Arteriovenous extracorporeal carbon dioxide removal: a mathematical model and experimental evaluation

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.

Blood Flow around Hollow Fibers

In an artificial lung, blood is oxygenated and flows around a bundle of hollow fibers while gas flows inside the fiber. The objective of this study is to understand the hydrodynamics of three different fiber banks (inline square IS, staggered square SS and equilateral triangle ET) and to investigate the influence of both a Newtonian and non-Newtonian Casson viscosity model on the flow field. A two-dimensional finite element model for permanent, isothermal, laminar blood flow perpendicular to hollow fibers is used. All fibers are assumed identical, straight and parallel. Porosity ranges from 0.4 to 0.6 and Reynolds number varies from 1 to 60. For a given Re, ET generates less resistance than SS, the latter being comparable with IS. A lower porosity increases resistance. Non-Newtonian viscosity affects velocity patterns only at low Re (<0.5) and higher porosity (>0.5). Resistance at low Re is significantly elevated in the fiber banks due to an overall increase in viscosity. This model makes it possible to study the influence of geometry and viscosity on hydrodynamics in fiber banks and may aid in the optimization of hollow fiber artificial lung design.

Seventy-two hour gas exchange performance and hemodynamic properties of NOVALUNG®iLA as a gas exchanger for arteriovenous carbon dioxide removal

Objective: Acute respiratory failure is complicated by acidosis and altered end-organ perfusion. NOVA-LUNG®iLA is an interventional lung assist (ILA) device for arteriovenous carbon dioxide removal (AVCO2R). The present study was conducted to evaluate the device for short-term CO2 removal performance and hemodynamic response.

Methods: Six adult sheep received cannulation of the jugular vein and carotid artery. The ILA-AVCO2R circuit was placed on the sheep for 72 hours. Hemodynamics and PaCO2 were measured; CO2 removal was calculated while varying sweep gas flow rates (Qg), device blood flow rates (Qb), and PaCO2.

Results: Hemo-dynamic variables remained normal throughout the 72 hour study. CO2 removal increased with increases in Qgor Qb. Mean CO2 removal was 119.3 ml/min for Qb 1L/min, Qg 5 L/min, and PaCO2 40 - 50 mmHg.PaCO2 was directly proportional to CO2 clearance (R-0.72, p B/0.001).

Conclusion: NOVALUNG®iLA can provide near total CO2 removal with Qb 1 - 2 L/min,Qg 5 L/min, and minimal flow resistance (3.889/0.82 mmHg/L/min). PaCO2 correlates with CO2 removal and is dependent on Qb and Qg.

Patient-specific modeling of cardiovascular and respiratory dynamics during hypercapnia

This study develops a lumped cardiovascular-respiratory system-level model that incorporates patient-specific data to predict cardiorespiratory response to hypercapnia (increased CO(2) partial pressure) for a patient with congestive heart failure (CHF). In particular, the study focuses on predicting cerebral CO(2) reactivity, which can be defined as the ability of vessels in the cerebral vasculature to expand or contract in response CO(2) induced challenges. It is difficult to characterize cerebral CO(2) reactivity directly from measurements, since no methods exist to dynamically measure vasomotion of vessels in the cerebral vasculature. In this study we show how mathematical modeling can be combined with available data to predict cerebral CO(2) reactivity via dynamic predictions of cerebral vascular resistance, which can be directly related to vasomotion of vessels in the cerebral vasculature. To this end we have developed a coupled cardiovascular and respiratory model that predicts blood pressure, flow, and concentration of gasses (CO(2) and O(2)) in the systemic, cerebral, and pulmonary arteries and veins. Cerebral vascular resistance is incorporated via a model parameter separating cerebral arteries and veins. The model was adapted to a specific patient using parameter estimation combined with sensitivity analysis and subset selection. These techniques allowed estimation of cerebral vascular resistance along with other cardiovascular and respiratory parameters. Parameter estimation was carried out during eucapnia (breathing room air), first for the cardiovascular model and then for the respiratory model. Then, hypercapnia was introduced by increasing inspired CO(2) partial pressure. During eucapnia, seven cardiovascular parameters and four respiratory parameters was be identified and estimated, including cerebral and systemic resistance. During the transition from eucapnia to hypercapnia, the model predicted a drop in cerebral vascular resistance consistent with cerebral vasodilation.

