A resuscitator is a device using positive pressure to inflate the lungs of an unconscious person who is not breathing, in order to keep them oxygenated and alive. There are three basic types: a manual version (also known as a bag valve mask) consisting of a mask and a large hand-squeezed plastic bulb using ambient air, or with supplemental oxygen from a high-pressure tank. The second type is the Expired Air or breath powered resuscitator. The first appearance of the second type was the Brooke Airway introduced in 1957. The third type is an oxygen powered resuscitator. These are driven by pressurized gas delivered by a regulator, and can either be automatic or manually controlled. The most popular type of gas powered resuscitator are Time Cycled, Volume Constant Ventilators. In the early days of pre-hospital emergency services, pressure cycled devices like the Pulmotor were popular but yielded less than satisfactory results. One of the first modern resuscitation ventilators was the HARV, later called the PneuPac 2R or Yellow Box. Most modern resuscitators are designed to allow the patient to breathe on his own should he recover the ability to do so. All resuscitation devices should be able to deliver >85% oxygen when a gas source is available.