Breathing circuits incorporating non-rebreathing valves carry inspiratory gas from ventilator to patient, and direct expired air into the atmosphere. Valves should maintain PEEP but provide low expiratory resistance to prevent gas trapping. Many ventilators use bespoke breathing circuits and valves which, if replaced by poor quality disposable circuits, can cause decreased tidal volume delivery and unstable or excessive PEEP. Poorly maintained, misassembled, or soiled patient valves are a recurrent cause of inadequate ventilation.