Mechanical cardiopulmonary support goes by many names under the general heading of extracorporeal life support. When the heart/lung machine is used in the operating room in venoarterial mode to provide total support of heart and lung function to facilitate cardiac operations, the technique is commonly called cardiopulmonary bypass (CPB). When used with extrathoracic cannulation for respiratory support the technique has been called extracorporeal membrane oxygenation (ECMO), extracorporeal lung assist (ECLA), and extracorporeal CO2 removal (ECCOR). When used with extrathoracic cannulation for emergency cardiac support the technique has been called cardiopulmonary support (CPS) or extracorporeal cardiopulmonary resuscitation (ECPR). Blood pumps alone can be used as left ventricular assist devices (LVAD), right ventricular assist devices (RVAD), or both (BiVAD. The abbreviations ECLS and ECMO are used synonymously to mean prolonged extracorporeal circulation with mechanical devices. Generally, all of these device applications include vascular access catheters, connecting tubing, servoregulating blood pump, an artificial lung (usually incorrectly called an oxygenator), a heat exchanger, and various measuring and monitoring devices. Some type of anticoagulation (usually heparin) is required, therefore bleeding is always a potential problem. ECLS can be used for mechanical assistance during cardiac or pulmonary failure occurring in newborn infants, older children, or adults. Depending on the application ECLS can be used in a venoarterial mode, venovenous mode, or rarely arterial venous mode.
The Extracorporeal Life Support Organization (ELSO) was founded in 1989 as a study group comprised of all the clinical centers where ECLS is used. The most important activity of ELSO is to maintain a large central data base including a registry of all ECLS cases (over 10,000), devices and complications, follow-up status, and active centers. ELSO coordinates prospective studies, publishes guidelines for ECLS referral and practice, facilitates teaching, standardization and communication, and serves as the professional voice for ECLS technology.
Figure 1: Typical ECLS Circuit with venovenous cannulation. Blood drains from the right atrium to the pump, and returns to the patient. The important monitors are shown in boxes.
A diagram of the circuit commonly used for ECLS is shown in Figure 1. The right atrium is cannulated through a large vein, usually the right internal jugular vein. Venous blood drains out of the right atrium, usually aspirated by a simple siphon. Blood passes directly to a self-regulating pump; unlike CPB for cardiac surgery there is no venous reservoir. Blood is pumped through a membrane lung where oxygen, CO2, and water vapor are transferred. This device is commonly referred to as the "oxygenator", although removal of CO2 is even more important than oxygenation for purposes of pulmonary support. The "arterialized" blood returns to the patient. If the application is purely for respiratory support, the arterialized blood is usually returned to the venous circulation, placing the membrane lung in series with the patient's native lungs. This is the application shown in Fig. 1. If the application requires cardiac support blood is returned to the arterial circulation through a catheter in a large artery, usually the right common carotid. In this application the pump and membrane lung are placed in parallel with the native heart and lungs.
Once the ECLS circuit is attached and functioning, blood flow through the circuit is regulated to provide part or all of the circulation and gas exchange. Total cardiopulmonary support is possible by extracorporeal circulation using only part of the venous blood arriving in the right atrium from the systemic circulation. Therefore ECLS is almost always "partial" bypass as opposed to "total" bypass which is required for cardiac operations. With circulation and gas exchange supported mechanically, the native heart and lungs are not needed for life support and are allowed to "rest". This means that ventilator settings and inotropic drugs are decreased to safe, low levels. Blood flow through the circuit continues until heart or lung function improves. The amount of blood flow is based on the degree of support required, which in turn is based on a series of physiologic monitors in the circuit and on the patient. Many days may be required for the native heart or lungs to regain adequate function. Continuous anticoagulation is required. This is done with continuous heparin infusion monitored by whole blood activated clotting time (ACT). Platelet dysfunction and thrombocytopenia occur during ECLS due to complex blood/surface interactions. A variety of mechanical and physiologic complications can occur requiring emergent management.
The details of ECLS management, the indications, and the results are quite different for cardiac and respiratory support, and also quite different for neonates, pediatric and adult applications. Each of these applications is discussed in subsequent chapters. In general, ECLS is indicated in acute severe reversible cardiac or respiratory failure when the risk of dying from the primary disease despite optimal conventional treatment is high (50 to 100%). Since ECLS is used only in patients who are most likely to die otherwise, the results are usually described in terms of simple survival. Long-term quality of life studies are ongoing. The current survival rate for neonatal respiratory failure is 80%; for pediatric respiratory failure 60%; adult respiratory failure 50%; pediatric cardiac failure 45%; and adult cardiac failure 40%.
The concept of extracorporeal life support is simple but the procedure itself is complex. ECLS is a cousin, but not a twin to operating room cardiopulmonary bypass. ECLS specialists are not operating room perfusionists, and perfusionists are not competent to manage ECLS without specialized training. The institution of ECLS in any hospital requires commitment at all levels of administration and practice, specialized equipment and supplies, carefully defined clinical protocols, and training in the animal laboratory. Involvement of a hospital with ECLS is expensive, but cost effective.
Further information regarding the various aspects of ECLS technique discussed above can be found in "ECMO: Extracorporeal Cardiopulmonary Support in Critical Care" by Joseph B. Zwischenberger, MD and Robert H. Bartlett, MD.