Effectiveness of pressure support ventilation for mechanical ventilatory support in patients with status asthmaticus.

We compared the effects of pressure support ventilation (PSV) with those of assist control ventilation (ACV) on breathing patterns and blood gas exchange in six patients with status asthmaticus. Both PSV and ACV delivered adequate minute ventilation (PSV: 7.5 +/- 1.4 l/min/m2, ACV: 7.3 +/- 1.3 l/min/m2) to correct respiratory acidosis (pH = 7.33 +/- 0.12 during both PSV and ACV) and prevent hypoxia. Peak airway pressure during PSV was significantly lower with the same tidal volume than that during ACV (PSV: 30 +/- 10 cmH2O (2.9 +/- 1.0 kPa), ACV: 50 +/- 13 cmH2O (4.9 +/- 1.3 kPa)). The lower airway pressure during PSV was due to persistent inspiratory muscle activity. The oxygen cost of breathing estimated by oxygen consumption was equivalent in both modes. We conclude that PSV is effective in supplying tidal volumes adequate to improve hypercarbia at markedly lower airway pressures than ACV.