built into most resuscitators built before the 1960s, including the Pulmotor and Emerson models. To ensure that the victim's lungs were not injured from being over-inflated, the resuscitator was pre-set to provide what was considered a safe pressure of oxygen. Once the unit reached this limit, it ceased to pump oxygen. For patients with chronic obstructive pulmonary disease (COPD), or any form of obstructive lung disease, the delivered pressure was insufficient pressure to fill the lungs with oxygen, meaning that, for patients with any sort of obstructive lung disease, units that pressure cycled did more harm than good. Pressure cycling also meant that cardiopulmonary resuscitation was impossible to perform if a patient's respiration was being supported by one of these units. If chest compressions were to be done, the cycle would be retarded and the resuscitator would be unable to provide oxygen as long as the chest was being compressed. For victims of smoke inhalation and drowning, however, the benefits outweighed the negatives, so these units found a home on ambulances around the world. The devices that cycled on the basis of upper and lower pressure limits are known as pressure cycled automatic resuscitators. In the UK the introduction of BS6850:1987 Ventilatory Resuscitators confirmed that "....automatic pressure-cycled gas-powered resuscitators are not considered suitable for such use (closed chest cardiac compression)..." and confirmed the standards required for gas powered resuscitators and operator powered resuscitators. The following year a similar ISO standard was introduced. Around this date most manufacturers supplied or introduced time - volume cycled resuscitators and pressure cycled devices were discontinued. Both the Pulmotor and the Emerson depended to a large extent upon the patient's ability to breathe the oxygen in order to be beneficial. Due to the limitations imposed by the cycling feature, this meant that patients in need of rescue breathing benefited little from the application of these devices. The Emerson and Pulmotor were utilized until the mid-1960s, when a breakthrough in the history of oxygen delivery was made: the demand valve.<ref name="Demand valve assembly for use with breathing or resuscitation equipment" The demand valve was a revolutionary new piece of equipment. At the simple push of a button, high-flow oxygen could be delivered into the lungs of the patient without the worry of the device cycling and, thus, ceasing to pump oxygen. Any amount of pressure that might be required to inflate the lungs could be achieved,