Pumpless arteriovenous carbon dioxide removal: A novel simplified strategy for severe asthma in children

Status asthmaticus unresponsive to pharmacotherapy is conventionally managed with mechanical ventilation, which has its inherent challenges due to barotrauma, dynamic hyperinflation and autopositive end-expiratory pressure (auto-PEEP). Extracorporeal membrane oxygenation has been used as a last resort in respiratory failure due to refractory asthma; however, it entails many complications. In contrast, arteriovenous carbon dioxide removal (AVCO₂R) is a novel strategy that has been shown to be highly effective in adults with acute respiratory failure. Only one pediatric case series of pediatric asthma managed with AVCO₂R have been published so far. We herein report a case of severe asthma in a 9-year-old boy who developed severe hypercapnia (Pco2 97 mmHg) and acidosis (pH 7.09) despite being on mechanical ventilation. Within 4 h of initiation of AVCO₂R, PCo₂ drastically reduced to near-normal levels. He was discharged on day 9 of hospital stay without any complications.

Modeling cardio-respiratory system response to inhaled CO2 in patients with congestive heart failure

In this paper we examine a cardiovascular-respiratory model of mid-level complexity designed to predict the dynamics of end-tidal carbon dioxide (CO(2)) and cerebral blood flow velocity in response to a CO(2) challenge. Respiratory problems often emerge as heart function diminishes in congestive heart failure patients. To assess system function, various tests can be performed including inhalation of a higher than normal CO(2) level. CO(2) is a key quantity firstly because any perturbation in system CO(2) quickly influences ventilation (oxygen perturbations need to be more severe). Secondly, the CO(2) response gain has been associated with respiratory system control instability. Thirdly, CO(2) in a short time impacts the degree of cerebral vascular constriction, allowing for the assessment of cerebral vasculature function. The presented model can be used to study key system characteristics including cerebral vessel CO(2) reactivity and ventilatory feedback factors influencing ventilatory stability in patients with congestive heart failure. Accurate modeling of the dynamics of system response to CO(2) challenge, in conjunction with robust parameter identification of key system parameters, can help in assessing patient system status.

Year in review 2009: Critical Care--cardiac arrest, trauma and disasters

During 2009, Critical Care published nine papers on various aspects of resuscitation, prehospital medicine, trauma care and disaster response. One article demonstrated that children as young as 9 years of age can learn cardiopulmonary resuscitation (CPR) effectively, although, depending on their size, some may have difficulty performing it. Another paper showed that while there was a trend toward mild therapeutic hypothermia reducing S-100 levels, there was no statistically significant change. Another predictor study also showed a strong link between acute kidney injury and neurologic outcome while another article described a program in which kidneys were harvested from cardiac arrest patients and showed an 89% graft survival rate. One experimental investigation indicated that when a pump-less interventional lung assist device is present, leaving the device open (unclamped) while performing CPR has no harmful effects on mean arterial pressures and it may have positive effects on blood oxygenation and CO2 clearance. One other study, conducted in the prehospital environment, found that end-tidal CO2 could be useful in diagnosing pulmonary embolism. Three articles addressed disaster medicine, the first of which described a triage system for use during pandemic influenza that demonstrated high reliability in delineating patients with a good chance of survival from those likely to die. The other two studies, both drawn from the 2008 Sichuan earthquake experience, showed success in treating crush injured patients in an on-site tent ICU and, in the second case, how the epidemiology of earthquake injuries and related factors predicted mortality.

