CPAP helps COPD patients keep their airways open by providing a continuous flow of air delivered through a face mask. The pressure level on CPAP is doctor-prescribed, single-level and constant. This means there is no difference in the pressures when breathing in or out, which is in direct contrast with Bi PAP (bilevel positive airway pressure), another type of non-invasive ventilation which provides a higher pressure when breathing in, and a lower one when breathing out.
CPAP is sometimes difficult for COPD patients to tolerate, as some find it difficult to breathe out against the continuous pressure of the CPAP machine and become anxious and short of breath. It is for this reason that Bi PAP is the preferred method of noninvasive ventilation for people with COPD.