To permit ventilation the mask must be attached to a ventilating bag.

In non-anaesthetic practice it is usually attached to a self-inflating bag. After emptying, the self-inflating bag refills automatically.

During anaesthetic practice it is attached to the reservoir bag of the anaesthetic circuit, filled with anaesthetic gases. If the seal between facemask and patient is not achieved, the anaesthetic circuit bag does not refill correctly and high gas flows are needed to keep the bag inflated.