Decreasing pulmonary ventilation through bicarbonate ultrafiltration: an experimental study

OBJECTIVE

: To demonstrate the technical feasibility of CO2 removal with a commercial hemofilter and a replacement solution containing sodium hydroxide to replace bicarbonate.

DESIGN

: Prospective animal experiment in sheep.

SUBJECTS

: Seven mixed-breed female sheep.

INTERVENTIONS

: Blood ultrafiltrate containing half of the metabolic production of CO2 was removed with a commercial hemofilter and a replacement solution containing sodium hydroxide was given as replacement. Minute ventilation was lowered to less than half of its baseline value. Ultrafiltration was stopped at 18 hrs, and Paco2 was allowed to increase for about 1 hr; at this time, the sheep were electively killed.

MEASUREMENTS AND MAIN RESULTS

: Every 6 hrs, blood was sampled from the carotid artery, the pulmonary artery, and from the extracorporeal perfusion circuit (before the hemofilter, immediately after the hemofilter, and after mixing with the replacement solution). To maintain normocapnia, minute ventilation was reduced from 3.8 +/- 0.1 L/min to 1.9 +/- 0.7 L/min; Paco2 remained near constant during the study. The average blood pH, after mixing with the replacement solution, was 7.64 +/- 0.12. One hour after the ultrafiltration had stopped, Paco2 had increased from 36.7 +/- 4.2 torr (4.9 +/- 0.6 kPa) to 59.6 +/- 9 torr (7.9 +/- 1.2 kPa) (p < .01) and blood pH had decreased from 7.317 +/- 0.041 to 7.151 +/- 0.051 (p < .01).

CONCLUSION

: CO2 removal with bicarbonate ultrafiltration may be an effective treatment for patients with respiratory failure.

Effects of interventional lung assist on haemodynamics and gas exchange in cardiopulmonary resuscitation: a prospective experimental study on animals with acute respiratory distress syndrome

Introduction

Interventional lung assist (ILA), based on the use of a pumpless extracorporeal membrane oxygenator, facilitates carbon dioxide (CO₂) elimination in acute respiratory distress syndrome (ARDS). It is unclear whether an ILA system should be clamped during cardiopulmonary resuscitation (CPR) in patients with ARDS or not. The aim of our study was to test the effects of an ILA on haemodynamics and gas exchange during CPR on animals with ARDS and to establish whether the ILA should be kept open or clamped under these circumstances.

Methods

The study was designed to be prospective and experimental. The experiments were performed on 12 anaesthetised and mechanically ventilated pigs (weighing 41 to 58 kg). One femoral artery and one femoral vein were cannulated and connected to an ILA. ARDS was induced by repeated bronchoalveolar lavage. An indwelling pacemaker was used to initiate ventricular fibrillation and chest compressions were immediately started and continued for 30 minutes. In six animals, the ILA was kept open and in the other six it was clamped.

Results

Systolic and mean arterial pressures did not differ significantly between the groups. With the ILA open mean ± standard deviation systolic blood pressures were 89 ± 26 mmHg at 5 minutes, 71 ± 28 mmHg at 10 minutes, 63 ± 33 mmHg at 20 minutes and 83 ± 23 mmHg at 30 minutes. The clamped ILA system resulted in systolic pressures of 77 ± 30 mmHg, 90 ± 23 mmHg, 72 ± 11 mmHg and 72 ± 22 mmHg, respectively. In the group with the ILA system open, arterial partial pressure of CO₂ was significantly lower after 10, 20 and 30 minutes of CPR and arterial partial pressure of oxygen was higher 20 minutes after the onset of CPR (191 ± 140 mmHg versus 57 ± 14 mmHg). End-tidal partial pressure of CO₂ decreased from 46 ± 23 Torr (ILA open) and 37 ± 9 Torr (ILA clamped) before intervention to 8 ± 5 Torr and 8 ± 10 Torr, respectively, in both groups after 30 minutes of CPR.

