Baseline characteristics included age, sex, length of mechanical ventilation before nosocomial infection, type of artificial airway, comorbidities, cause of respiratory failure(leading to prolonged mechanical ventilation), and source of nosocomial infection. Previous antibiotic exposure was determined as any antibiotic, intravenous, or oral, administered for the last infection. Comorbidities other than prolonged mechanical ventilation were identified by reviewing charts and imaging reports. Recent use of systemiccorticosteroids (ie, prednisolone N0.5 mg kg−1 d−1 for N2 weeks) was also recorded