It is thought by many, that<unk>  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 body's 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 Bernoulli's 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 body's 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<unk>  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<unk> , 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.
Ventilator-associated pneumonia
Hospital-acquired pneumonia
Community-acquired pneumonia
Occult pneumonia
Idiopathic pneumonia syndrome
Ventilator-associated lung injury
Aspiration pneumonia
Hantavirus pulmonary syndrome