New insights into the pathogenesis of ventilator-associated pneumonia.

Colonization of the lower respiratory tract of critically ill patients with Gram-negative bacilli is an early stage in the pathogenesis of ventilator-associated pneumonia. Recent studies have emphasized the endogenous source of these bacteria and have pointed to the upper gastrointestinal tract as a major source. Gastric bacterial overgrowth is known to be promoted by raised intragastric pH. Further evidence suggests that duodenal reflux is just as important in gastric bacterial overgrowth and subsequent colonization of the mechanically ventilated lung. Work on the biomaterials lining the inside of the tracheal tube in critically ill patients has shown that this biofilm can be scattered many centimetres as a result of ventilator gasflow. In vitro and in vivo evidence has been found to support a fluid dynamic process that results in dissemination of tracheal tube biofilm, which also contains neutrophils and their contents. The dissemination of combined bacteria, effete neutrophils and their breakdown products into already compromised lungs may explain the complexity of the clinical condition known as ventilator-associated pneumonia. It may thus be more correct to refer to the condition as 'ventilator-induced pneumonitis'.