An Introduction to Extracorporeal Membrane Oxygenation (ECMO)
Introduction
Extracorporeal Membrane Oxygenation (ECMO) and Extracorporeal Life Support (ECLS), a broader term used synonymously, both include various modalities of temporary mechanical cardiopulmonary assistance used to support patients with severe heart and/or lung failure which is unresponsive to optimal conventional care. The main objective is to provide systemic perfusion and gas exchange allowing the heart and/or lungs to rest and recover or to bridge a patient to a different modality of support or to transplantation. This technology involves redirecting the blood flow from the patient’s body through cannulas and connecting tubing to a gas exchange membrane and then returning the blood by means of a pump back to the patient’s circulation.
ECMO intervention has evolved from its pioneering days of utilization mostly from the pediatric patient population. Advances in technology and the pioneers who made tireless efforts in this arena provided successful and improved ECMO development . Dr. John Gibbon’s development of a roller pump in the 1930’s led to the first successful extracorporeal assist ASD repair in 1953. In 1957 Kammermeyer revolutionized the artificial lung with the development of synthesis of silicone rubber. Bartlett and Drinker developed an approach to continuously titrate coagulation with Heparin dosing using the ACT which is still in wide use today. These developments in extracorporeal support offered during a procedure in children with congenital heart disease encouraged physicians to expand ECMO utilization outside the operating room theater. In 1972 Bartlett, Gazzaniga et al. reported the first successful cardiac ECMO run of 36 hours in a 2 year old infant with cardiac failure following a Mustard Procedure for correction of transposition of the great vessels. The improvements in technology are testimony to the collaboration of physicians, biomedical engineers, physiologists and a plethora of health care specialists. The continued evolution of this technology has been improving with the ability to make simpler, smaller and safer extracorporeal circuits.
Rationale / Techniques
ECMO is used when heart or lung failure is so severe that DO2 ( Oxygen delivery ) : VO2 ( O2 tissue consumption = 120cc/min/m2 ) ratio DO2:VO2 is less than 2:1 or when interventions required to maintain DO2 twice the VO2 are damaging as seen with extremely high airway pressures or extreme high vasoactive drug dosing. ECMO maintains normal DO2:VO2 by draining most of the venous blood from the patient, pumping it through a membrane oxygenator and into the patient’s circulation. A significant volume of blood bypasses the heart and/or lungs to the circuit’s pump and oxygenator and replaces the function of either the compromised heart and or lungs.
There are several modes of ECLS available. The exact configuration of the extracorporeal circuit and it’s corresponding cannulation strategy depends on the organ which needs to be supported. Respiratory or Veno-Venous ECMO (VV-ECMO), Extracorporeal CO2 removal (ECCO2R ), and Veno-Arterial ECMO (V-A ECMO, CVA-ECMO) are the three main modalities of conventional ECMO. In addition there are extracorporeal circuits available for bridging to short term BiVAD or long term LVAD support in patients with advanced heart/lung failure. Emergent extracorporeal cardiopulmonary resuscitation (ECPR) can involve percutaneous cannulation or elective surgical procedure via sternotomy or re-sternotomy (central cannulation ECMO ).
Patient selection is ultimately the key criteria upon for which a successful ECMO run is based. ECMO is primarily indicated in patients with such severe oxygenation and/or ventilation difficulties whose prognosis is poor despite conventional mechanical ventilation. ECMO is only useful in cases where the primary lung insult may be reversible. Historically, adult ECMO was considered futile however with newer technology and a better understanding of the lung’s apparent ability to heal with time, the adult ARDS patient reversible regional ischemia and even sepsis is now often successfully weaned from ECMO therapy.
The evolution of ECMO’s application needs to be inclusive of all body systems for the potential to save a patient’s life. The most successful ECMO candidates will be treated by a multi-disiplinary team of professionals who can tailor the extracorporeal oxygenation circuit to each patient’s specific needs. This includes appropriate circuit and cannulation sizes and techniques depending on the patient’s size and primary diagnosis.
Complications
Bleeding and micro clotting continue to be the most often reported complications associated with ECMO procedures. The blood should be anticoagulated utilizing heparin that is often measured by the ACT, anti Xa, and ApTT, or more recent and possibly better-controlled medications such as the direct thrombin inhibitors (DTI) Bivalrudin (Angiomax) or Argatroban. DTI’s can be routinely utilized or is often prescribed in patients with known Heparin InducedThrombocytopenia (HIT). Activity and dosages should be monitored by the ApTT. Occasionally, anticoagulants are temporarily discontinued in biocompatible-coated circuits for patients with an existing or potential high expectation of hemorrhaging due to recent surgical intervention or physiological complications.
