow rates to both oxygenate the blood and remove carbon dioxide, may be considered in patients with severe ARDS whose oxygenation or ventilation cannot be maintained adequately with best practice conventional mechanical ventilation and adjunctive therapies, including prone positioning. Extracorporeal carbon dioxide removal (ECCO₂R) uses lower blood flow rates through smaller cannulae and provides substantial CO₂ elimination (~ 20–70% of total CO₂ production), albeit with marginal improvement in oxygenation. The rationale for using ECCO₂R in ARDS is to facilitate lung-protective ventilation by allowing a reduction of tidal volume, respiratory rate, plateau pressure, driving pressure and mechanical power delivered by the mechanical ventilator. This narrative review summarizes physiological concepts related to ECLS, as well as the rationale and evidence supporting ECMO and ECCO₂R for the treatment of ARDS. It also reviews complications, limitations, and the ethical dilemmas that can arise in treating patients with ECLS. Finally, it discusses future key research questions and challenges for this technology.

Phototherapy and extracorporeal membrane oxygenation facilitate removal of carbon monoxide in rats

Inhaled carbon monoxide (CO) displaces oxygen from hemoglobin, reducing the capacity of blood to carry oxygen. Current treatments for CO-poisoned patients involve administration of 100% oxygen; however, when CO poisoning is associated with acute lung injury secondary to smoke inhalation, burns, or trauma, breathing 100% oxygen may be ineffective. Visible light dissociates CO from hemoglobin. We hypothesized that the exposure of blood to visible light while passing through a membrane oxygenator would increase the rate of CO elimination in vivo. We developed a membrane oxygenator with optimal characteristics to facilitate exposure of blood to visible light and tested the device in a rat model of CO poisoning, with or without concomitant lung injury. Compared to ventilation with 100% oxygen, the addition of extracorporeal removal of CO with phototherapy (ECCOR-P) doubled the rate of CO elimination in CO-poisoned rats with normal lungs. In CO-poisoned rats with acute lung injury, treatment with ECCOR-P increased the rate of CO removal by threefold compared to ventilation with 100% oxygen alone and was associated with improved survival. Further development and adaptation of this extracorporeal CO photo-removal device for clinical use may provide additional benefits for CO-poisoned patients, especially for those with concurrent acute lung injury.

Extracorporeal Membrane Oxygenation for Respiratory Failure

This review focuses on the use of veno-venous extracorporeal membrane oxygenation for respiratory failure across all blood flow ranges. Starting with a short overview of historical development, aspects of the physiology of gas exchange (i.e., oxygenation and decarboxylation) during extracorporeal circulation are discussed. The mechanisms of phenomena such as recirculation and shunt playing an important role in daily clinical practice are explained.Treatment of refractory and symptomatic hypoxemic respiratory failure (e.g., acute respiratory distress syndrome [ARDS]) currently represents the main indication for high-flow veno-venous-extracorporeal membrane oxygenation. On the other hand, lower-flow extracorporeal carbon dioxide removal might potentially help to avoid or attenuate ventilator-induced lung injury by allowing reduction of the energy load (i.e., driving pressure, mechanical power) transmitted to the lungs during mechanical ventilation or spontaneous ventilation. In the latter context, extracorporeal carbon dioxide removal plays an emerging role in the treatment of chronic obstructive pulmonary disease patients during acute exacerbations. Both applications of extracorporeal lung support raise important ethical considerations, such as likelihood of ultimate futility and end-of-life decision-making. The review concludes with a brief overview of potential technical developments and persistent challenges.

Extracorporeal and advanced therapies for progressive refractory near-fatal acute severe asthma in children

Asthma is the most common chronic illness and is one of the most common medical emergencies in children. Progressive refractory near-fatal asthma requiring intubation and mechanical ventilation can lead to death. Extracorporeal membrane oxygenation (ECMO) can provide adequate gas exchange during acute respiratory failure although data on outcomes in children requiring ECMO support for status asthmaticus is sparse with one study reporting survival rates of nearly 85% with asthma being one of the best outcome subsets for patients with refractory respiratory failure requiring ECMO support. We describe the current literature on the use of ECMO and other advanced extracorporeal therapies available for children with acute severe asthma. We also review other advanced invasive and noninvasive therapies in acute severe asthma both before and while on ECMO support.

Extracorporeal Blood Purification and Organ Support in the Critically Ill Patient during COVID-19 Pandemic: Expert Review and Recommendation

Critically ill COVID-19 patients are generally admitted to the ICU for respiratory insufficiency which can evolve into a multiple-organ dysfunction syndrome requiring extracorporeal organ support. Ongoing advances in technology and science and progress in information technology support the development of integrated multi-organ support platforms for personalized treatment according to the changing needs of the patient. Based on pathophysiological derangements observed in COVID-19 patients, a rationale emerges for sequential extracorporeal therapies designed to remove inflammatory mediators and support different organ systems. In the absence of vaccines or direct therapy for COVID-19, extracorporeal therapies could represent an option to prevent organ failure and improve survival. The enormous demand in care for COVID-19 patients requires an immediate response from the scientific community. Thus, a detailed review of the available technology is provided by experts followed by a series of recommendation based on current experience and opinions, while waiting for generation of robust evidence from trials.

