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Extracorporeal membrane oxygenation: a breath of fresh air or yesterday's treatment?

Extracorporeal oxygenation during cardiopulmonary bypass has become a routine procedure during the last 30 years, though a "pump oxygenator" for patients with respiratory failure was first developed as long ago as 1885.1 Extracorporeal membrane oxygenation (ECMO) has been shown to be beneficial in selected cases of neonatal respiratory failure and the ECMO Central Registry in the United States has now documented over 800 neonates treated in this way with an overall survival rate of 79%.2 3 How effective is ECMO for acute respiratory failure in adults and is it likely to become a useful and widely applied therapeutic technique? The deficiencies in conventional mechanical ventilatory support provided the initial spur for trials of ECMO in patients with the adult respiratory distress syndrome, particularly as evidence accumulated that mechanical ventilation might itself contribute to the pathophysiology of lung injury and influence the course of the disease adversely.4 The role of high peak inspiratory pressure in the development of barotrauma is generally accepted, but experimental work has also shown that peak inspiratory pressures as low as 30 cm H,O may be associated with pulmonary damage.5 In addition, the high volume ventilation required for patients with the adult respiratory distress syndrome, regardless of whether the peak inspiratory pressure is high or low, may lead to increased lung microvascular permeability,6 or to exacerbations of pre-existing lung injury.7 The theoretical attractions of extracorporeal oxygenation are therefore considerable. Several studies have been carried out in patients with the adult respiratory distress syndrome in recent years using various approaches; but the success rate has varied considerably, suggesting that the precise indications for the technique have yet to be determined. As parallel advances are made with other techniques of respiratory support, the use of ECMO, with its high cost and complication rate, may never become widespread. Extracorporeal gas exchange (ECGE) has emerged as the blanket term describing all techniques of extracorporeal respiratory support, but the specific approach for a particular patient depends on the desired therapeutic goal. In true ECMO oxygenation is the prime objective and the transfer of carbon dioxide occurs as a secondary effect. Blood flow rates exceeding 50% of cardiac output are required. The objective in extracorporeal carbon dioxide removal (ECCO2R) is to reduce the need for alveolar ventilation, thereby diminishing the potential for further pulmonary injury due to barotrauma. Partial carbon dioxide removal (PECCO2R) can be achieved with blood flow at as little as 0-8-1 1/min and only one gas exchanging membrane, whereas total removal of carbon dioxide (TECCO2R) usually requires higher flow rates (1-2 1/min) and the use of two membranes in series. As this technique provides only 20-30% of total body oxygen requirements, some ventilatory support is still required. This can be provided in one ofthree ways: spontaneous ventilation with or without the aid of continuous positive airways pressure (the term extracorporeal lung assist has been used recently89 to describe this technique); controlled conventional positive pressure ventilation at low frequency (for example, 4-6 breaths a minute) with positive end expiratory pressure; and "apnoeic" intratracheal oxygen insufflation with positive end expiratory pressure.