Inclusion Criteria:  major patients (>18 years) admitted in ICU for an acute brain injury defined by a Glasgow Coma Scale < 8 and the presence of a cerebral lesion identified on a referent Tomodensitometry (TDM) or Magnetic Resonance Imaging (MRI) requiring mechanical ventilation during >48 hours eligible for a ventilatory weaning test  Exclusion Criteria:  preexisting neurologic evolutive or degenerative affection or preexisting diaphragmatic dysfunction preexisting decision of therapeutic limitation impossibility to collect an informed consent patients aged <18 years pregnancy mental illness impossibility for the subject to have a good comprehension of the study lack of health insurance coverage