Update on extracorporeal carbon dioxide removal: a comprehensive review on principles, indications, efficiency, and complications

TECHNOLOGY

Extracorporeal carbon dioxide removal means the removal of carbon dioxide from the blood across a gas exchange membrane without substantially improving oxygenation. Carbon dioxide removal is possible with substantially less extracorporeal blood flow than needed for oxygenation. Techniques for extracorporeal carbon dioxide removal include (1) pumpless arterio-venous circuits, (2) low-flow venovenous circuits based on the technology of continuous renal replacement therapy, and (3) venovenous circuits based on extracorporeal membrane oxygenation technology.

INDICATIONS

Extracorporeal carbon dioxide removal has been shown to enable more protective ventilation in acute respiratory distress syndrome patients, even beyond the so-called "protective" level. Although experimental data suggest a benefit on ventilator induced lung injury, no hard clinical evidence with respect to improved outcome exists. In addition, extracorporeal carbon dioxide removal is a tool to avoid intubation and mechanical ventilation in patients with acute exacerbated chronic obstructive pulmonary disease failing non-invasive ventilation. This concept has been shown to be effective in 56-90% of patients. Extracorporeal carbon dioxide removal has also been used in ventilated patients with hypercapnic respiratory failure to correct acidosis, unload respiratory muscle burden, and facilitate weaning. In patients suffering from terminal fibrosis awaiting lung transplantation, extracorporeal carbon dioxide removal is able to correct acidosis and enable spontaneous breathing during bridging. Keeping these patients awake, ambulatory, and breathing spontaneously is associated with favorable outcome.

COMPLICATIONS

Complications of extracorporeal carbon dioxide removal are mostly associated with vascular access and deranged hemostasis leading to bleeding. Although the spectrum of complications may differ, no technology offers advantages with respect to rate and severity of complications. So called "high-extraction systems" working with higher blood flows and larger membranes may be more effective with respect to clinical goals.

Options in extracorporeal support of multiple organ failure

Multiorgan failure is among the most frequent reasons of death in critically ill patients. Based on extensive and long-term use of renal replacement therapy, extracorporeal organ support became available for other organ failures. Initially, most of these techniques (e.g. extracorporeal membrane oxygenation, extracorporeal CO₂ removal [ECCO2R] and extracorporeal liver support) were used as stand-alone single organ support systems. Considering multiple interactions between native organs (“crosstalk”), combined or integrated extracorporeal organ support (ECOS) devices are intriguing. The concept of multiple organ support therapy (MOST) providing simultaneous and combined support for different failing organs was described more than 15 years ago by Ronco and Bellomo. This concept also implicates overcoming the “compartmentalized” approach provided by different single organ specialized professionals by a multidisciplinary and multiprofessional strategy. The idea of MOST is supported by the failure of several recent studies on single organ support including liver and lung support. Improvement of outcome by ECOS necessarily depends on optimized patient selection, integrated organ support and limitation of its side effects. This implicates challenges for engineers, industry and healthcare professionals. From a technical viewpoint, modular combination of pre-existing technologies such as renal replacement, albumin-dialysis, ECCO2R and potentially cytokine elimination can be considered as a first step. While this allows for stepwise and individual combination of standard organ support facilities, it carries the disadvantage of large extracorporeal blood volume and surfaces as well as additive costs. The more intriguing next step is an integrated platform providing the capacity of multiple organ support within one device. (This article is freely available.)

Low-flow assessment of current ECMO/ECCO2R rotary blood pumps and the potential effect on hemocompatibility

Background

Extracorporeal carbon dioxide removal (ECCO₂R) uses an extracorporeal circuit to directly remove carbon dioxide from the blood either in lieu of mechanical ventilation or in combination with it. While the potential benefits of the technology are leading to increasing use, there are very real risks associated with it. Several studies demonstrated major bleeding and clotting complications, often associated with hemolysis and poorer outcomes in patients receiving ECCO₂R. A better understanding of the risks originating specifically from the rotary blood pump component of the circuit is urgently needed.

Methods

High-resolution computational fluid dynamics was used to calculate the hemodynamics and hemocompatibility of three current rotary blood pumps for various pump flow rates.

Results

The hydraulic efficiency dramatically decreases to 5–10% if operating at blood flow rates below 1 L/min, the pump internal flow recirculation rate increases 6–12-fold in these flow ranges, and adverse effects are increased due to multiple exposures to high shear stress. The deleterious consequences include a steep increase in hemolysis and destruction of platelets.

