Characterization of an animal model of ventilator-acquired pneumonia

To delelop an experimental model of ventilator-acquired pneumonia (VAP), we investigated whether healthy piglets could develop endogenously acquired p ulmonary infection as a result of prolonged mechanical ventilation (MV). Th irty-three piglets underwent MV with anesthesia analgesia, and paralysis pr oduced by continuous infusion of midazolam, fentanyl, and pancuronium bromi de. Ten animals received antibioprophlaxis with ceftriaxone (ATB group) and 23 received no antibiotics (control group). Eighteen control animals and 9 ceftriaxone-treated animals completed the 4-day study protocol. The presen ce of pneumonia on day 4 was ascertained by multiple pulmonary biopsy speci mens, processed for microscopic examination and quantitative cultures, The anesthetic regimen provided satisfactory electrolyte balance and cardiovasc ular stability. Under these circumstances, 17 of 18 animals and 4 of 9 anim als del developed VAP in the control and the ATB groups, respectively. Lesi ons of different grades of severity were unevenly distributed through both lungs with a predominance and a higher severity in dependent lung segments. Noninfectious lesions frequently associated with VAP in humans were not ob served. Pneumonia was usually polymicrobial with a predominance of Gram-neg ative organisms. Most of the causative organisms originated from the oropha rynx. Histologic lesions and lung bacterial concentrations were less in the ATB group than in control animals. We then investigated the effects of int rabronchial challenge with bacterial pathogens in die absence of MV. Intrab ronchial bacterial inoculation resulted in the development of pneumonia tha t spontaneously resolved even when using very highly titrated inocula. Ther efore, MV seems to be the main predisposing factor in the development of pn eumonia in this model. This model that resembles human VAP in its histologi c, bacteriologic, and pathogenic aspects may be useful to further study pat hogenesis, diagnosis, prevention, and therapy of VAP.