Conclusions

Our results indicate that in an animal model of ARDS, blood pressures were not impaired by keeping the ILA system open during CPR compared with the immediate clamping of the ILA with the onset of CPR. The effect of ILA on gas exchange implied a beneficial effect.

Near-fatal pediatric asthma managed with pumpless arteriovenous carbon dioxide removal

OBJECTIVE

To describe the use of pumpless arteriovenous carbon dioxide removal in support of four pediatric patients with near-fatal asthma.

DESIGN

Report of four cases.

SETTING

Tertiary care university pediatric intensive care unit.

PATIENTS

Four pediatric patients, ages 4, 10, 12, and 13, were intubated and initially managed with positive pressure ventilation for severe respiratory failure. Despite pharmacologic therapy with inhaled beta-agonists, inhaled anticholinergics, systemic corticosteroids, and intravenous magnesium, marked progressive hypercapnia and acidosis rapidly developed requiring high levels of positive pressure ventilation.

INTERVENTIONS

Application of pumpless arteriovenous carbon dioxide removal (AVCO2R) via percutaneous femoral cannulation.

MEASUREMENT AND MAIN RESULTS

Arterial or mixed venous carbon dioxide partial pressure (PCO2) and pH were measured before and at intervals following initiation of AVCO2R. Before cannulation, PCO2 was elevated to 100, 108, 90, and 186 mm Hg in the four patients, with corresponding pH of 7.07, 6.96, 7.09, and 6.80, respectively. Pco2 levels were reduced to more acceptable levels (37-57 mm Hg) within 2-4 hrs of initiation of AVCO2R, with corresponding improvements in pH despite reductions in ventilatory frequency and tidal volumes to safe levels. Duration of support ranged from 18 hrs to 5 days during resolution of bronchospasm. No red blood cell or platelet transfusions were required, and no complications resulted from AVCO2R or from mechanical ventilation. All patients were discharged from the hospital without sequelae.

CONCLUSIONS

Percutaneous cannulation with a simplified pumpless extracorporeal circuit is capable of removing sufficient carbon dioxide to allow application of a protective ventilatory strategy in severe hypercapnic pediatric respiratory failure. The procedure was safely applied without complications in four pediatric patients as young as 4 yrs of age.

Oxygenation effect of interventional lung assist in a lavage model of acute lung injury: a prospective experimental study

Introduction

The aim of the study was to test the hypothesis that a pumpless arteriovenous extracorporeal membrane oxygenator (interventional lung assist (ILA)) does not significantly improve oxygenation in a lavage model of acute lung injury.

Methods

The study was designed as a prospective experimental study. The experiments were performed on seven pigs (48–60 kg body weight). The pigs were anesthetized and mechanically ventilated. Both femoral arteries and one femoral vein were cannulated and connected with ILA. Acute lung injury was induced by repeated bronchoalveolar lavage until the arterial partial pressure of O₂ was lower than 100 Torr for at least 30 minutes during ventilation with 100% O₂.

Results

ILA was applied with different blood flow rates through either one or both femoral arteries. Measurements were repeated at different degrees of pulmonary gas exchange impairment with the pulmonary venous admixture ranging from 35.0% to 70.6%. The mean (± standard deviation) blood flow through ILA was 15.5 (± 3.9)% and 21.7 (± 4.9)% of cardiac output with one and both arteries open, respectively. ILA significantly increased the arterial partial pressure of O₂ from 64 (± 13) Torr to 71 (± 14) Torr and 74 (± 17) Torr with blood flow through one and both femoral arteries, respectively. O₂ delivery through ILA increased with extracorporeal shunt flow (36 (± 14) ml O₂/min versus 47 (± 17) ml O₂/min) and reduced arterialization of the inlet blood. Pulmonary artery pressures were significantly reduced when ILA was in operation.

