Extracorporeal gas exchange

During extracorporeal membrane oxygenation (ECMO) venous blood is removed via a cannula in the inferior vena cava or right atrium, passed through a heart/lung machine, and is returned to either the right atrium (veno-venous bypass) or aorta (veno-arterial bypass). In veno-venous bypass, pulmonary and systemic haemodynamics are maintained by the patient's own cardiovascular function. Veno-arterial bypass allows systemic haemodynamic support as well as gas exchange. Institution of ECMO allows ventilator pressures and volumes to be decreased to prevent further ventilator induced lung injury. In addition, the reduction in intrathoracic pressure allows fluid removal to be carried out with less risk of haemodynamic instability. A pumpless form of extracorporeal gas exchange using arteriovenous cannulation has recently been described.47

ECMO has proven mortality benefit in neonatal ARDS. In adults a single prospective randomised study failed to show a survival advantage over conventional support.48 However, overall survival in both groups was extremely low and the results are not applicable to current practice. Extracorporeal CO2 removal (ECCOR) involves use of an extracorporeal veno-venous circuit with lower blood flows and oxygenation still occurring via the patient's lungs. A randomised prospective study of ECCOR compared with conventional support in patients with severe ARDS reported no significant difference in survival.49 Several centres have recently reported observational studies showing high survival rates in adult patients managed with extracorporeal support (table 9.4). These encouraging survival rates should be interpreted, however, in the context of improved survival without ECMO.50 51 A

Table 9.4 Recent observational studies reporting survival of patients with ARDS managed with extracorporeal membrane oxygenation (ECMO)

Reference

n

Survival (%)

Lewandowski (1997)55

122

75*

Ullrich (1999)56

13

62*

Bartlett (2000)57

86

61

Linden (2000)S8

16

76

*Patients managed with a protocol that included ECMO if necessary.

*Patients managed with a protocol that included ECMO if necessary.

randomised prospective controlled study of ECMO in adult patients is currently underway in Leicester, UK (CESAR trial).

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