Respiratory muscle pressure analysis in pressure-support ventilation.

The extent to which respiratory muscles are exerted during partially supported ventilation is difficult to differentiate, because these muscles and the ventilator work simultaneously to produce ventilation. We have developed a new method for determining the pressure developed by the respiratory muscles in partially supported ventilation. In seven patients on pressure-support ventilation (PSV), pressure, flow, and lung volume change were measured at the airway opening. Various PSV levels (0-15 cmH2O) were applied to each patient in random order. By utilizing a model of respiratory mechanics, we calculated the pressure developed by the respiratory muscles and the inspiratory work performed by the muscles from the measured parameters by use of the resistance and elastance of the respiratory system obtained during controlled ventilation. Increasing PSV from 0 to 15 cmH2O modulated the resultant breathing pattern, i.e., increasing tidal volume and decreasing respiratory rate. The respiratory muscle pressure, although less negative, had a shape that corresponded to the shape of airway occlusion pressure at each PSV level, and both pressures decreased concomitantly with increasing PSV. The respiratory muscle work progressively decreased with increasing PSV. This analysis enabled clear and continuous quantifications of the respiratory muscle force generation and inspiratory work during partially supported ventilation.