Endobronchial valves are inserted to reduce the pressure effects of bullae on adjacent more normal lung (in a similar way that surgical excision of large bullae has been undertaken in the past), in order to improve overall lung ventilation in patients with severe emphysema. They are "one-way" valves, i.e. air in the bronchus is prevented from entering the bullae but can escape causing them to deflate. This is effectively a bronchoscopic technique of lung volume reduction.