113: Artificial Lung

Introduction

Extracorporeal membrane oxygenation (ECMO) has continued to evolve since the pioneers of cardiac surgery, Gibbon and Lillehei, developed cardiopulmonary bypass in the 1950s. The term ECMO applies to the use of an extracorporeal circuit, consisting of tubing, oxygenator and blood pump, in the setting of cardiopulmonary failure. The original ECMO was veno-arterial (VA) as popularized by Bartlett in the early 1980s. Over the last three decades ECMO has evolved into several forms including VA, veno-venous (VV), arterio-venous (AV), right atrium to aorta (RA–Ao), and pulmonary artery to left atrium (PA–LA). ECMO in some form may be indicated for acute cardiac failure, respiratory failure, or a mixed presentation; the specific application of the therapy will depend on the presentation of the patient. Likewise, several programs have developed ambulatory capability of most forms of ECMO to aid recovery or suitability for transplant. Ambulatory ECMO is often referred to as the “artificial lung.”

General Principles

Historically, ECMO in the adult was considered “salvage” therapy for patients “dying” from severe respiratory failure despite maximal medical therapy. Some of the oft-quoted trials in the 1980s compared ECMO with conventional medical therapy and failed to show survival advantage. However, there have been significant advances in ECMO, and evidence now favors earlier utilization of the technology to minimize end-organ damage that might occur during prolonged maximal ventilatory and medical support. This is particularly apparent in the case of respiratory failure associated with H1N1 influenza. Survival is approximately 72% for patients placed on ECMO within 6 days of intubation compared with only 30% for patients on ECMO 7 or more days after intubation.¹

Patient Selection

ECMO is indicated for the short-to-medium-term management of respiratory failure, cardiac failure, or both. Specifically, patients are evaluated when the native disease process is thought to have an estimated mortality of 50% or greater, is reversible, and/or requires a bridge to transplant. The Extracorporeal Life Support Organization (ELSO) publishes guidelines which cover application of ECMO in the adult. In the case of acute respiratory distress syndrome (ARDS), this would include a PaO₂/FiO₂ ratio of less than 100 despite optimization of ventilator settings.

Special scenarios include H1N1 infection, where early deployment of ECMO is associated with better outcomes, and delay may adversely affect the risk/benefit ratio. In addition, use of ECMO as a bridge-to-transplant for lung recipients should be considered when maximal medical therapy is insufficient. Many programs utilize ambulatory ECMO to improve the respiratory status and conditioning prior to transplant.

ECMO as a bridge to lung transplantation has significantly increased during the last 10 years. A recent analysis of the UNOS database showed that the use of ECMO at the time of lung transplantation has grown 150% in the last 24 months compared to all previous decades (1970–2010). This increase in utilization is reflected in the growing success reported with the ...