A mathematical model of CO2, O2 and N2 exchange during venovenous extracorporeal membrane oxygenation

Therapeutic Hypothermia for Refractory Hypoxemia on Venovenous Extracorporeal Membrane Oxygenation: An In Silico Study

Patients with respiratory failure may remain hypoxemic despite treatment with venovenous extracorporeal membrane oxygenation (VV-ECMO). Therapeutic hypothermia is a potential treatment for such hypoxia as it reduces cardiac output ( ) and oxygen consumption. We modified a previously published mathematical model of gas exchange to investigate the effects of hypothermia during VV-ECMO. Partial pressures were expressed as measured at 37°C (α-stat). The effect of hypothermia on gas exchange was examined in four clinical scenarios of hypoxemia on VV-ECMO, each with different physiological derangements. All scenarios had arterial partial pressure of oxygen (PaO 2 ) ≤ 46 mm Hg and arterial oxygen saturation of hemoglobin (SaO 2 ) ≤ 81%. Three had high with low extracorporeal blood flow to ratio ( ). The problem in the fourth scenario was recirculation, with normal . Cooling to 33°C increased SaO 2 to > 89% and PaO 2 to > 50 mm Hg in all scenarios. Mixed venous oxygen saturation of hemoglobin as % ( ) increased to > 70% and mixed venous partial pressure of oxygen in mm Hg ( ) increased to > 34 mm Hg in scenarios with low . In the scenario with high recirculation, and increased, but to < 50% and < 27 mm Hg, respectively. This in silico study predicted cooling to 33°C will improve oxygenation in refractory hypoxemia on VV-ECMO, but the improvement will be less when the problem is recirculation.

[When mechanical ventilation fails-Venovenous extracorporeal membrane oxygenation]

Venovenous extracorporeal membrane oxygenation (VV-ECMO) is predominantly being used as a rescue strategy in patients with acute lung failure, suffering from severe oxygenation and/or decarboxylation impairment. Cannulas introduced into the central veins lead blood through a membrane oxygenator in which it is oxygenated via sweep gas (pO2 up to 600 mm Hg) flow, eliminating CO2. According to the largest randomized studies carried out so far, the two most important indications for VV-ECMO are hypoxic respiratory failure (paO2 < 80 mm Hg for more than 6 h) and refractory hypercapnia (pH < 7.25 und pCO2 > 60 mm Hg with a breathing frequency of >30/min) despite optimal protective mechanical ventilation settings (ARDS, Δp < 14 mbar, plateau pressure < 30 mbar, tidal volume VT < 6 ml/kg idealized body weight). Relative contraindications are life-limiting comorbidities and terminal pulmonary diseases that cannot be treated by lung transplantation. Advanced patient age is not regarded as an absolute contraindication, though it highly impacts ARDS survival rates, especially for pneumonia associated with coronavirus disease 2019 (COVID-19). The most frequent complications of VV-ECMO include bleeding, thrombus formation and rare cases of cannula-associated infections. Its use in nonintubated patients (awake ECMO) is possible in specific cases and has proven valuable as a bridge to lung transplant approach. Some ECMO centers offer cannulation of a patient at primary care hospitals, facilitating subsequent transport to the center (ECMO transport). The COVID-19 pandemic not only caused the number of VV-ECMO runs to skyrocket but has also drawn public attention to this extracorporeal procedure. Strict quality control to improve vvECMO outcomes according to the German hospital reform is urgently needed, especially so since the technique has a high demand in resources and bears significant risks when performed by untrained personnel.

A numerical study of the hemodynamic behavior and gas transport in cardiovascular systems with severe cardiac or cardiopulmonary failure supported by venoarterial extracorporeal membrane oxygenation

Venoarterial extracorporeal membrane oxygenation (VA-ECMO) has been extensively demonstrated as an effective means of bridge-to-destination in the treatment of patients with severe ventricular failure or cardiopulmonary failure. However, appropriate selection of candidates and management of patients during Extracorporeal membrane oxygenation (ECMO) support remain challenging in clinical practice, due partly to insufficient understanding of the complex influences of extracorporeal membrane oxygenation support on the native cardiovascular system. In addition, questions remain as to how central and peripheral venoarterial extracorporeal membrane oxygenation modalities differ with respect to their hemodynamic impact and effectiveness of compensatory oxygen supply to end-organs. In this work, we developed a computational model to quantitatively address the hemodynamic interaction between the extracorporeal membrane oxygenation and cardiovascular systems and associated gas transport. Model-based numerical simulations were performed for cardiovascular systems with severe cardiac or cardiopulmonary failure and supported by central or peripheral venoarterial extracorporeal membrane oxygenation. Obtained results revealed that: 1) central and peripheral venoarterial extracorporeal membrane oxygenation modalities had a comparable capacity for elevating arterial blood pressure and delivering oxygenated blood to important organs/tissues, but induced differential changes of blood flow waveforms in some arteries; 2) increasing the rotation speed of extracorporeal membrane oxygenation pump (ω) could effectively improve arterial blood oxygenation, with the efficiency being especially high when ω was low and cardiopulmonary failure was severe; 3) blood oxygen indices (i.e., oxygen saturation and partial pressure) monitored at the right radial artery could be taken as surrogates for diagnosing potential hypoxemia in other arteries irrespective of the modality of extracorporeal membrane oxygenation; and 4) Left ventricular (LV) overloading could occur when ω was high, but the threshold of ω for inducing clinically significant left ventricular overloading depended strongly on the residual cardiac function. In summary, the study demonstrated the differential hemodynamic influences while comparable oxygen delivery performance of the central and peripheral venoarterial extracorporeal membrane oxygenation modalities in the management of patients with severe cardiac or cardiopulmonary failure and elucidated how the status of arterial blood oxygenation and severity of left ventricular overloading change in response to variations in ω. These model-based findings may serve as theoretical references for guiding the application of venoarterial extracorporeal membrane oxygenation or interpreting in vivo measurements in clinical practice.

