Automatic increases in mean airway pressure during mechanical ventilation.

The response of a time-cycled, continuous-flow, pressure-limited infant ventilator to changes in lung compliance was assessed by use of a variable-compliance test lung. A 50% reduction in lung compliance produced an automatic increase in mean airway pressure (MAP). Increases in MAP averaged 17% over baseline values. Factors that favored a large increase in MAP when lung compliance was reduced were a high original lung compliance, a low ventilator compliance, a high I:E, and ventilation with a triangular inspiratory pressure wave. The increase in MAP was accompanied by a change in the inspiratory pressure wave from triangular to square. Increases in MAP were not seen when the pressure wave was initially square. Automatic increases in MAP when lung compliance decreases may help to minimize decreases in PaO2; however, pulmonary barotrauma and increases in intracranial pressure could also result. The factors that allow for automatic MAP increases may also allow for automatic decreases in MAP as lung compliance improves. The advantages and disadvantages of automatic changes in MAP merit further study.