A resuscitator is a device using positive pressure to inflate the lungs of an unconscious person who is not breathing, in order to keep him 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 a pulmonary or breath powered resuscitator. The first one developed was the White Pulmonary Resuscitator or W.P.R. introduced in 1981. 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. The workings of most modern resuscitators are arranged so that the patient will be able to breathe on his own on should he resume the ability to do so. All resuscitation devices should be able to deliver >85% oxygen when a gas source is available.