Histology and microbiology of ventilator-associated pneumonias.

A good knowledge of histological and bacteriological characteristics of experimental and human ventilator-associated bronchopneumonias (BPN) is of critical importance for the intensivist. BPN can be experimentally produced by intratracheal inoculation of microorganisms in high concentrations and ventilator-associated BPN by ventilating baboons with oleic-acid lung injury. Experimental ventilator-associated BPN is frequently polymicrobial, and bacterial lung burden increases with the severity of lung infection. In human ventilator-associated BPN, gross examination is of poor value for diagnosing lung infection. Four histologic categories of increasing severity have been described: bronchiolitis, focal bronchopneumonia, confluent bronchopneumonia, and lung abscess. Nonspecific inflammatory lesions are always associated with histologic lung infection: primary lung infection causes secondary inflammatory lung damage, whereas non-specific alveolar injury is rapidly superinfected when the lungs are mechanically ventilated. Infectious pulmonary lesions are disseminated within all pulmonary segments but preferentially found in the dependent segments. This fact suggests that ventilator-associated BPN has a bronchogenic origin and that gravity plays an important role in the dissemination of microorganisms within lung parenchyma. Ventilator-associated BPN is a nosocomial infection with a predominance of gram-negative bacteria, staphylococci species, and yeasts. It is frequently polymicrobial, and the lung bacterial burden depends on the histologic grade, the administration of topical and intravenous antibiotics, and the host's local antibacterial defenses. The bacterial complexity of human lung infection does not support the concept of a threshold for the diagnosis of nosocomial BPN. Intensivists should always keep in mind that human ventilator-associated BPN is a complex and rapidly changing entity.