VAP occurrence : the diagnosis of VAP will be based on the positive results of a protected specimen brush (over 103 cfu/mL) or a broncho-alveolar lavage (over 2 % infected cells or over 104 cfu/mL). Distal bronchial samples will be performed under fiberoptic bronchoscopy when several of the following criteria suggestive VAP will be present, corresponding to a CPIS score at least equal to 6 : fever, new chest X-ray opacity, hyper-leucocytosis, arterial oxygenation alteration.