An Ambu bag, also known as a bag valve mask (BVM) or manual resuscitator, is a handheld device used to provide emergency respiratory support to patients who cannot breathe effectively on their own. It consists of a self-inflating bag, a one-way valve, and a mask or mouthpiece. By manually squeezing the bag, a rescuer delivers air or oxygen to the patient's lungs until spontaneous breathing is restored or more advanced support is available.