Progression of EELV in individual patients over three stepwise reductions in PEEP. Mean EELV values at each PEEP level are presented as black dots. Patients are divided according to the type of lung condition. Patients in group N had normal lungs, those in group P had a primary lung disorder, and those in group S had a secondary lung disorder. EELV, end-expiratory lung volume; PBW, predicted body weight; PEEP, positive end-expiratory pressure.