Optimal Settings at Initiation of Veno-Venous Extracorporeal Membrane Oxygenation: An Exploratory In-Silico Study

The Extracorporeal Life Support Organisation (ELSO) recommends initiating veno-venous extracorporeal membrane oxygenation (ECMO) with sweep gas flow rate () of 2 L/min and extracorporeal circuit blood flow () of 2 L/min. We used an in-silico model to examine the effect on gas exchange of initiating ECMO with different and , and the effect of including 5% in sweep gas. This was done using a set of patient examples, each with different physiological derangements at baseline (before ECMO). When ECMO was initiated following ELSO recommendations in the patient examples with significant hypercapnia at baseline, sometimes fell to < 50% of the baseline , a magnitude of fall associated with adverse neurological outcomes. In patient examples with very low baseline arterial oxygen saturation (), initiation of ECMO did not always increase to > 80%. Initiating ECMO with of 1 L/min and of 4 L/min, or with sweep gas containing 5% , of 2 L/min, and of 4 L/min, reduced the fall in and increased the rise in compared to the ELSO strategy. While ELSO recommendations may suit most patients, they may not suit patients with severe physiological derangements at baseline.

A model of the pulmonary acinar circulatory system with gas exchange function to explore the influence of alveolar diameter

Lung diseases such as acute respiratory distress syndrome affect the patient's lung compliance, which in turn affects the ability of gas exchange. Changes in alveolar diameter relate to local lung compliance. How alveolar diameter affects gas exchange, particularly oxygen concentrations in alveolar capillaries, is a topic of concern for researchers, and can be studied using mathematical models. The level of small-scale mathematical models of the pulmonary circulatory system was the alveolar capillaries, but existing models do not consider the gas-exchange function and fail to reflect the influence of alveolar diameter. Therefore, we proposed a pulmonary acinar capillary model with gas exchange function, and most importantly, introduced alveolar diameter into the model, to analyze the effect of alveolar diameter on the gas exchange function of the pulmonary acini. The model was tested by three respiratory function simulation experiments. According to the simulation results of changing diameter, we found that the alveolar diameter mainly affects the alveolar gas exchange function of lung acinar inlets and the middle section compared with the peripheral section.

What Determines the Arterial Partial Pressure of Carbon Dioxide on Venovenous Extracorporeal Membrane Oxygenation?

Rapid reductions in PaCO2 during extracorporeal membrane oxygenation (ECMO) are associated with poor neurologic outcomes. Understanding what factors determine PaCO2 may allow a gradual reduction, potentially improving neurologic outcome. A simple and intuitive arithmetic expression was developed, to describe the interactions between the major factors determining PaCO2 during venovenous ECMO. This expression was tested using a wide range of input parameters from clinically feasible scenarios. The difference between PaCO2 predicted by the arithmetic equation and PaCO2 predicted by a more robust and complex in-silico mathematical model, was <10 mm Hg for more than 95% of the scenarios tested. With no CO2 in the sweep gas, PaCO2 is proportional to metabolic CO2 production and inversely proportional to the "total effective expired ventilation" (sum of alveolar ventilation and oxygenator ventilation). Extracorporeal blood flow has a small effect on PaCO2, which becomes more important at low blood flows and high recirculation fractions. With CO2 in the sweep gas, the increase in PaCO2 is proportional to the concentration of CO2 administered. PaCO2 also depends on the fraction of the total effective expired ventilation provided via the oxygenator. This relationship offers a simple intervention to control PaCO2 using titration of CO2 in the sweep gas.

