Resuscitation. Doctor using an Ambu bag to resuscitate an unconscious man. An Ambu bag is used when a patient is breathing weakly, or not at all. It comprises a mask (centre left) connected to a bag (right) by a one-way valve. Oxygen is supplied to the bag. Squeezing the bag forces a mixture of air and oxygen into the patient's lungs, maintaining oxygenation of the blood and stimulating natural breathing.