Conclusions

The role of blood pumps in contributing to adverse effects at the lower blood flow rates used during ECCO₂R is shown here to be significant. Current rotary blood pumps should be used with caution if operated at blood flow rates below 2 L/min, because of significant and high recirculation, shear stress, and hemolysis. There is a clear and urgent need to design dedicated blood pumps which are optimized for blood flow rates in the range of 0.5–1.5 L/min.

A CO2 removal system using extracorporeal lung and renal assist device with an acid and alkaline infusion

The patients with respiratory failure need high tidal volume by mechanical ventilation, which lead to the ventilator-induced lung injury. We developed an extracorporeal lung and renal assist device (ELRAD), comprising acid infusion, membrane lung, continuous hemodiafiltration and alkaline infusion. To evaluate this system, we conducted in vivo studies using experimental swine which were connected to the new system. In vivo experiments consist of four protocols; baseline = hemodiafiltration only (no O₂ gas flow to membrane lung); membrane lung = "Baseline" plus O₂ gas flow to membrane lung; "Acid infusion" = "Membrane lung" plus continuous acid infusion; ELRAD = "Acid infusion" plus continuous alkaline infusion. We changed the ventilatory rate of the mechanical ventilation to maintain PCO₂ at 50-55 mmHg during the four protocols. The results showed that there was statistically no significant difference in the levels of pH, HCO₃^-, and base excess when each study protocol was initiated. The amount of CO₂ eliminated by the membrane lung significantly increased by 1.6 times in the acid infusion protocol and the ELRAD protocol compared to the conventional membrane lung protocol. Minute ventilation in the ELRAD protocol significantly decreased by 0.5 times compared with the hemodiafiltration only protocol (P < 0.0001), the membrane lung (P = 0.0006) and acid infusion protocol (P = 0.0017), respectively. In conclusion, a developed CO₂ removal system efficiently removed CO₂ at low blood flow and reduced minute ventilation, while maintaining acid-base balance within the normal range.

Extracorporeal carbon dioxide removal for lowering the risk of mechanical ventilation: research questions and clinical potential for the future

As a result of technical improvements, extracorporeal carbon dioxide removal (ECCO₂R) now has the potential to play an important role in the management of adults with acute respiratory failure. There is growing interest in the use of ECCO₂R for the management of both hypoxaemic and hypercapnic respiratory failure. However, evidence to support its use is scarce and several questions remain about the best way to implement this therapy, which can be associated with serious side-effects. This Review reflects the consensus opinion of an international group of clinician scientists with expertise in managing acute respiratory failure and in using ECCO₂R therapies in this setting. We concisely review clinically relevant aspects of ECCO₂R, and provide a series of recommendations for clinical practice and future research, covering topics that include the practicalities of ECCO₂R delivery, indications for use, and service delivery.

[Current techniques for extracorporeal decarboxylation]

The widespread use of extracorporeal lung assist (ECLA) in recent years has led to the introduction of different decarboxylation systems into clinical practice. Due to the large CO₂ transport capacity of the blood such systems require considerably lower extracorporeal blood flows and therefore allow for effective decarboxylation with reduced invasiveness and complexity. While systems derived from classical lung assist are mainly used to control severe acute hypercapnic respiratory failure, recently a growing number of therapies based on renal replacement platforms have become available ("respiratory dialysis"). Such low-flow systems still allow for effective partial CO₂ elimination and can control respiratory acidosis as well as facilitate or even enable protective and ultraprotective ventilation strategies in acute lung failure (ARDS). While the use of extracorporeal CO₂ elimination (ECCO₂R) has been shown to decrease ventilator-induced lung injury (VILI), positive effects on hard clinical endpoints such as mortality or duration of mechanical ventilation are still unproven. In light of limited evidence, ECCO₂R must be regarded as an experimental procedure. Its use should therefore at present be restricted to centers with appropriate experience.

Carbon dioxide removal using low bicarbonate dialysis in rodents

Background:

Extracorporeal carbon dioxide removal may be used to manage hypercapnia, but compared to dialysis, it’s not widely available. A recent in vitro study showed that dialysis with low bicarbonate dialysates removes CO2.

Objective:

To show that bicarbonate dialysis removes CO2 in an animal model to validate in-vitro findings and quantify the effect on arterial pH.

Methods:

Male Sprague-Dawley hypercapnic rats were dialyzed with either a conventional dialysate (PrismasolTM) or a bicarbonate-free dialysate (Bicarb0). The effect of dialysis on standard blood gases and electrolytes was measured.

Results:

Partial pressure of CO2 and bicarbonate concentration in blood decreased significantly after exposure to Bicarb0 compared to PrismasolTM (filter outflow values 12.8 vs 81.1 mmHg; p < 0.01 for CO2 and 3.5 vs 22.0 mmol/L; p < 0.01 for bicarbonate). Total CO2 content of blood was reduced by 459 mL/L during dialysis with Bicarb0 (filter inflow 546 ± 91 vs filter outflow 87 ± 52 mL/L; p < 0.01), but was not significantly reduced with PrismasolTM.

