Patients admitted to the PACU after surgery may require mechanical ventilation. Knowledge about the anatomy and physiology of the diaphragm and its association with ventilator modes may be helpful in the management of this patient. As the acuity of PACU patients increase, more patients may also be on higher levels of positive end-expiratory pressure (PEEP), requiring PACU nurses to understand the relationship between PEEP and diaphragm function to facilitate weaning. This article provides a review of the mechanical ventilation mode of PEEP and its relationship to diaphragmatic performance. The physiological effects associated with the use of PEEP are also reviewed.