The invention relates to a method and an apparatus to identify alveolar opening and collapse of a lung. To automatically generate the settings of ventilator parameters in a simple and gentle way, the hemoglobin oxygen saturation and/or the endtidal CO.sub.2 concentration and/or the CO.sub.2 output are-measured and processing to detect alveolar opening and closing. From the knowledge of the corresponding airway pressures, a central processing unit may generate the settings of ventilation parameters such that gas exchange is maximal while the mechanical stress of the lung tissue is minimal.