Conclusions:

Bicarbonate dialysis removes CO2 at rates comparable to existing low-flow ECCO2R.

Impact of sweep gas flow on extracorporeal CO2 removal (ECCO2R)

BACKGROUND

Veno-venous extracorporeal carbon dioxide (CO₂) removal (vv-ECCO₂R) is increasingly being used in the setting of acute respiratory failure. Blood flow rates range in clinical practice from 200 mL/min to more than 1500 mL/min, and sweep gas flow rates range from less than 1 to more than 10 L/min. The present porcine model study was aimed at determining the impact of varying sweep gas flow rates on CO₂ removal under different blood flow conditions and membrane lung surface areas.

METHODS

Two different membrane lungs, with surface areas of 0.4 and 0.8m², were used in nine pigs with experimentally-induced hypercapnia. During each experiment, the blood flow was increased stepwise from 300 to 900 mL/min, with further increases up to 1800 mL/min with the larger membrane lung in steps of 300 mL/min. Sweep gas was titrated under each condition from 2 to 8 L/min in steps of 2 L/min. Extracorporeal CO₂ elimination was normalized to a PaCO₂ of 45 mmHg before the membrane lung.

RESULTS

Reversal of hypercapnia was only feasible when blood flow rates above 900 mL/min were used with a membrane lung surface area of at least 0.8m². The membrane lung with a surface of 0.4m² allowed a maximum normalized CO₂ elimination rate of 41 ± 6 mL/min with 8 L/min sweep gas flow and 900 mL blood flow/min. The increase in sweep gas flow from 2 to 8 L/min increased normalized CO₂ elimination from 35 ± 5 to 41 ± 6 with 900 mL blood flow/min, whereas with lower blood flow rates, any increase was less effective, levelling out at 4 L sweep gas flow/min. The membrane lung with a surface area of 0.8m² allowed a maximum normalized CO₂ elimination rate of 101 ± 12 mL/min with increasing influence of sweep gas flow. The delta of normalized CO₂ elimination increased from 4 ± 2 to 26 ± 7 mL/min with blood flow rates being increased from 300 to 1800 mL/min, respectively.

CONCLUSIONS

The influence of sweep gas flow on the CO₂ removal capacity of ECCO₂R systems depends predominantly on blood flow rate and membrane lung surface area. In this model, considerable CO₂ removal occurred only with the larger membrane lung surface of 0.8m² and when blood flow rates of ≥ 900 mL/min were used.

Physiological and Technical Considerations of Extracorporeal CO2 Removal

This article is one of ten reviews selected from the Annual Update in Intensive Care and Emergency Medicine 2019. Other selected articles can be found online at https://www.biomedcentral.com/collections/annualupdate2019. Further information about the Annual Update in Intensive Care and Emergency Medicine is available from http://www.springer.com/series/8901.

Extracorporeal Carbon Dioxide Removal

Mechanical ventilation (MV) represents a lifesaving treatment for patients with respiratory failure, but it could be harmful through the development of ventilator-induced lung injury (VILI). In patients with acute respiratory distress syndrome (ARDS), protective MV strategies with low tidal volume to minimize VILI have been demonstrated to reduce lung injury and mortality. However, they can be limited by the emergence of uncontrolled hypercapnia. Similarly, in COPD patients, noninvasive MV failure often is associated with a progressive rise in arterial CO₂ and need for endotracheal intubation, with higher risk of hospital mortality. Minimally invasive extracorporeal CO₂ removal systems (ECCO₂R) theoretically can remove the entire amount of the CO₂ produced in the body per minute. In ARDS patients, ECCO₂R may further reduce the risk of VILI ensuring ultraprotective MV and avoiding hypercapnia. In patients with exacerbation of COPD, ECCO₂R may help to avoid intubation or facilitate weaning from invasive MV. In intensive care unit, concomitant renal and respiratory failure with MV is one of the strongest risk factors for hospital mortality. Combining ECCO₂R and renal replacement therapy may support respiratory and renal functions and limit the side effects of MV. However, the need for systemic anticoagulation and the related risk of bleeding still represent a concern for a wider application of ECCO₂R devices.

In conclusion, ECCO₂R is an effective support therapy to MV to limit its invasiveness and side effects, but its efficacy and safety must be proven in well-designed clinical trials.

This chapter will:1

Explain the physiology of CO₂ removal during extracorporeal support.

2

Describe potential clinical applications of extracorporeal CO₂ removal systems (ECCO₂R) support therapy in patients with acute respiratory distress syndrome (ARDS) and chronic obstructive pulmonary disease (COPD) as well as in those with acute kidney injury requiring renal replacement therapy.

