Aim 2: The primary outcome of this aim is weaning duration. For respiratory muscle strength the investigators will compare the first measured aPiMax (after resolution of the acute phase, before the first SBT), the trajectory and value of the daily aPiMax during the weaning phase prior to extubation, the lowest and highest measured aPiMax, and aPiMax on the day of extubation against weaning duration. For analysis, aPiMax will be dichotomized at 30 cmH2O, and weaning duration will be compared between patients with aPiMax > 30 versus ≤ 30 cmH2O using a t-test with or without log-transformation, or Mann-Whitney U test, depending on the distribution. From preliminary data, it is anticipated at least 35% of patients (n=84) will have aPiMax ≤ 30 cmH2O. Based on a similar power analysis as presented above, this would allow the investigators to determine whether low aPiMax is associated with a ≥ 1-day increase in weaning duration, with an alpha of 0.05 and power of 0.8. The investigators will perform identical analysis for ePiMax. Diaphragm Thickness analysis will compound daily ultrasound measures to detect the relative change in diaphragm thickness from study day 1 until passage of an SBT. The investigators will compare the change in thickness after resolution of the acute phase (on the day of the first SBT) against weaning duration, in a similar manner as proposed above for aPiMax. In addition to weaning duration, the investigators will also examine whether the respiratory measures taken just prior to or during each SBT are associated with the patient passing the SBT. For example with aPiMax and ePiMax, the investigators will examine if there is a dose response relationship between PiMax measured just before the SBT and the rate of passage of the subsequent SBT.