Expiratory Pressure Relief (EPR)
EPR (Expiratory Pressure Relief) is a ResMed brand-specific term for CPAP exhalation pressure relief.
According to the ResMed website: "EPR (expiratory pressure relief) is an effective alternative to continuous positive airway pressure (CPAP) therapy. It features several options that enable you to reliably and predictably control patient therapy while delivering a higher level of patient comfort".  EPR reduces the pressure during exhale from 1 to 3 cm-H2O corresponding to the setting. In the image below the mask pressure (blue) is shown below the Respiratory flow rate graph (black). The changes in pressure show how EPR works with the top of the mask pressure corresponding to IPAP and the bottom of the curve to EPAP.