Extracorporeal Gas Exchange

Extracorporeal gas exchange is increasingly used for various indications. Among these are refractory acute respiratory failure, including the acute respiratory distress syndrome (ARDS), and the avoidance of ventilator-induced lung injury (VILI) by enabling lung-protective ventilation. Additionally, extracorporeal gas exchange allows the treatment of hypercapnic respiratory failure while helping to unload the respiratory muscles and avoid intubation and invasive ventilation, as well as facilitating weaning from the ventilator. These indications are based on a reasonable physiologic rationale but must be weighed against the costs and complications associated with the technique. This article summarizes current evidence and indications for extracorporeal gas exchange.

Bench Validation of a Compact Low-Flow CO2 Removal Device

Background

There is increasing evidence demonstrating the value of partial extracorporeal CO₂ removal (ECCO₂R) for the treatment of hypercapnia in patients with acute exacerbations of chronic obstructive pulmonary disease and acute respiratory distress syndrome. Mechanical ventilation has traditionally been used to treat hypercapnia in these patients, however, it has been well-established that aggressive ventilator settings can lead to ventilator-induced lung injury. ECCO₂R removes CO₂ independently of the lungs and has been used to permit lung protective ventilation to prevent ventilator-induced lung injury, prevent intubation, and aid in ventilator weaning. The Low-Flow Pittsburgh Ambulatory Lung (LF-PAL) is a low-flow ECCO₂R device that integrates the fiber bundle (0.65 m²) and centrifugal pump into a compact unit to permit patient ambulation.

Methods

A blood analog was used to evaluate the performance of the pump at various impeller rotation rates. In vitro CO₂ removal tested under normocapnic conditions and 6-h hemolysis testing were completed using bovine blood. Computational fluid dynamics and a mass-transfer model were also used to evaluate the performance of the LF-PAL.

Results

The integrated pump was able to generate flows up to 700 mL/min against the Hemolung 15.5 Fr dual lumen catheter. The maximum vCO₂ of 105 mL/min was achieved at a blood flow rate of 700 mL/min. The therapeutic index of hemolysis was 0.080 g/(100 min). The normalized index of hemolysis was 0.158 g/(100 L).

Conclusions

The LF-PAL met pumping, CO₂ removal, and hemolysis design targets and has the potential to enable ambulation while on ECCO₂R.

Intermittent extracorporeal CO2 removal in chronic obstructive pulmonary disease patients: a fiction or an option

PURPOSE OF REVIEW

Aim of this article is to review evidence recently generated on the application of extracorporeal carbon dioxide removal (ECCO2R) in patients with acute exacerbation of chronic obstructive pulmonary disease (COPD) requiring mechanical ventilation (invasive and non invasive) for hypercapnic respiratory failure.

RECENT FINDINGS

To date, the paucity of evidences on ECCO2R to decrease the rate of noninvasive ventilation (NIV) failure and to wean hypercapnic patients from invasive mechanical ventilation (IMV) precludes to systematically apply this technology to COPD patients.

SUMMARY

Although several efforts have been made to reduce invasiveness and to improve the efficiency of extracorporeal systems, further randomized studies are needed to assess the effects of this technique on both short-term and long-term clinical outcomes.

Efficient CO2 removal using extracorporeal lung and renal assist device

We developed a novel system comprising acid infusion, membrane lung, and a continuous renal replacement therapy console for efficient CO₂ removal at a low blood flow. To evaluate the new system, we used an ex vivo experimental model using swine blood. A liter of aliquoted blood adjusted to pH 7.25 and pCO₂ 65 mm Hg was mixed with acid (0, 10, or 20 mL of lactic or hydrochloric acid [1 mol/L]) and was immediately delivered to the system in a single pass. We collected blood samples at each point of the circuit and calculated the amount of CO₂ eliminated by the membrane lung. The new system removed 13.2 ± 0.8, 32.0 ± 2.1, and 51.6 ± 3.7 mL/min of CO₂ (with 0, 10, and 20 mEq/L of lactic acid) and 21.2 ± 1.2, 27.3 ± 0.3, and 42.0 ± 1.3 mL/min (with 0, 10, and 20 mEq/L of hydrochloric acid), respectively. The levels of lactate and Cl^- ions for acid-base equilibrium were restored after continuous hemodiafiltration. Thus, the amount of CO₂ eliminated by the membrane lung was 3.9 times higher with lactic acid and 2.0 times higher with hydrochloric acid compared with non-acid controls. In conclusion, this easy-to-setup CO₂ removal system was safe, effective, and removed CO₂ at a low blood flow.

Extracorporeal carbon dioxide removal in acute exacerbations of chronic obstructive pulmonary disease

Extracorporeal carbon dioxide removal (ECCO2R) has been proposed as an adjunctive intervention to avoid worsening respiratory acidosis, thereby preventing or shortening the duration of invasive mechanical ventilation (IMV) in patients with exacerbation of chronic obstructive pulmonary disease (COPD). This review will present a comprehensive summary of the pathophysiological rationale and clinical evidence of ECCO2R in patients suffering from severe COPD exacerbations.

