In summary, most cases of endemic VAP are acquired through the aspiration of
microorganism-containing oropharyngeal, gastric, or tracheal secretions around the cuffed
endotracheal tube into the normally sterile lower respiratory tract.
On the other hand, epidemic VAP infection is most commonly contracted via
contaminated respiratory treatment equipment, such as bronchoscopes or medical aerosols;
water (e.g.,Legionella); or air (e.g.,Aspergillus).
Direct inoculation with pathogens through ventilation devices is possible if no
preventive measures are taken. Bacterial contamination of equipment accounted for several
VAP outbreaks in the 1970s, although today’s improved hygiene has meant that this route is
only responsible for a few isolated outbreaks. Water condensing in the ventilation circuit is a
potential source of contamination, and several preventive measures are specifically recom-
mended (see section Prevention) to avoid the risk of contamination via this route (2,26–29,31).
Theinhalation of pathogens,such as viruses, fungi (Aspergillusspp.), or evenLegionella
spp., from the environment (2,16,26) has also been described.
Pneumonia can also be acquired by thespreadof infection from adjacent infected tissue,
such as the pleura or mediastinum, but this occurs very rarely.
Bacterial translocationfrom the gastrointestinal tract is another pathogenic mechanism
described for VAP. The intestinal wall of critically ill patients loses its capacity to prevent the
systemic absorption of bacteria and toxins. This in turn leads to impaired intestinal function,
promoting the invasion of the blood system with intestinal pathogens and thus metastatic
infections (40,41). The hematogenous spread of pathogens from intravascular catheters seems
to be rare.
An exception to the idea that “pathogenesis always starts with oropharyngeal
colonization” is the case of infection byPseudomonasspp. Thus, the findings of several studies
have indicated that tracheal colonization by these pathogens may occur without previous
oropharyngeal colonization (42–44).
MICROBIOLOGY
Approximately two-thirds of nosocomial pneumonias are caused by gram-negative bacilli (45),
although infections by gram-positive cocci are on the rise (45,46).
There is much paucity of data regarding whether the pathogens that cause VAP differ
from those causing HAP in patients who are not mechanically ventilated. One prospective
observational study evaluated 158,519 patients admitted to a single center over a four-year
period (46). A total of 327 episodes of VAP and 261 episodes of HAP were identified in non-
ventilated patients. Pathogens in ventilated patients included gram-negative bacilli (59%—
P. aeruginosa, 17%;Stenotrophomonas maltophilia, 7%;Acinetobacterspp., 8%); gram-positive cocci
(32%—methicillin-susceptibleStaphylococcus aureus(MSSA), 9%; methicillin-resistantS. aureus
(MRSA), 18%); and miscellaneous pathogens (9%). Pathogens in non-ventilated patients were
similar, except for non-Enterobacteriaceae bacilli, which were less frequent: gram-positive
cocci (43%—MSSA, 13%; MRSA, 20%); gram-negative bacilli (40%—P. aeruginosa,9%;
S. maltophilia, 1%;Acinetobacterspp., 3%); and miscellaneous pathogens (18%).
In a prospective, multicenter, observational study performed in 398 ICU patients with
suspected VAP, a similar distribution of pathogens was observed. Major pathogens were
identified in 197 patients (49.5%) through either tracheal aspirate or BAL fluid and included
primarily MRSA (14.8%),P. aeruginosa(14.3%), and otherStaphylococcusspecies (8.8%) (47).
Multidrug-resistant (MDR)–related VAP rates have recently undergone a dramatic
increase in hospitalized patients. These pathogens are more likely to infect patients with late-
onset HAP and VAP. The following risk factors for colonization and infection with MDR
pathogens have been identified (2,20,24,48–52):
- Antimicrobial therapy in the preceding 90 days
- A length of hospital stay of five days or more
- An existing high incidence of resistance to antibiotics in the hospital area or unit
- Risk factors for health care–associated pneumonia:
a. Hospitalization for 2 days or more in the preceding 90 days
b. Stay in a nursing home or an extended care facility
180 Bouza and Burillo