pRotective vEntilation With Veno-venouS Lung assisT in Respiratory Failure (REST)

Acute hypoxaemic respiratory failure requiring mechanical ventilation is a major cause of morbidity and mortality. A significant proportion of affected patients will have the Acute Respiratory Distress Syndrome (ARDS). Mechanical ventilation is often required to provide adequate gas exchange and although it is life-saving in this setting, it is also now known to contribute to the morbidity and mortality in the condition. Ventilators delivering high pressures and volumes cause regional over distension in the injured lung resulting in further inflammation and non-cardiogenic pulmonary oedema. The release of inflammatory mediators from the damaged lung causes systemic inflammation leading to multi-organ failure and death.

The few interventions that have been shown to reduce the high mortality in these patients have targeted ventilator-induced lung injury (VILI). A landmark trial by the ARDSNet trials group found that ventilating patients with acute hypoxaemic respiratory failure secondary to ARDS with a lung protective strategy aiming for a reduced tidal volume of 6ml/kg predicted body weight (PBW) and a maximum end-inspiratory plateau pressure (Pplat) ≤ 30cmH2O decreased mortality from 40% (in the conventional arm treated with tidal volume less than 12ml/kg PBW) to 31%.

Extracorporeal carbon dioxide removal (ECCO2R) in association with mechanical ventilation offers a potentially attractive solution to permit tidal volume reduction to less than 6ml/kg PBW and to achieve low plateau pressures (< 25cmH2O). Using these extracorporeal circuits, carbon dioxide can be 'dialysed' out of the blood while the lungs are ventilated in a more protective manner. In recent years, more efficient veno-venous devices have become available. These have replaced arterio-venous devices and have the advantage of not requiring arterial puncture. These can achieve carbon dioxide removal with relatively low extracorporeal blood flows (0.4-1 l/min) requiring only a smaller dual lumen venous catheter. In addition these ECCO2R devices use more biocompatible materials making the device more resistant to clot formation and cause less platelet and clotting factor consumption. Therefore only minimal systemic anticoagulation is required which reduces the likelihood of bleeding complications. These devices are now comparable to renal dialysis equipment, which is routinely used safely as standard care in ICUs in the United Kingdom.

Together this highlights the need for a large randomised controlled trial to establish whether VV-ECCO2R in acute hypoxaemic respiratory failure can allow the use of a more protective ventilatory strategy and is associated with improved patient outcomes. Importantly, if there was no benefit, the trial would provide evidence to stop the widespread adoption of an expensive and ineffective or potentially harmful treatment in this setting.