Industrial PM2.5 cause pulmonary adverse effect through RhoA/ROCK pathway

According to the Chinese Ministry of Health, industrial pollution-induced health impacts have been the leading cause of death in China. While industrial fine particulate matter (PM_2.5) is associated with adverse health effects, the major action mechanisms of different compositions of PM_2.5 are currently unclear. In this study, we treated normal human lung epithelial BEAS-2B cells with industrial organic and water-soluble PM_2.5 extracts under daily alveolar deposition dose to elucidate the molecular mechanisms underlying adverse pulmonary effects induced by PM_2.5, including oxidative damage, inflammatory response, lung epithelial barrier dysfunction, and the recruitment of macrophages. We found that water-soluble PM_2.5 extracts caused more severe cytotoxic effects on BEAS-2B cells compared with that of organic extracts. Both organic and water-soluble PM_2.5 extracts induced activation of the RhoA/ROCK pathway. Inflammatory response, epithelial barrier dysfunction, and the activation of NF-кB caused by both PM_2.5 extracts were attenuated by ROCK inhibitor Y-27632. This indicated that both PM_2.5 extracts could cause damage to epithelial cells through RhoA/ROCK-dependent NF-кB activation. Furthermore, the upregulation of macrophage adhesion induced by both PM_2.5 extracts was also attenuated by Y-27632 in a co-culture model of macrophages and the epithelial cells. Therefore, our results support that industrial PM_2.5 extracts-induced activation of the RhoA/ROCK-dependent NF-кB pathway induces pulmonary adverse effect. Thus, pharmacological inhibition of ROCK activation might have therapeutic potential in preventing lung disease associated with PM_2.5.

Extracorporeal Gas Exchange: The Expanding Role of Extracorporeal Support in Respiratory Failure

The use of extracorporeal support is expanding quickly in adult respiratory failure. Extracorporeal gas exchange is an accepted rescue therapy for severe acute respiratory distress syndrome (ARDS) in select patients. Extracorporeal carbon dioxide removal is also being investigated as a preventative, preemptive, and management platform in patients with respiratory failure other than severe ARDS. The non-ARDS patient population is much larger, so the potential for rapid growth is high. This article hopes to inform decisions about the use of extracorporeal support by increasing understanding concerning the past and present practice of extracorporeal gas exchange.

An extracorporeal carbon dioxide removal (ECCO2R) device operating at hemodialysis blood flow rates

Background

Extracorporeal carbon dioxide removal (ECCO₂R) systems have gained clinical appeal as supplemental therapy in the treatment of acute and chronic respiratory injuries with low tidal volume or non-invasive ventilation. We have developed an ultra-low-flow ECCO₂R device (ULFED) capable of operating at blood flows comparable to renal hemodialysis (250 mL/min). Comparable operating conditions allow use of minimally invasive dialysis cannulation strategies with potential for direct integration to existing dialysis circuitry.

Methods

A carbon dioxide (CO₂) removal device was fabricated with rotating impellers inside an annular hollow fiber membrane bundle to disrupt blood flow patterns and enhance gas exchange. In vitro gas exchange and hemolysis testing was conducted at hemodialysis blood flows (250 mL/min).

Results

In vitro carbon dioxide removal rates up to 75 mL/min were achieved in blood at normocapnia (pCO₂ = 45 mmHg). In vitro hemolysis (including cannula and blood pump) was comparable to a Medtronic Minimax oxygenator control loop using a time-of-therapy normalized index of hemolysis (0.19 ± 0.04 g/100 min versus 0.12 ± 0.01 g/100 min, p = 0.169).

Conclusions

In vitro performance suggests a new ultra-low-flow extracorporeal CO₂ removal device could be utilized for safe and effective CO₂ removal at hemodialysis flow rates using simplified and minimally invasive connection strategies.

Impact of membrane lung surface area and blood flow on extracorporeal CO2 removal during severe respiratory acidosis

BACKGROUND

Veno-venous extracorporeal CO₂ removal (vv-ECCO₂R) is increasingly being used in the setting of acute respiratory failure. Blood flow rates through the device range from 200 ml/min to more than 1500 ml/min, and the membrane surface areas range from 0.35 to 1.3 m². The present study in an animal model with similar CO₂ production as an adult patient was aimed at determining the optimal membrane lung surface area and technical requirements for successful vv-ECCO₂R.

METHODS

Four different membrane lungs, with varying lung surface areas of 0.4, 0.8, 1.0, and 1.3m² were used to perform vv-ECCO₂R in seven anesthetized, mechanically ventilated, pigs with experimentally induced severe respiratory acidosis (pH 7.0-7.1) using a 20Fr double-lumen catheter with a sweep gas flow rate of 8 L/min. During each experiment, the blood flow was increased stepwise from 250 to 1000 ml/min.

