ECMO (Extracorporeal Membrane Oxygenation)

ECMO was first introduced in early 1970, mostly were for prolonged CPB in adults. Later on, were used for respiratory failure in adult with <10% survival rate. With the initial failure in adults, ECMO now being used limitedly in reversible neonatal respiratory failure. Most common causes including meconium aspiration syndrome, congenital diaphragmatic hernia, persistent pulmonary hypertension of the newborn.

Technique

Venovenous bypass will provide gas exchange support, but for circulatory support, the venoarterial cannulation is chosen. When used in neonate, the peripheral cannulation may not provide adequate blood flow, due to small size of jugular or femoral vessles, but successful ECMO using neck cannulation has been reported. The peripheral cannulation also limit only right side drainage, study in dogs with MI has shown the ECMO is more effective with left side drainage. Chest cannulation can be performed through a median sternotomy with venous return from Rt. Atrium (or Lt. Atrium if left side drainage needed), the arterial in flow go through the ascending aortic cannula.

Picture from reference (1) * see below.

The circuit functions similarly to CPB circuit. Continuous heparinization is needed to keep ACT 180-220. The venous blood drained by a gravity to the distensible reservoir (similar to the reservoir on a CPB). The blood then passes through a pump (roller or centrifugal), enters a membrane oxygenator with O2 sat of 95%. Blood then will passes through the countercurrent heat exchanger, the bride (between arterial inflow tubing and venous outflow tubing) will provide the recirculation of perfusate (which is principally used during cannulation and weaning). The circuit volume is about 350-400 ml in young infants.

Once the ECMO is started, the Hct should be maintained between 40-45%, minimum plts count of 50-70K and fibrinogen level of 150-200 mg/dL. The membrane oxygenator must be large enough to provide a flow that is greater than patient's CO.

Complications

Bleeding (48%), renal insufficiency (29%), neurologic injury (17%), arrhythmia (17%), infection (10%0, circuit problems (10%), mediastial bleeding and mediastinitis is a particular concern for chest cannulations.

Overall survival rate is about 59%. The patient group who can be weaned from CPB has greater survival (74%) compare to the patients who need ECMO initiated in the OR (14%).

References

1.) Frank H. Kern et al : Extracorporeal Circulation and Circulatory Assist Devices in the Pediatric Patient in Carol L. Lake : Pediatric Cardiac Anesthesia , Appleton & Lange, 1998, pp 245-248

2.) Brent A. Graham : ECMO in Michael F. Roizen : Essence of Anesthesia Practice, W.B. Saunders, 1997 pp 380