Pathophysiology of respiratory failure and physiology of gas exchange during ECMO

Lungs play a key role in sustaining cellular respiration by regulating the levels of oxygen and carbon dioxide in the blood. This is achieved by exchanging these gases between blood and ambient air across the alveolar capillary membrane by the process of diffusion. In the microstructure of the lung, gas exchange is compartmentalized and happens in millions of microscopic alveolar units. In situations of lung injury, this structural complexity is disrupted resulting in impaired gas exchange. Depending on the severity and the type of lung injury, different aspects of pulmonary physiology are affected. If the respiratory failure is refractory to ventilator support, extracorporeal membrane oxygenation (ECMO) can be utilized to support the gas exchange needs of the body. In ECMO, thin hollow fiber membranes made up of polymethylpentene act as blood-gas interface for diffusion. Decades of innovative engineering with membranes and their alignment with blood and gas flows has enabled modern oxygenators to achieve clinically and physiologically significant amount of gas exchange.

Extracorporeal carbon dioxide removal requirements for ultraprotective mechanical ventilation: Mathematical model predictions

Extracorporeal carbon dioxide (CO₂) removal (ECCO₂R) facilitates the use of low tidal volumes during protective or ultraprotective mechanical ventilation when managing patients with acute respiratory distress syndrome (ARDS); however, the rate of ECCO₂R required to avoid hypercapnia remains unclear. We calculated ECCO₂R rate requirements to maintain arterial partial pressure of CO₂ (PaCO₂) at clinically desirable levels in mechanically ventilated ARDS patients using a six‐compartment mathematical model of CO₂ and oxygen (O₂) biochemistry and whole‐body transport with the inclusion of an ECCO₂R device for extracorporeal veno‐venous removal of CO₂. The model assumes steady state conditions. Model compartments were lung capillary blood, arterial blood, venous blood, post‐ECCO₂R venous blood, interstitial fluid and tissue cells, with CO₂ and O₂ distribution within each compartment; biochemistry included equilibrium among bicarbonate and non‐bicarbonate buffers and CO₂ and O₂ binding to hemoglobin to elucidate Bohr and Haldane effects. O₂ consumption and CO₂ production rates were assumed proportional to predicted body weight (PBW) and adjusted to achieve reported arterial partial pressure of O₂ and a PaCO₂ level of 46 mmHg at a tidal volume of 7.6 mL/kg PBW in the absence of an ECCO₂R device based on average data from LUNG SAFE. Model calculations showed that ECCO₂R rates required to achieve mild permissive hypercapnia (PaCO₂ of 46 mmHg) at a ventilation frequency or respiratory rate of 20.8/min during mechanical ventilation increased when tidal volumes decreased from 7.6 to 3 mL/kg PBW. Higher ECCO2R rates were required to achieve normocapnia (PaCO2 of 40 mmHg). Model calculations also showed that required ECCO2R rates were lower when ventilation frequencies were increased from 20.8/min to 26/min. The current mathematical model predicts that ECCO2R rates resulting in clinically desirable PaCO2 levels at tidal volumes of 5‐6 mL/kg PBW can likely be achieved in mechanically ventilated ARDS patients with current technologies; use of ultraprotective tidal volumes (3‐4 mL/kg PBW) may be challenging unless high mechanical ventilation frequencies are used.

In-vitro performance of a low flow extracorporeal carbon dioxide removal circuit

INTRODUCTION

Extracorporeal gas exchange requires the passage of oxygen and carbon dioxide (CO₂) across an artificial membrane. Current European Union regulations do not require the transfer to be assessed in models using clinically relevant haemoglobin, making it difficult for clinicians to understand the CO₂ clearance of a membrane, and how it changes in relation to sweep gas flow through the membrane. The characteristics of membrane CO₂ clearance are described using a single membrane at different sweep gas flows in an in vitro model with clinically relevant haemoglobin concentrations using three separate methods of calculating CO₂ clearance.

METHODS

To define the CO₂ removal characteristics of the extra-corporeal CO₂ removal (ECCO₂R) device, we devised an in-vitro gas exchange circuit formed by a dedicated ECCO₂R circuit (ALung, Pittsburgh, USA) in series with two membrane oxygenators. The system was primed with donated expired human red cells provided by the local blood bank. The experimental set-up allowed constant CO₂ input (via one membrane oxygenator) with variable removal from a portion of the blood in a manner which was analogous to that seen in vivo. Blood gases were measured from different ports in the circuit in order to measure the experimental membrane CO₂ clearance (VCO₂).

RESULTS

Results demonstrate that the relationship between VCO₂ and gas flow at a constant blood flow of 0.4 L/minute with a haemoglobin of 7 g/dL increases sharply from a gas flow of 0 to 2 L/min but plateaus at gas flows >4 L/minute. VCO₂, calculated using three different methods, showed a strong linear correlation with minimal bias.

CONCLUSIONS

The CO₂ clearance of the membrane used in this bench test is non-linear. This has implications for clinical practice, especially during the weaning phase of the device.

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.