RESULTS

Amelioration of severe respiratory acidosis was only feasible when blood flow rates from 750 to 1000 ml/min were used with a membrane lung surface area of at least 0.8 m². Maximal CO₂ elimination was 150.8 ml/min, with pH increasing from 7.01 to 7.30 (blood flow 1000 ml/min; membrane lung 1.3 m²). The membrane lung with a surface of 0.4 m² allowed a maximum CO₂ elimination rate of 71.7 mL/min, which did not result in the normalization of pH, even with a blood flow rate of 1000 ml/min. Also of note, an increase of the surface area above 1.0 m² did not result in substantially higher CO₂ elimination rates. The pressure drop across the oxygenator was considerably lower (<10 mmHg) in the largest membrane lung, whereas the smallest revealed a pressure drop of more than 50 mmHg with 1000 ml blood flow/min.

CONCLUSIONS

In this porcine model, vv-ECCO₂R was most effective when using blood flow rates ranging between 750 and 1000 ml/min, with a membrane lung surface of at least 0.8 m². In contrast, low blood flow rates (250-500 ml/min) were not sufficient to completely correct severe respiratory acidosis, irrespective of the surface area of the membrane lung being used. The converse was also true, low surface membrane lungs (0.4 m²) were not capable of completely correcting severe respiratory acidosis across the range of blood flows used in this study.

Update in Critical Care 2015

This review documents important progress made in 2015 in the field of critical care. Significant advances in 2015 included further evidence for early implementation of low tidal volume ventilation as well as new insights into the role of open lung biopsy, diaphragmatic dysfunction, and a potential mechanism for ventilator-induced fibroproliferation. New therapies, including a novel low-flow extracorporeal CO2 removal technique and mesenchymal stem cell-derived microparticles, have also been studied. Several studies examining the role of improved diagnosis and prevention of ventilator-associated pneumonia also showed relevant results. This review examines articles published in the American Journal of Respiratory and Critical Care Medicine and other major journals that have made significant advances in the field of critical care in 2015.

Fifty Years of Research in ARDS. Is Extracorporeal Circulation the Future of Acute Respiratory Distress Syndrome Management?

Mechanical ventilation (MV) remains the cornerstone of acute respiratory distress syndrome (ARDS) management. It guarantees sufficient alveolar ventilation, high Fi_O2 concentration, and high positive end-expiratory pressure levels. However, experimental and clinical studies have accumulated, demonstrating that MV also contributes to the high mortality observed in patients with ARDS by creating ventilator-induced lung injury. Under these circumstances, extracorporeal lung support (ECLS) may be beneficial in two distinct clinical settings: to rescue patients from the high risk for death associated with severe hypoxemia, hypercapnia, or both not responding to maximized conventional MV, and to replace MV and minimize/abolish the harmful effects of ventilator-induced lung injury. High extracorporeal blood flow venovenous extracorporeal membrane oxygenation (ECMO) may therefore rescue the sickest patients with ARDS from the high risk for death associated with severe hypoxemia, hypercapnia, or both not responding to maximized conventional MV. Successful venovenous ECMO treatment in patients with extremely severe H1N1-associated ARDS and positive results of the CESAR trial have led to an exponential use of the technology in recent years. Alternatively, lower-flow extracorporeal CO₂ removal devices may be used to reduce the intensity of MV (by reducing Vt from 6 to 3-4 ml/kg) and to minimize or even abolish the harmful effects of ventilator-induced lung injury if used as an alternative to conventional MV in nonintubated, nonsedated, and spontaneously breathing patients. Although conceptually very attractive, the use of ECLS in patients with ARDS remains controversial, and high-quality research is needed to further advance our knowledge in the field.

Spontaneous Breathing during Extracorporeal Membrane Oxygenation in Acute Respiratory Failure

BACKGROUND

We evaluate the clinical feasibility of spontaneous breathing on extracorporeal membrane oxygenation and the interactions between artificial and native lungs in patients bridged to lung transplant or with acute exacerbation of chronic obstructive pulmonary disease (COPD) or acute respiratory distress syndrome.

METHODS

The clinical course of a total of 48 patients was analyzed. Twenty-three of 48 patients were enrolled in the prospective study (nine bridged to lung transplant, six COPD, and eight acute respiratory distress syndrome). The response to the carbon dioxide removal was evaluated in terms of respiratory rate and esophageal pressure swings by increasing ("relief" threshold) and decreasing ("distress" threshold) the extracorporeal membrane oxygenation gas flow, starting from baseline condition.

RESULTS

Considering all 48 patients, spontaneous breathing extracorporeal membrane oxygenation was performed in 100% bridge to lung transplant (9 of 9 extubated), 86% COPD (5 of 6 extubated), but 27% acute respiratory distress syndrome patients (6 of 8 extubated; P < 0.001) and was maintained for 92, 69, and 38% of the extracorporeal membrane oxygenation days (P = 0.021), respectively. In all the 23 patients enrolled in the study, gas flow increase (from 2.3 ± 2.2 to 9.2 ± 3.2 l/min) determined a decrease of both respiratory rate (from 29 ± 6 to 8 ± 9 breaths/min) and esophageal pressure swings (from 20 ± 9 to 4 ± 4 cm H2O; P < 0.001 for all). All COPD and bridge to lung transplant patients were responders (reached the relief threshold), while 50% of acute respiratory distress syndrome patients were nonresponders.