Conclusion

Oxygenation is increased by ILA in severe lung injury. This effect is significant but small. The results indicate that the ILA use may not be justified if the improvement of oxygenation is the primary therapy goal.

Cardiovascular stability during arteriovenous extracorporeal therapy: a randomized controlled study in lambs with acute lung injury

INTRODUCTION

Clinical application of arteriovenous (AV) extracorporeal membrane oxygenation (ECMO) requires assessment of cardiovascular ability to respond adequately to the presence of an AV shunt in the face of acute lung injury (ALI). This ability may be age dependent and vary with the experimental model. We studied cardiovascular stability in a lamb model of severe ALI, comparing conventional mechanical ventilation (CMV) with AV-ECMO therapy.

METHODS

Seventeen lambs were anesthetized, tracheotomized, paralyzed, and ventilated to maintain normocapnia. Femoral and jugular veins, and femoral and carotid arteries were instrumented for the AV-ECMO circuit, systemic and pulmonary artery blood pressure monitoring, gas exchange, and cardiac output determination (thermodilution technique). A severe ALI (arterial oxygen tension/inspired fractional oxygen <200) was induced by lung lavage (repeated three times, each with 5 ml/kg saline) followed by tracheal instillation of 2.5 ml/kg of 0.1 N HCl. Lambs were consecutively assigned to CMV treatment (n = 8) or CMV plus AV-ECMO therapy using up to 15% of the cardiac output for the AV shunt flow during a 6-hour study period (n = 9). The outcome measures were the degree of inotropic and ventilator support needed to maintain hemodynamic stability and normocapnia, respectively.

RESULTS

Five of the nine lambs subjected to AV-ECMO therapy (56%) died before completion of the 6-hour study period, as compared with two out of eight lambs (25%) in the CMV group (P > 0.05; Fisher's exact test). Surviving and nonsurviving lambs in the AV-ECMO group, unlike the CMV group, required continuous volume expansion and inotropic support (P < 0.001; Fisher's exact test). Lambs in the AV-ECMO group were able to maintain normocapnia with a maximum of 30% reduction in the minute ventilation, as compared with the CMV group (P < 0.05).

CONCLUSION

AV-ECMO therapy in lambs subjected to severe ALI requires continuous hemodynamic support to maintain cardiovascular stability and normocapnia, as compared with lambs receiving CMV support.

New technologies for respiratory assist

"The artificial lung especially has lingered behind progress with artificial hearts and ventricular assist devices, not because the need for lungs has not been recognized, but because we have not had a full understanding of the engineering problems and the unique material requirements until recent years." Brack Hattler, MD PhD. The development from the first clinical use of haemodialysis over five decades ago to widespread chronic treatment took more than two decades. The histories of other artificial organ technologies, such as artificial hearts, follow similar long development paths. For five decades, due to a lack of technology, artificial lungs have been limited to use with a heart-lung machine for cardiopulmonary bypass (CPB) or extracorporeal membrane oxygenation (ECMO). The advent of pumpless biocompatible artificial lungs will open new treatment options for patients with acute or chronic lung failure.

Smoke/burn injury-induced respiratory failure elicits apoptosis in ovine lungs and cultured lung cells, ameliorated with arteriovenous CO2 removal

STUDY OBJECTIVE

s: The purpose of this study was to examine the effects of two supportive therapies, conventional mechanical ventilation (CMV) and arteriovenous CO(2) removal (AVCO(2)R), during treatment of severe smoke/burn injury-induced ARDS.

DESIGN

Sheep were exposed to a smoke/burn injury (lethal dose causing death in 40% of animals); lung tissue and blood was collected prior to injury (control), when an ARDS criterion was met (PaO(2)/fraction of inspired oxygen ratio < 200), then after 72 h of either CMV (group 1) or AVCO(2)R (group 2). Lung tissue was studied by standard histopathologic techniques; cultured lung cells were studied in media supplemented with serum from all four groups.

