Should a full face mask fit properly if the patient's mouth is slightly open?

A full face mask is designed to cover both the nose and mouth, ensuring that positive pressure during noninvasive mechanical ventilation is effectively delivered through both airways. If the mask fits properly, it will create an adequate seal even if the patient's mouth is slightly open. The key point is that a properly fitted mask should maintain an effective seal and prevent air leaks, allowing for effective ventilation regardless of the patient's mouth position.

The rationale behind this stems from the construction of the mask itself, which is intended to accommodate a range of facial structures and movements, including slight mouth openings. While comfort is an important factor in patient compliance, the primary function of the mask remains to ensure that air can flow properly to support ventilation needs. Thus, under normal circumstances, a full face mask should fit effectively even if the patient's mouth is slightly open, provided adjustments have been made to account for the individual's facial features and the mask's design.