External negative-pressure ventilation was introduced by Dalziel in 1843. Alfred Jones manufactured the first 'body-enclosing lung', but almost 100 years elapsed before the first clinically practical motorized device was developed. Drinker in 1929 and Emerson 3 years later introduced versions of the famous tank respirator known as the 'iron lung'; this was used successfully between 1930 and 1960, and greatly reduced mortality during the poliomyelitis epidemics in the 1950s and 1960s.

Despite this success the iron lung had many limitations, in particular size and limited access to the immobilized patient. A further disadvantage was that only one phase of the respiratory cycle, inspiration, was 'active'. Expiration remained passive, being achieved by elastic recoil of the lungs and chest wall. This limited the frequency of breathing to less than 30 cycles/min. These limitations, as well as the increasing demand for more sophisticated methods of artificial ventilatory support, led to the introduction of positive-pressure ventilation in the 1950s. Over the next 40 years developments in ventilator technology were focused on positive-pressure ventilation, almost to the total exclusion of negative-pressure ventilation.

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