Some OSA patients while using CPAP/BiPAP therapy complain of difficulty in exhaling against the pressure which can affect compliance to therapy. One of the comfort features in PAP machines that can be used to make exhalation comfortable is pressure drop during exhalation. It means PAP machine will deliver a set pressure during inhalation but drop the pressure during exhalation and increase pressure again during inhalation.
Different manufacturers have given different names to this function of pressure drop during exhalation.
| Manufacturer | Technical name for pressure drop during Expiration | Settings | What does settings mean? |
| Philips Respironics | Flex Flex function in fixed CPAP is called C-Flex Flex function in APAP is called A-Flex Flex function in BiPAP is called Bi-Flex | 3 settings- 1, 2 & 3 | Higher setting of flex leads to higher pressure drop proportional to patients airflow Philips do not clearly specify how much pressure drop will happen at each flex setting |
| Resmed | EPR- Expiratory pressure relief | 4 settings- 0, 1, 2 & 3 | EPR setting of 1, 2 & 3 will lead to 1cmH2O, 2cmH2O AND 3cmH2O pressure drop during exhalation respectively EPR setting of zero means EPR is off |
| Loewenstein | Soft PAP | 3 settings- off, 1 & 2 | Higher Soft PAP setting means higher pressure drop.  I could not find how much pressure drop happens at each setting on manufacturers website |
| BMC | Reslex | 4 settings- 0, 1, 2 & 3 | The RESlex setting can be adjusted to levels 0–3, with 3 being the greatest pressure relief. The default setting is 0, or “off |
Keep in mind that the use of pressure relief during expiration function during CPAP treatment reduces peak inspiratory flow and narrows the airway and requires higher therapeutic CPAP (https://journal.chestnet.org/article/S0012-3692(24)04740-8/fulltext).
Pressure-relief features may attenuate CPAP efficacy if not adjusted for at the time of their introduction. In clinical practice, efficacy can be ensured by increasing the therapeutic pressure delivered by fixed CPAP or by enabling the pressure-relief features prior to initial pressure titration. Device-reported pressures in APAP devices with pressure relief activated may overstate delivered pressures (https://doi.org/10.5664/jcsm.5590).
Every patient do not need pressure to drop during expiration. This comfort feature is only meant for those patients who have difficulty in exhaling out against the pressure. Its use does not improve compliance in every patient (https://doi.org/10.1002/14651858.cd003531.pub3).
Clinical tips for using pressure drop during expiration-
- Use this function only in patients complaining of difficulty in exhaling against the pressure
- After switching this function on, closely follow up the patient for increasing AHI
- If AHI is increasing, increase the PAP pressure by 1-2cmH2O and follow up for reduction in AHI
- Avoid setting of 3
- Let the patient decide the setting at which patient can expire comfortably against the pressure