By Leon Gussow, MD
Extracorporeal membrane oxygenation had attendees at the recent North American Congress of Clinical Toxicology meeting buzzing about its use for treating poisoned patients. I knew little about ECMO so I contacted Scott Weingart, MD, the chief of emergency critical care at Stony Brook University Medical Center in New York, who said ECMO could be extremely valuable in treating certain severely ill poisoned patients because they are frequently young, otherwise healthy, and have “a heart too good to die.” My conversation with Dr. Weingart yielded these key points that may prompt you to think about ECMO.
Extracorporeal membrane oxygenation is an intervention that can support failing cardiac and respiratory function from a reversible cause until a specific end-point can be reached. It is often considered a bridge to recovery, buying time until, for example, a deteriorating patient with a myocardial infarction can be brought to the cath lab or a patient with a massive pulmonary embolus can undergo thrombectomy. ECMO may also maintain tissue perfusion and oxygenation in overdoses until the drug can be eliminated by the body’s natural metabolic and excretory processes, possibly enhanced by use of renal replacement therapy.
Hypoxic blood is removed from a large vessel such as the femoral vein, oxygenated mechanically, and returned to the venous circulation in veno-venous ECMO. This technique can maintain oxygen delivery to tissues while avoiding dangerously high ventilator pressures or high FiO2 in patients with severe acute respiratory distress syndrome. VV-ECMO also allows for pulsatile flow through the pulmonary vessels, letting the lungs continue their natural role as a filter for small emboli. It does not, however, provide any circulatory support.
On the other hand, hypoxic blood is removed from near the right atrium, oxygenated, and pumped back into the aorta during veno-arterial ECMO (VA-ECMO). This supports respiratory and circulatory functions.
Use of ECMO goes back almost half a century. Initially, it could be started only in the operating room because access to the great vessels near the heart required thoracotomy. Today, access can be secured via large peripheral vessels such as the femoral vein and artery. This technique can be facilitated using ultrasound guidance, and it builds on a skill set common to emergency physicians.
Early on, the machinery required was large, cumbersome, and difficult to manage. The original pumps operated on an occlusive roller mechanism that frequently broke down, and, more importantly, caused platelet dysfunction. Newer pumps use a magnetically suspended centrifugal design that is nonocclusive and maintains blood flow by producing pressure differentials. As technology improved, ECMO units have become easier to set up and are more portable and dependable than before.
During the A/H1N1 flu pandemic in 2009, VV-ECMO was used as a bridge to recovery for many patients with severe ARDS. The surprisingly low mortality rate in these extremely sick patients created renewed interest in all aspects of the modality.
ECMO may be indicated in patients with a reversible cause of pulmonary or cardiac failure that has not responded to conventional therapy. It is now generally considered a last-gasp intervention for patients who seem destined to die without it, but continued research and experience may find that results improve in certain situations when ECMO is initiated at an earlier stage.
VV-ECMO has been used to treat children with lung injury from inhalation of hydrocarbon products. VA-ECMO has potential use in patients poisoned with cardiotoxic agents such as calcium channel blockers, beta blockers, venlafaxine, or bupropion who are suffering from hypotension and shock that has not responded to standard therapy with fluids and inotropes. Supporting failing cardiac function in these patients can not only buy time while the body’s natural processes eliminate the toxin and promote systemic distribution of specific antidotes.
It is important to remember that drugs such as calcium channel blockers can cause hypotension from cardiogenic shock or vasodilation. Because VA-ECMO is not indicated in the latter, it is prudent to obtain an echocardiogram before initiating the procedure to confirm myocardial dysfunction.
Uncontrolled coagulopathy and severe intracranial bleeding are considered absolute contraindications to ECMO. The presence of a significant irreversible process such as hypoxic brain injury or terminal metastatic cancer may also make heroic measures futile.
Bleeding at access sites occurs in up to a third of patients, intracranial hemorrhage in about six percent. The relatively large size of the catheter required for arterial access can result in ischemia of the lower extremity. Because oxygenated blood is returned directly to the aorta without being filtered through the lungs in VA-ECMO, patients are at risk for systemic emboli.
An excellent article by de Lange, et al. comprehensively reviewed the literature to date about use of ECMO in poisoned patients. (Clin Toxicol 2013;51:385.) It includes 81 references and a list of published reports of patients treated with ECMO for toxic exposure.
An excellent resource for all things ECMO in emergency critical care is the website of the ED-ECMO project (www.edecmo.org) by Joe Bellezzo, MD, and Zack Shinar, MD, from San Diego, and Dr. Weingart. The site includes a discussion of basic concepts, protocols, video tutorials, and an up-to-date list of relevant medical literature. It also has a blog and podcast. Anyone interested in the potential of ECMO for treating poisoned patients will want to follow this site.
Physicians still have relatively little experience treating toxicology patients with ECMO. The National Poison Data System reported that 14 patients in their database were treated with ECMO in 2010. That had increased to 29 by 2013. This is still just a handful, but that will continue to grow as the modality becomes more commonly used in emergency and critical care.