The demand valve could also provide oxygen at any flow rate required to a conscious patient in respiratory distress. Conserving the often limited reserves of oxygen was easier with a demand valve, as oxygen was designed only to flow when either the button was depressed or the casualty inhaled. Later medical opinion decided that getting high flow oxygen into a patients airway was a factor in causing vomiting and aspiration. Demand valve resuscitators were introduced with restrictors to limit flow rates to 40 lpm. Use of the demand valve resuscitator in Europe was limited by the lack of pressure relief valve or audible alarm for high pressure. The ambu-bag was a further advancement in resuscitation. Introduced in the 1960s by the Danish company Ambu, this device allowed two rescuers to perform CPR and ventilation on a non-breathing patient with an acceptable chance of success. The ambu-bag has now mostly replaced the demand valve as the primary method of ventilation, largely due to concerns of potential over-inflation with the demand valve by untrained rescuers. The ambu-bag, unlike the older version of the demand valve (all new models of demand valve now have pressure relief valves set at 60 cm of water to prevent accidental overinflation of the lungs), has a "pop-off" valve to prevent inflation at greater than 40 pounds -per-square-inch (275.79 kilo-pascals), with the result being that it is generally more common in the pre-hospital setting than the demand valve. However, the demand valve remains popular with BLS providers, and in situations where conserving supplies of oxygen is of paramount importance. The demand valve, while less popular today than it was previously, still remains in service, albeit with important safety features added, including the addition of a pressure-relief valve to prevent over-inflation and the restriction of its flow to 40 liters a minute. Even newer products have been developed and are now available. In 1992 the Genesis(R) II time/volume cycled resuscitator (now upgraded to meet the current, International, resuscitation guidelines and called the CAREvent(R) ALS and CA)provide the SIMV automatic ventilation mode with demand breathing for the spontaneously breathing patient. These devices work like full blown transport ventilators yet are simple enough to operate that they can be used in an emergency situation by pre-hospital healthcare providers and are small enough to be easily transportable. Having a manual override control for use during mask CPR they meet the requirements of the current resuscitation standards.