Changes in breathing variables during a 30-minute spontaneous breathing trial.

BACKGROUND Spontaneous breathing trials (SBTs) are increasingly performed. Significant changes in monitored breathing variables and the timing of those changes during the trial have important implications for its outcome determination and supervision. We aimed to study the magnitude and timing of change in breathing variables during the course of a 30-min SBT. METHODS Breathing variables were continuously measured and averaged by minute during the SBT in 32 subjects with trial success and 8 subjects with trial failure from a general ICU population. Percentage changes in breathing variables during the trial and proportions of subjects showing a ≥20% change at different time points relative to the second minute of the trial were calculated. RESULTS The commonly monitored breathing variables (frequency, tidal volume, their ratio, and minute ventilation) showed median coefficients of variation of <15% throughout the trial and a median change of less than ±20% by the end of the trial. Changes in a detrimental direction of ≥20% at the end of the trial but not already present at 10 min were noted in ≤5% of all subjects. CONCLUSIONS During the course of a 30-min SBT, breathing variables remain relatively constant, and potentially significant changes in these variables after 10 min into the trial are uncommon. These findings should be considered when addressing aspects of duration and supervision of SBTs in weaning protocols.