Effect of PEEP on Lung Recruitment and Homogeneity Over Time in Moderate to Severe ARDS

Ventilator-induced lung injury (VILI) is a well-recognised problem of ventilation in patients with ARDS, and is currently treated with lung-protective ventilation, which limits tidal volumes and airway pressures by applying higher levels of positive-end expiratory pressure (PEEP). However, it is not known whether higher levels of PEEP increases recruitment and homogeneity of ventilation within the lungs. APRV is a mode of inverse ventilation, where high levels of PEEP are maintained with brief releases of pressure, and has been proposed as an appropriate method of ventilation in patients with ARDS. This study will assess homogeneity of ventilation and recruitment in 15 patients before APRV is started, and 30, 60 and 120 mins after commencing APRV. It will do this using Electrical Impedance Tomography (EIT), nitrogen wash in/wash out technique, and lung strain.