Mechanical ventilators have only two functions: to provide a flux to eliminate carbon dioxide from those who will not or cannot breathe and to establish an adequate gas-exchanging volume to reduce shunting. The concept of volume recruitment to reduce shunting goes back at least to Mead and Collier in 1959,1 who showed that without periodic inflations there was a progressive fall in compliance during prolonged mechanical ventilation. Much of the subsequent history of mechanical ventilation in acute lung disease has really been the search for better methods of volume recruitment. The lung has to be inflated past the pressure at which atelectatic lung begins to open and be maintained above its closing pressure (that pressure below which alveoli and airways start to close again).

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