Cause
Risk factors
Risk factors for VAP include underlying heart or lung disease, neurologic disease, and trauma, as well as modifiable risk factors such as whether the head of the bed is flat (increased risk) or raised, whether the patient had an aspiration event before intubation, and prior antibiotic exposure. As a result of intubation many of the bodys defenses against infections are reduced or impaired; this can result in an ability for microorganisms to enter and cause infection. Patients who are in the ICU for head trauma or other severe neurologic illness, as well as patients who are in the ICU for blunt or penetrating trauma, are at especially high risk of developing VAP. Further, patients hospitalized for blunt trauma are at a higher risk of developing VAP compared to patients with penetrating trauma.Ventilator-associated tracheobronchitis may be a risk factor for VAP, though not all cases of VAT progress to VAP.Recent studies have also linked the overall oral health of a patient to the potential development of VAP; suggesting that bacteria found in plaque can "migrate to the respiratory system." Microbiology
The microbiologic flora responsible for VAP is different from that of the more common community-acquired pneumonia (CAP). In particular, viruses and fungi are uncommon causes in people who do not have underlying immune deficiencies. Though any microorganism that causes CAP can cause VAP, there are several bacteria which are particularly important causes of VAP because of their resistance to commonly used antibiotics. These bacteria are referred to as multidrug resistant (MDR).
If a person has previously had episodes of pneumonia, information may be available about prior causative bacteria. The choice of initial therapy is therefore entirely dependent on knowledge of local flora and will vary from hospital to hospital. Treatment of VAP with a single antibiotic has been reported to result in similar outcomes as with a combination of more than one antibiotics, in terms of cure rates, duration of ICU stay, mortality and adverse effects.Risk factors for infection with an MDR strain include ventilation for more than five days, recent hospitalization (last 90 days), residence in a nursing home, treatment in a hemodialysis clinic, and prior antibiotic use (last 90 days). Possible empirical therapy combinations include (but are not limited to):

vancomycin/linezolid and ciprofloxacin,
cefepime and gentamicin/amikacin/tobramycin
vancomycin/linezolid and ceftazidime
Ureidopenicillin plus β-lactamase inhibitor such as piperacillin/tazobactam or ticarcillin/clavulanate
a carbapenem (e.g., imipenem or meropenem)Therapy is typically changed once the causative bacteria are known and continued until symptoms resolve (often 7 to 14 days). For patients with VAP not caused by nonfermenting Gram-negative bacilli (like Acinetobacter, Pseudomonas aeruginosa) the available evidence seems to support the use of short-course antimicrobial treatments (< or =10 days).People who do not have risk factors for MDR organisms may be treated differently depending on local knowledge of prevalent bacteria. Appropriate antibiotics may include ceftriaxone, ciprofloxacin, levofloxacin, or ampicillin/sulbactam. As of 2005, there is ongoing research into inhaled antibiotics as an adjunct to conventional therapy. Tobramycin and polymyxin B are commonly used in certain centres but there is no clinical evidence to support their use.