MEASUREMENTS AND RESULTS

In vivo assays demonstrate less apoptotic cell death, and in vitro assays show significantly greater (p < 0.05) cell survival in group 2 (AVCO(2)R) than in group 1 (CMV) or baseline. Differential gene expression demonstrates significantly higher messenger RNA levels of proapoptotic and tumor necrosis factor (TNF)-alpha in cells incubated in baseline media. After exposure of cultured lung cells to conditioned media, protein expression assay of the culture medium revealed no TNF-alpha, TNF receptor (TNFR)-1, or TNFR-2, however, cultured cell lysate reveals elevated levels of TNF-alpha, TNFR-1 and caspase-3 in all groups; most occurred in cells incubated in baseline media (p < 0.05). HOECHST stain, DNA fragmentation, and caspase-3 cleavage show that AVCO(2)R ameliorates apoptosis in this model.

CONCLUSIONS

This in vitro work specifically examines cell death in lung cells as a result of smoke/burn injury and effects of therapeutic interventions. Our in vivo studies temporally correlate the clinical pathology to that studied in these lung cells and show that both in vivo and in vitro cell death is predominantly apoptotic and is significantly reduced by AVCO(2)R.

Extracorporeal carbon dioxide removal to control arterial pH and PACO2 in a heart-beating donor with acute lung injury

BACKGROUND

Arteriovenous carbon dioxide (AVCO2R) removal is a technique of pumpless extracorporeal carbon dioxide removal. This system has been used successively to control pH and PaCO2 in patients with acute lung injury who could not be adequately ventilated. This report describes the use of this technology in an organ donor awaiting harvesting.

METHODS

AVCO2R was implanted using a hollow-fiber oxygenator attached to 12 F and 14 F vascular cannulas that were inserted into the femoral artery and vein, respectively. Oxygen was attached to the oxygenator to provide the sweep gas.

RESULTS

The PaCO2 and arterial pH promptly corrected after support was initiated (from 83-42 mm Hg and 7.18-7.38, respectively).

CONCLUSION

This case describes the successful use of pumpless arteriovenous extracorporeal removal of CO2 in a heart-beating donor awaiting organ harvest.

Toward Ambulatory Arteriovenous CO2 Removal: Initial Studies and Prototype Development

Extracorporeal arteriovenous carbon dioxide removal (AVCO2R) using percutaneous cannulae and a low resistance gas exchanger achieves near total CO2 removal, allowing lung rest and potentially improving survival. AVCO2R, redesigned to allow ambulation, has potential as treatment for severe chronic obstructive pulmonary disease or rehabilitation before lung transplant. The purposes of this study were to 1) determine the optimal ambulatory access for AVCO2 removal and 2) develop a prototype Ambulatory-AVCO2R gas exchanger. Initially, reinforced Gore-Tex 6 mm (two) and 8 mm (four) grafts were anastomosed to sheep carotid arteries and jugular veins as a loop in parallel to the cranial circulation to determine blood flow capabilities. Blood flow was 100-150 ml/min with a 14 gauge dialysis needle, and transected 6 mm Gore-Tex grafts achieved 500-900 ml blood flow, whereas transected 8 mm grafts achieved up to 2000 ml/min flow. The polytetrafluoroethylene (PTFE) loops were then connected to our newly developed ultra low resistance pumpless gas exchanger for ambulatory AVCO2R. The average pressure gradient across the prototype Ambulatory-AVCO2R gas exchangers (n = 5) was 2.8 +/- 0.8 mm Hg, and mean CO2 removal was 104.8 +/- 14.0 ml/min, with an average blood flow of 900 ml/min. We conclude that an 8 mm Gore-Tex reinforced graft arteriovenous loop supplies ample blood flow for our new ultra low resistance Ambulatory-AVCO2R to achieve near total CO2 removal.

