A control unit (105), software, and breathing apparatus (1) are disclosed that are adapted to determine a change in transpulomonary pressure in a patient and operable to set a first mode of operation for ventilating said patient with a first Positive End Expiratory Pressure (PEEP) level; set a second mode of operation for ventilating said patient with a second PEEP starting from the first PEEP; and determine said change transpulmonary pressure (ΔPtp) based on a difference between the first PEEP and the second PEEP (ΔPEEP).