An Ambu bag, also known as a bag valve mask (BVM), is a handheld device used to provide positive pressure ventilation to patients unable to breathe effectively on their own. It consists of a self-inflating bag, one-way valve, mask, and optional oxygen reservoir. The Ambu bag is used to manually ventilate a patient's lungs until they can breathe spontaneously or more advanced ventilation support is available. Complications can include aspiration, hypoventilation, hyperventilation, and pneumothorax if not used properly.