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BACKGROUND: Patients with a tracheostomy and difficult weaning from invasive mechanical ventilation constitute a challenging problem in critical care. An increased duration of ventilation may lead to diaphragmatic dysfunction and a noninvasive assessment of the diaphragm, such as ultrasound, attracts interest in the clinical practice. We evaluated the relationship of ultrasound-derived indices with weaning outcome and with established indices of respiratory strength and load in subjects who are tracheostomized and undergoing weaning. METHODS: This prospective study was conducted at an academic ICU in Greece. Twenty subjects with tracheostomy and difficult weaning, during a spontaneous breathing trial, underwent time synchronous diaphragmatic sonography and esophageal manometry, to assess diaphragmatic excursion and thickening fraction, esophageal and transdiaphragmatic pressures, pressure-time product of the esophageal pressure, and maximum inspiratory pressure. The primary outcome was liberation from mechanical ventilation at 48 h. The relationship of diaphragmatic ultrasound with esophageal pressure-derived indices was also evaluated. RESULTS: Weaning from invasive ventilation failed in 10 subjects. Diaphragmatic excursion exhibited a significant difference between weaning success and failure (1.34 ± 0.56 versus 0.79 ± 0.44; P = .044), a strong correlation with transdiaphragmatic pressure (r = 0.7, P = .02), and a moderate correlation with the pressure-time product of the esophageal pressure (r = 0.65, P = .02) and the maximum inspiratory pressure (r = 0.66, P = .02). Transdiaphragmatic pressure presented the highest area under the curve (0.97). However, when transdiaphragmatic pressure was compared with diaphragmatic excursion (area under the curve, 0.84) for predictive accuracy, no significant difference was found. CONCLUSIONS: Diaphragmatic excursion is a valuable tool for the assessment of diaphragmatic strength, respiratory load, and weaning prediction. [ABSTRACT FROM AUTHOR]