The development of molecular diagnostic techniques is changing the understanding of the microbiology of VAP, with an increasing appreciation of the role of hard to culture bacteria and the change in the lung microbiome. A recent finding has highlighted the presence of Mycoplasma in the lavage of patients with VAP, a finding which was largely absent from ventilated patients without VAP and healthy controls. The Mycoplasma species most commonly identified, Mycoplasma salivarium, was able to impair the antibacterial functions of monocytes and macrophages. Pathophysiology
It is thought by many, that VAP primarily occurs because the endotracheal or tracheostomy tube allows free passage of bacteria into the lower segments of the lung in a person who often has underlying lung or immune problems. Bacteria travel in small droplets both through the endotracheal tube and around the cuff. Often, bacteria colonize the endotracheal or tracheostomy tube and are embolized into the lungs with each breath. Bacteria may also be brought down into the lungs with procedures such as deep suctioning or bronchoscopy. Another possibility is that the bacteria already exist in the mucus lining the bronchial tree, and are just kept in check by the bodys first line of defenses. Ciliary action of the cells lining the trachea drive the mucus superiorly, leading to a build-up of fluids around the inflated cuff where there is little to no airway clearance. The bacteria can then colonize easily without disturbance and then rise in numbers enough to become infective.
The droplets that are driven into the airstream and into the lung fields are lofted by way of Bernoullis principle. There is also a condition called oxidative damage that occurs when concentrations of pure oxygen come into prolonged contact with cells and this damages the cilia of the cells, thus inhibiting their action as part of the bodys first line of defense. Whether bacteria also travel from the sinuses or the stomach into the lungs is, as of 2005, controversial. However, spread to the lungs from the blood stream or the gut is uncommon. Once inside the lungs, bacteria then take advantage of any deficiencies in the immune system (such as due to malnutrition or chemotherapy) and multiply. Patients with VAP demonstrate impaired function of key immune cells, including the neutrophil, both in the blood and in the alveolar space, with this impairment being driven by pro-inflammatory molecules such as C5a. These defects in immune function appear to be causally linked to the development of VAP, as they are seen before clinical infection develops. A combination of bacterial damage and consequences of the immune response lead to disruption of gas exchange with resulting symptoms. Diagnosis
Diagnosis of ventilator-associated pneumonia is difficult and is not standardized.