Extracorporeal CO2 removal in a child with a single ventricle by the addition of an oxygenator to a dialysis circuit

OBJECTIVE

To optimize CO2 removal in a child with a single ventricle.

DESIGN

Case report.

SETTING

A ten-bed pediatric intensive care unit in a university-affiliated hospital.

PATIENT

A 6-yr-old boy with a single ventricle who underwent a Fontan procedure.

INTERVENTIONS

Addition of a membrane oxygenator to a venovenous hemodialysis circuit.

MEASUREMENTS

Patient respiratory variables were measured.

MAIN RESULTS

A clinically significant improvement in CO2 removal was achieved.

CONCLUSIONS

Addition of a membrane oxygenator to a dialysis circuit facilitates CO2 removal.

Effects of arteriovenous extracorporeal therapy on hemodynamic stability, ventilation, and oxygenation in normal lambs

OBJECTIVE

To evaluate hemodynamic stability and gas exchange in a neonatal animal model of pumpless arteriovenous extracorporeal membrane oxygenation (AV-ECMO) with extracorporeal shunt flow of up to 15% of cardiac output during variable ventilation and oxygenation.

DESIGN

Prospective study.

SETTING

Research laboratory in a hospital.

SUBJECTS

Seven lambs (5.5 +/- 0.6 kg, mean +/- sd).

INTERVENTIONS

The lambs initially were anesthetized by 50 mg/kg ketamine intravenously. After tracheostomy, the lambs were mechanically ventilated and paralyzed by using 1 mg/kg vecuronium bromide followed by 0.1 mg.kg(-1).hr(-1). One femoral vein was cannulated with a pulmonary artery flotation catheter and used for cardiac output and pulmonary artery pressure measurements. A femoral artery was cannulated for measuring mean arterial blood pressure, measuring heart rate, and blood sampling for gas exchange analyses. Finally, the right internal jugular vein and carotid artery were cannulated and used for the AV-ECMO. Normothermia (38 +/- 0.5 degrees C), fluid balance (5 mL.kg(-1).hr(-1) normal saline), and anesthesia (5 mg.kg(-1).hr(-1), intravenous ketamine) were maintained. Ventilator settings were adjusted to establish a baseline Paco2 (25-35 mm Hg) at an Fio2 of 0.4. The AV-ECMO circuit was established by using a hollow fiber oxygenator, primed with maternal sheep blood (150-200 mL).

MEASUREMENTS AND MAIN RESULTS

The physiologic effects of the AV-ECMO shunt were evaluated at 15, 25, and 40 mL.kg(-1).hr(-1) ECMO flow, corresponding roughly to 4%, 8%, and 15% of the cardiac output values. The baseline minute volume was maintained during stepwise increases in arteriovenous shunt. A significant increase in endogenous cardiac output occurred at arteriovenous shunt of 25 and 40 mL.kg(-1).hr(-1) (analysis of variance followed by Tukey-Kramer multiple comparisons test), which was attributed to a significant increase of 30% in the heart rate. Effective cardiac output (difference between the thermodilution value and the AV-ECMO flow rate) and mean arterial blood pressure were not significantly changed. CO2 removal, measured at 15% arteriovenous shunt, was significantly increased with decreasing ventilation to 25% and 50% of the baseline (analysis of variance and Tukey-Kramer test). Oxygenation through the membrane was measured after reducing inspired Fio2 from 0.4 to 0.21, 0.15, and 0.10 with 15% arteriovenous shunt and baseline minute ventilation. Oxygen delivery by the oxygenator was significantly increased at Fio2 of 0.10, providing a maximum of 19.5% of the total oxygen consumption at an arterial hemoglobin-oxygen saturation of 60%.

CONCLUSIONS

Healthy lambs are capable of maintaining effective cardiac output in the presence of moderate arteriovenous shunts (15%). AV-ECMO may provide efficient ventilatory support in the neonatal population with hypercapnia. The amount of oxygen delivery with AV-ECMO depends on arterial desaturation.