Effects of Different Levels of Pressure Support on Intra-Individual Breath-to-Breath Variability

BACKGROUND: Evidence exists that during pressure support ventilation (PSV), the addition of an extrinsic (ie, ventilator-generated) breath-to-breath variability (BBV) of breathing pattern im-proves respiratory function. If BBV is beneficial per se, choosing the PS level that maximizes it could be considered a valid strategy for conventional PSV. In this study, we evaluated the effect of different PS levels on intrinsic BBV in acutely ill, mechanically ventilated subjects to determine whether a significant relationship exists between PS level and BBV magnitude. METHODS: Four-teen invasively mechanically ventilated subjects were prospectively studied. PS was adjusted at 20 cm H2O and sequentially reduced to 15, 10, and 5 cm H2O. Arterial blood gas analysis and pressure at 0.1 s after the onset of inspiration (P0.1) were measured at each PS level. Airway and esophageal pressure and air flow were continuously recorded. Peak inspiratory flow, tidal volume (VT), breathing frequency, and pressure-time product (PTP) were calculated on a breath-by-breath basis. The breathing pattern variability was assessed by the coefficient of variation of the time series of VT, peak inspiratory flow, and breathing frequency from 60 consecutive breath cycles at each PS level. A general linear model for repeated measures was applied, with PS as an independent factor. A significance level of.05 was considered. RESULTS: Despite a large inter-individua