Continuous Positive Airway Pressure (CPAP) delivers pressurized air to keep the throat open, preventing airway collapses caused by Obstructive Sleep Apnea (OSA). While highly effective, the constant air pressure can make it difficult or uncomfortable for users to exhale, leading to poor adherence to therapy. Expiratory Pressure Relief (EPR) is a feature developed by manufacturers to address this comfort issue.
The Mechanism of Expiratory Pressure Relief (EPR)
CPAP machines maintain a single, therapeutic pressure level throughout the entire breathing cycle. Exhaling against this continuous, fixed pressure can feel unnatural and cause some users to feel strained. EPR mitigates this feeling by momentarily reducing the air pressure when the user begins to breathe out.
The device uses algorithms to detect the precise moment the user transitions from inhalation to exhalation. Upon sensing the change in airflow, the machine temporarily drops the delivered pressure by a set amount for the duration of the exhale.
This pressure reduction is measured in centimeters of water (cm H2O) and is adjustable in three levels. For example, if the therapy pressure is 10 cm H2O, an EPR setting of 2 reduces the pressure to 8 cm H2O during exhalation. The machine instantly returns to the full prescribed pressure level at the start of the next inhalation to ensure the airway remains protected.
EPR and Treatment Efficacy
The primary function of CPAP therapy is to maintain a positive airway pressure sufficient to prevent the throat from collapsing, which is measured clinically by the Apnea-Hypopnea Index (AHI). The prescribed pressure is the minimum required to keep the airway fully open during the most vulnerable moments of sleep. Because EPR reduces the pressure below this minimum for a portion of the breathing cycle, it introduces a trade-off between comfort and therapeutic effectiveness.
Reducing the airway pressure during exhalation, even briefly, can create a small window of opportunity for the airway to momentarily narrow or collapse, potentially increasing the user’s AHI. A bench study demonstrated that using the maximum EPR setting (EPR 3) on a fixed-pressure CPAP machine resulted in a mean delivered pressure that was 2.6 cm H2O lower than the set pressure. This drop in pressure can be particularly problematic for users who require high-pressure settings, where even a slight reduction might negate the required therapeutic effect.
The goal of CPAP is to achieve an AHI of less than five events per hour. For those with more severe OSA, the pressure drop might allow a brief obstruction to occur, even if the user finds the relief more comfortable. While EPR can significantly improve comfort and adherence to therapy, it must be used with the understanding that it could potentially compromise the efficacy of the treatment for some individuals.
Practical Guide to EPR Settings and Use
EPR settings are offered in levels 1, 2, and 3, corresponding to a reduction of 1, 2, or 3 cm H2O during exhalation. Level 1 provides the least pressure reduction and is the setting least likely to interfere with the therapeutic pressure. Users can find this setting within the comfort options menu of their CPAP device.
A common recommendation is to start by trying the therapy without EPR to see if the standard pressure is tolerable. If exhaling feels challenging, start with the lowest setting, Level 1, and only increase it if comfort remains an issue.
It is important to monitor your AHI data closely following any change to the EPR setting. If your AHI begins to rise above your established therapeutic level, the pressure relief may be compromising the treatment. Any decision to use or change EPR settings should be discussed with a sleep physician or respiratory therapist. They can review your recent therapy data to ensure that the comfort feature is not inadvertently increasing the number of breathing events you experience each night.