A patient ventilator for assisting a clinician in determining a suitable PEEP for the patient. The amount of the lung volume recruited/de-recruited at various levels of PEEP may be determined for use in selecting a desired PEEP. To this end, the functional residual capacity of the lungs is determined for a first PEEP level. The PEEP is then altered to a second level and a spirometry dynostatic curve of lung volume and pressure data is obtained. The lung volume on the dynostatic curve at a lung pressure corresponding to the first PEEP value is obtained. The difference between the functional residual capacity of the lungs at the first PEEP level and that determined from the dynostatic curve represents the lung volume recruited/de-recruited when changing between said first and second PEEPs.
