Extracorporeal Membrane Oxygenation (ECMO)

Background

The system consists of a circuit - often heparin-coated- composed of 1) a membrane oxygenator, 2) centrifugal pump, and if patient needs temperature modification, 3) a heat exchanger. The centrifugal pump has a non-occlusive design that promotes laminar flow, improving blood handling capabilities and decreasing blood trauma often associated with extracorporeal circulatory support experienced with cardiopulmonary bypass. Coated circuits provide a more biocompatible surface that minimises platelet activation and systemic inflammatory response. It also seeks to reduce or eliminate the heparin requirement allowing the ECMO circuit to be used for several days. 

1 - Types of ECMO

The two types are different in terms of effects on the circulation; these are detailed in table 1.
There is 1) veno-arterial (VA) ECMO - supports the heart and lungs: requires two cannulae-1) one in right atrium via peripheral vein and 2) one in an artery and the 2) Veno-venous (VV) ECMO – supports the lungs only: requires one cannula in the jugular vein or a second cannula through the femoral vien.

Table 1. Types of ECMO in regards to their effects on circulation.

2 - Indications

ECMO support is useful when 1) the primary lung injury is accompanied by oxygen toxicity or barotrauma caused by mechanical ventilatory support or 2) the primary underlying cardiac insult is potentially reversible.
The following conditions are possible indications.   

• Post-cardiotomy: when unable to get patient off cardiopulmonary bypass following cardiac surgery
• Post-heart transplant: usually due to primary graft failure
• Severe cardiac failure due to almost any other cause: decompensated cardiomyopathy, myocarditis (acute coronary syndrome with cardiogenic shock, profound cardiac depression due to drug overdose or sepsis).

3 - Implantation technique

Femoral arterial cannulation can be performed 1) via percutaneous Seldinger’s technique or 2) an open surgical approach.
The percutaneous approach is adequate 1) when digital palpation of the femoral pulse can be obtained. It is performed under ultrasound/Doppler guidance. 
Percutaneous femoral cannulation carries the advantage of allowing an expeditious procedure in an emergency situation. After heparinisation, puncture of the common femoral artery (CFA) is done, then a soft tipped guide-wire is passed through the needle. Dilatators are passed stepwise over the guide-wire to enlarge the access until the cannula size is achieved. The cannula is inserted and connected to the ECMO system (3-5).
The open approach for arterial cannulation allows visualisation of the artery to ensure adequate size of cannula, proper placement, good hemostasis and opportunity to choose an alternative cannulation site in case of arterial calcifications. 

4 - Arterial vascular complications

Early arterial vascular complications in peripheral ECMO support remain important with rates between 3.2% and 28% occurring in case of femoral cannulation site. The most significant vascular complication reported is limb ischemia. Causes of limb ischemia after ECMO are listed in table 2.
Table 2 : Etiology of ischemia after femoral artery.

5 - Management

ECMO requires a multidisciplinary team approach involving surgeons, anesthetists and intensive care physicians and nursing staff to achieve the best clinical outcome. There is a considerable risk of bleeding with ECMO.  This is attributed to a low patient platelet count which may occur due to 1) Platelets adhering to surface fibrinogen and are activated 2) Resultant platelet aggregation and clumping causes numbers to drop.
The degree of bleeding is out of proportion to the severity of thrombocytopenia and further coagulopathy is related to the continuous activation of contact and fibrinolytic systems by the circuit and the consumption and dilution of factors within minutes of initiation of ECMO. 
Consequently, particular attention should be made to monitoring and managing hemostasis. This includes:
regular blood tests (Q6-Q8h), coagulation profile (Activating clotting time (ACT), heparin levels, activated factor X assessment), platelet count (above 50,000), hemoglobin (around 10 g/dl) and aggressive replacement of clotting factors, electrolytes, PRBC (packed red blood cells).

Conclusion

Extracorporeal membrane oxygenation provides short-term cardiopulmonary support <3 weeks and a bridge to decision regarding the next step which may be recovery, transplantation long-term device (ventricular assist device), or operation (CABG, pulmonary embolectomy...).