CONCLUSIONS

Carbon dioxide removal through extracorporeal membrane oxygenation relieves work of breathing and permits extubation in many patients, mainly bridge to lung transplant and COPD. Only few patients with acute respiratory distress syndrome were able to perform the spontaneous breathing trial, and in about 50% of these, removal of large amount of patient's carbon dioxide production was not sufficient to prevent potentially harmful spontaneous respiratory effort.

[The role of extracorporeal removal of CO2 (ECCO2R) in the management of respiratory diseases]

INTRODUCTION

The aim of extracorporeal removal of CO₂ (ECCO₂R) is to ensure the removal of CO₂ without any significant effect on oxygenation. ECCO₂R makes use of low to moderate extracorporeal blood flow rates, whereas extracorporeal membrane oxygenation (ECMO) requires high blood flows.

STATE OF THE ART

For each ECCO₂R device it is important to consider not only performance in terms of CO₂ removal, but also cost and safety, including the incidence of hemolysis and of hemorrhagic and thrombotic complications. In addition, it is possible that the benefits of such techniques may extend beyond simple removal of CO₂. There have been preliminary reports of benefits in terms of reduced respiratory muscle workload. Mobilization of endothelial progenitor cells could also occur, in analogy to the data reported with ECMO, with a potential benefit in term of pulmonary repair. The most convincing clinical experience has been reported in the context of the acute respiratory distress syndrome (ARDS) and severe acute exacerbations of chronic obstructive pulmonary disease (COPD), especially in patients at high risk of failure of non-invasive ventilation.

PERSPECTIVES

Preliminary results prompt the initiation of randomized controlled trials in these two main indications. Finally, the development of these technologies opens new perspectives in terms of long-term ventilatory support.

Extracorporeal carbon dioxide removal (ECCO2R) in patients with acute respiratory failure

PURPOSE

To review the available knowledge related to the use of ECCO₂R as adjuvant strategy to mechanical ventilation (MV) in various clinical settings of acute respiratory failure (ARF).

METHODS

Expert opinion and review of the literature.

RESULTS

ECCO₂R may be a promising adjuvant therapeutic strategy for the management of patients with severe exacerbations of COPD and for the achievement of protective or ultra-protective ventilation in patients with ARDS without life-threatening hypoxemia. Given the observational nature of most of the available clinical data and differences in technical features and performances of current devices, the balance of risks and benefits for or against ECCO₂R in such patient populations remains unclear CONCLUSIONS: ECCO₂R is currently an experimental technique rather than an accepted therapeutic strategy in ARF-its safety and efficacy require confirmation in clinical trials.

Extracorporeal CO2 Removal by Respiratory Electrodialysis: An In Vitro Study

We previously described a highly efficient extracorporeal CO2 removal technique called respiratory electrodialysis (R-ED). Respiratory electrodialysis was composed of a hemodiafilter and a membrane lung (ML) positioned along the extracorporeal blood circuit, and an electrodialysis (ED) cell positioned on the hemodiafiltrate. The ED regionally increased blood chloride concentration to convert bicarbonate to CO2 upstream the ML, thus enhancing ML CO2 extraction (VCO2ML). In this in vitro study, with an aqueous polyelectrolytic carbonated solution mimicking blood, we tested a new R-ED setup, featuring an ML positioned on the hemodiafiltrate after the ED, at increasing ED current levels (0, 2, 4, 6, and 8 A). We measured VCO2ML, electrolytes concentrations, and pH of the extracorporeal circuit. Raising levels of ED-current increased chloride concentration from 107.5 ± 1.6 to 114.6 ± 1.3 mEq/L (0 vs. 8 A, p < 0.001) and reduced pH from 7.48 ± 0.01 to 6.51 ± 0.05 (0 vs. 8 A, p < 0.001) of the hemodiafiltrate entering the ML. Subsequently, VCO2ML increased from 27 ± 1.7 to 91.3 ± 1.5 ml/min (0 vs. 8 A, p < 0.001). Respiratory electrodialysis is efficient in increasing VCO2ML of an extracorporeal circuit featuring an ML perfused by hemodiafiltrate. During R-ED, the VCO2ML can be significantly enhanced by increasing the ED current.

Extracorporeal Support for Chronic Obstructive Pulmonary Disease: A Bright Future

In the past the only option for the treatment of respiratory failure due to acute exacerbation of chronic obstructive pulmonary disease (aeCOPD) was invasive mechanical ventilation. In recent decades, the potential for extracorporeal carbon dioxide (CO₂) removal has been realized. We review the various types of extracorporeal CO₂ removal, outline the optimal use of these therapies for aeCOPD, and make suggestions for future controlled trials. We also describe the advantages and requirements for an ideal long-term ambulatory CO₂ removal system for palliation of COPD.