Complex sentence: If the patients PEEPi is caused by dynamic hyperinflation, external PEEP will reduce the patient's work of breathing and if it is caused by expiratory muscles it will add an elastic load and it will increase the operating lung volume.
Split into simpler sentences: If the patients PEEPi is caused by dynamic hyperinflation, external PEEP will reduce the patient's work of breathing. If it is caused by expiratory muscles, it will add an elastic load and it will increase the operating lung volume.