For the majority of patients, however, the ability to generate minute ventilation or to inflate poorly compliant lung, is inadequate. Inspiratory assistance using one of a multitude of modes of ventilation is required.
The mechanics of inspiratory support are more complex than previously considered. It has been established that cyclical inflation and deflation injures lung parenchyma and worsens outcome. Large tidal volume ventilation, to "normalize" blood gases has been shown to worsen outcome in lung injury, presumably due to excessive pressure induced stretch injury of the parenchyma. Modern ventilation strategy involves attempting to achieve an adequate minute volume with the lowest possible airway pressure. The pressure that we are interested in minimizing is at the level of the alveolus, the plateau pressure.
The rate, pattern and duration of gas flow control the interplay between volume and pressure. In volume controlled modes, a desired tidal volume is delivered at a specific flow (peak flow) rate, using constant, decelerating or sinusoidal flow patterns: the airway pressure generated may be higher than is desirable. In pressure controlled modes, flow occurs until a preset peak pressure is met over a specified inspiratory period, the flow pattern is always decelerating: the tidal volume may be lower than that desired. Moreover, as pulmonary mechanics change, so too does the delivered tidal volume.