9. Initial oscillation frequency will be randomly set at either at 3.5-4 Hz or at 7 Hz. The low frequency setting will be combined with a 3-5 cmH2O cuff leak and TGI for 60 min followed by "no-TGI" for another 60 min in random order. The high frequency setting will not be combined with either a cuff leak or TGI and will be maintained for an additional 60 min. Oscillatory pressure amplitude (ΔP) will be set at 90 cmH2O.