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There are two approaches to ECLS: VV and VA. With VV ECMO, blood is removed from a major vein, passed through a pump and oxygenator, and back to the patient via a major vein. In VA ECMO, blood is removed from a major vein but, after passing through a pump and oxygenator, is returned via a major artery.
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This approach to ECMO is primarily designed to support the respiratory system. Blood is removed from a large vein, usually a femoral vein with the catheter frequently extending into the inferior vena cava, and returned to another large vein, the contralateral femoral or the superior vena cava via the jugular vein (Figure 38-1). Since the arterial circulation is not affected, normal pulsatile blood flow is maintained. This approach requires a normal functioning heart. Thus, blood flows through the pulmonary circulation. Gas exchange is a combination of the effect of gas exchange via the ECMO system and the respiratory system. The amount of gas exchange occurring in each area is dependent on the amount of blood flow diverted through the ECMO circuit. The greater the ECMO blood flow, the less contribution to gas exchange by the respiratory system.
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VV ECMO has the advantage of not invading the arterial circulation, so the risk of air embolism is minimized. Pulmonary perfusion is also maintained, and pulsatile blood flow is maintained to the kidneys. The major disadvantages to this approach are that cardiac function must be relatively normal. Patients with severely compromised cardiac function are not candidates for VV ECMO. Catheter position is critical. Recirculation can occur, nullifying the effects of ECMO. Careful monitoring of the oxygenation and ventilation is important. The ability of VV ECMO to oxygenate the patient is less than that of VA ECMO for the same blood volume diverted to the ECMO system. The patient must be anticoagulated, and bleeding is a major potential complication.
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Venoarterial (VA) ECMO
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With venoarterial (VA) ECMO, blood is removed from a large vein (femoral or jugular) similar to VV ECMO, but blood is returned to circulation via a large artery, frequently the carotid artery in neonates and children or the femoral artery in adults (Figure 38-2). VA ECMO is designed to support the failing heart. With this approach, essentially 100% of the cardiac output can be diverted through the ECMO system. As a result, pulsatile blood flow is lost. All gas exchange occurs via the ECMO system.
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The advantages of this approach are independence from cardiac function, ability to divert the entire blood volume, maximized oxygenation capability, and a marked decrease in the need for ventilatory support. The major disadvantages are the need to invade the arterial circulation and the risk of air embolism. Pulsatile blood flow is lost, and thus, pulse oximeters do not work. Normal pulmonary and renal pulsatile flow is lost. The patient must be anticoagulated, and thus, bleeding is a major potential complication.