Advances in extracorporeal ventilation

Dynamics of blood flow and thrombus formation in a multi-bypass microfluidic ladder network

The reaction dynamics of a complex mixture of cells and proteins, such as blood, in branched circulatory networks within the human microvasculature or extravascular therapeutic devices such as extracorporeal oxygenation machine (ECMO) remains ill-defined. In this report we utilize a multi-bypass microfluidics ladder network design with dimensions mimicking venules to study patterns of blood platelet aggregation and fibrin formation under complex shear. Complex blood fluid dynamics within multi-bypass networks under flow were modeled using COMSOL. Red blood cells and platelets were assumed to be non-interacting spherical particles transported by the bulk fluid flow, and convection of the activated coagulation factor II, thrombin, was assumed to be governed by mass transfer. This model served as the basis for predicting formation of local shear rate gradients, stagnation points and recirculation zones as dictated by the bypass geometry. Based on the insights from these models, we were able to predict the patterns of blood clot formation at specific locations in the device. Our experimental data was then used to adjust the model to account for the dynamical presence of thrombus formation in the biorheology of blood flow. The model predictions were then compared to results from experiments using recalcified whole human blood. Microfluidic devices were coated with the extracellular matrix protein, fibrillar collagen, and the initiator of the extrinsic pathway of coagulation, tissue factor. Blood was perfused through the devices at a flow rate of 2 µL/min, translating to physiologically relevant initial shear rates of 300 and 700 s^-1 for main channels and bypasses, respectively. Using fluorescent and light microscopy, we observed distinct flow and thrombus formation patterns near channel intersections at bypass points, within recirculation zones and at stagnation points. Findings from this proof-of-principle ladder network model suggest a specific correlation between microvascular geometry and thrombus formation dynamics under shear. This model holds potential for use as an integrative approach to identify regions susceptible to intravascular thrombus formation within the microvasculature as well as extravascular devices such as ECMO.

Management of refractory hypoxemia

Mechanical ventilation remains the cornerstone in the management of severe acute respiratory failure. Acute respiratory distress syndrome (ARDS) is the most common cause of respiratory failure. It is associated with substantial mortality, and unmanageable refractory hypoxemia remains the most feared clinical possibility. If hypoxemia persists despite application of lung protective ventilation, additional therapies including inhaled vasodilators, prone positioning, recruitment maneuvers, high-frequency oscillatory ventilation, neuromuscular blockade (NMB), and extracorporeal membrane oxygenation may be needed. NMB and prone ventilation are modalities that have been clearly linked to reduced mortality in ARDS. Rescue therapies pose a clinical challenge requiring a precarious balance of risks and benefits, as well as, in-depth knowledge of therapeutic limitations.

Veno-venous ECMO: a synopsis of nine key potential challenges, considerations, and controversies

Background

Following the 2009 H1N1 Influenza pandemic, extracorporeal membrane oxygenation (ECMO) emerged as a viable alternative in selected, severe cases of ARDS. Acute Respiratory Distress Syndrome (ARDS) is a major public health problem. Average medical costs for ARDS survivors on an annual basis are multiple times those dedicated to a healthy individual. Advances in medical and ventilatory management of severe lung injury and ARDS have improved outcomes in some patients, but these advances fail to consistently “rescue” a significant proportion of those affected.

Discussion

Here we present a synopsis of the challenges, considerations, and potential controversies regarding veno-venous ECMO that will be of benefit to anesthesiologists, surgeons, and intensivists, especially those newly confronted with care of the ECMO patient. We outline a number of points related to ECMO, particularly regarding cannulation, pump/oxygenator design, anticoagulation, and intravascular fluid management of patients. We then address these challenges/considerations/controversies in the context of their potential future implications on clinical approaches to ECMO patients, focusing on the development and advancement of standardized ECMO clinical practices.

Summary

Since the 2009 H1N1 pandemic ECMO has gained a wider acceptance. There are challenges that still must be overcome. Further investigations of the benefits and effects of ECMO need to be undertaken in order to facilitate the implementation of this technology on a larger scale.

Advances in therapy for acute lung injury

Despite advances in the therapy for acute lung injury and adult respiratory distress syndrome, mortality remains high. The iatrogenic risk of ventilator-induced lung injury might contribute to this high mortality because the lungs are hyperinflated. Low tidal volume and inspiratory pressure are surrogates for the stress and strain concept; but lung compliance, transpulmonary pressure, and chest wall elastance might differ in individual patients. In previous published studies, an increasing number of patients were treated successfully with extracorporeal support. Extracorporeal membrane oxygenation and interventional lung assist allow ultraprotective ventilation strategies. However, these assists have different technical aspects and different indications.

Extracorporeal membrane oxygenation for acute respiratory distress syndrome: is the configuration mode an important predictor for the outcome?

Extracorporeal membrane oxygenation (ECMO) is increasingly applied as rescue-therapy for patients with severe acute respiratory distress syndrome (ARDS). Here, we evaluate the effect of different configuration strategies (venovenous vs. venoarterial vs. veno-venoarterial) on the outcome. From 2006 to 2008, 30 patients received ECMO for severe ARDS. Patients were divided into three groups according to the configuration: veno-venous (vv; n = 11), venoarterial (va; n=8) or veno-venoarterial (vva; n = 11). Data were prospectively collected and endpoint was 30-day mortality. To identify independent risk factors, univariate analysis was performed for clinical parameters, such as age, body mass index, gender, configuration, low-pH, oxygenation index (pO(2)/FiO(2)) and underlying disease. Thirty-day mortality was 53% (n = 16) for all comers: 63% (n = 7) died in the vv-group, 75% (n = 6) in the va-group and 27% (n = 3) in the vva-group. Although univariate analysis could not rule out a significant predictor for the outcome, there was a trend visible to decreased mortality in the vva-group when compared to vv- and va-groups (27% vs. 63% vs. 75%; P = 0.057). ECMO provides a survival benefit in patients when considering a predicted mortality rate of 80% in ARDS. The configuration mode appears to impact the outcome as the veno-venoarterial appears to further improve the survival in this subset of patients.

Bridge to thoracic organ transplantation in patients with pulmonary arterial hypertension using a pumpless lung assist device

We describe a novel technique of pumpless extracorporeal life support in four patients with cardiogenic shock due to end-stage pulmonary hypertension (PH) including patients with veno-occlusive disease (PVOD) using a pumpless lung assist device (LAD). The device was connected via the pulmonary arterial main trunk and the left atrium, thereby creating a septostomy-like shunt with the unique addition of gas exchange abilities in parallel to the lung. Using this approach, all four patients were successfully bridged to bilateral lung transplantation and combined heart-lung transplantation, respectively. Although all patients presented in cardiogenic shock, hemodynamic unloading of the right ventricle using the low-resistance LAD stabilized the hemodynamic situation immediately so that no pump support was subsequently required.