Other ECMO complications may include vascular disruptions, infection, air or clot emboli, acute neurologic injury, and acute kidney dysfunction to name a few. Mechanical complications include dysfunction of circuit components and accidental blood exsanguination. Because of these potential complications, a highly trained ECMO specialist should always be available in addition to the patient’s primary nursing care staff. Where once contraindicated, sepsis as well as kidney failure are now successfully treated while the patient is on ECMO. Managing an ECMO patient necessitates a thorough understanding of cardiopulmonary physiology, pathophysiology, hemostasis management and ECMO circuit physiology.
Patient Selection
As mentioned previously, usual ECMO criteria include patients with a severe reversible process that would result in a very high predicted mortality with conventional ventilatory support. The best candidates are those without multi-system organ failure that are otherwise considered to be salvageable. It would be expected, that once ECMO becomes safer and more accepted by physicians, that these criteria would be revised to eliminate the requirement for a very high predicted pre-treatment mortality. There are several ways to evaluate patients to determine if they are ‘sick enough’ for ECMO. APACHE and other scoring systems could be used to predict mortality. More frequently, indexes to assess refractory hypoxemia are used. The a:A ratio or A-a gradient can be used, but the oxygenation index (OI) is currently in more common use: OI = (FIO2) (MAP) / PaO2 An OI greater than 0.40-0.55 is thought to predict an 80% predicted mortality. Unfortunately, mortality predictors have been notoriously pessimistic since they are derived by looking at retrospective data and tend to do a poor job of predicting prospective results.
Evolving Trends
The management of an ECMO patient as described requires 24 hour daily monitoring by a trained professional. Collaborative rounding is essential by ECMO trained Physicians, Perfusion/ECMO specialist teams, Respiratory Therapy, Pharmacists, Blood Bank staff, Social workers, Nutritionists, Infectious Disease practitioners, or any other patient associated specialists. One ECMO patient can tap the resources of an intensive care unit very quickly especially when these cases happen infrequently and the institution is not accustom to treating these patients. An ECMO run can last a few days to several months depending on many factors. Institutions have a difficult time deciphering what level of care and length of time they can provide adequate maintenance of an ECMO patient without jeopardizing other staffing needs.
An evolving trend is the Centralization and Regionalization of ECLS resources across large health systems. Similar to trauma and stroke care, ECMO requires comprehensive, multidisciplinary teams of providers able to offer a continuum of advanced interventions and procedures. High volume ECMO centers generally have more experience and success in treating multiple ECMO patients simultaneously as they may have abundant resources and ECMO specialist teams. Many community hospitals are now setting up individual ECMO programs with success. Proper training of ALL staff is mandatory and shared information between centers is suggested in order not to repeat similar negative outcomes.
Another trend that ECMO has experienced in recent years is extubation while on pump. Frequently a tracheostomy is performed to the removal of endotracheal tube. In addition, by utilizing dual-lumen VV catheters placed in the jugular position, respiratory patients are encouraged to strengthen muscle development and often become ambulatory while on ECMO. These patients have improved multi-organ failure and their prognosis is encouraging with reversible ischemia for recovery or eventual transplantation.
Conclusions
The Extracorporeal Life Support Organization (ELSO) is a non-profit organization established in 1989 supporting health care professionals and scientists involved in ECMO. ELSO maintains a registry where procedural data is collected from individual facilities. This information can be requested and extracted to assist in benchmarking, quality improvement, and research. Global chapters of ELSO have developed and report to the same registry, and the ELSO organization has created a very friendly core of health care professionals willing to share ideas and help one another with problematic issues.
At present, the ELSO Registry shows the rapid growth from 80 centers in 1990 to over 467 active centers in 2019. The registry database has captured detailed information from over 112,000 patients and provided data for many publications, while also offering assistance to manufacturers in the development and approval of products.
ECMO is a complex service, which is labor intensive and requires collaboration across multiple specialties and services. ECMO shares many of the same characteristics as that of a chess match. The two players involved are the patient and the entire health care team of professionals all working collaboratively together to decipher and plan the best strategic course of action to improve the patient’s condition. Both require excellent critical thinking skills (creative and strategic), problem solving ability, abstract reasoning, calmness under pressure and patience. When every member of each team freely shares their knowledge and expertise then a continuity of care exists to produce the best possible outcomes for these patients.
