What causes VAP?

Secretions pool above the ETT cuff,
resulting in gradual aspiration.

The first studies linking pneumonia and ventilators started coming out in 1972. While it was first thought pathogens came from the equipment, subsequent studies confirmed that the pathogens came from the patient.  

By 2002, it was known that the pathogens came from either secretions that pooled in the upper airway, or those accumulated in the upper GI tract, and then were aspirated, thereby increasing the risk for developing Ventilator Associated Pneumonia (VAP). 

It was at this time that VAP was also broken down into early onset and late onset (1, page 3):

1. Early Onset: The infection occurs within 48-96 hours of admission, and is generally associated antibiotic sensitive agents, such as:
• Staphylococcus aureus (gram positive)
• Haemophilus influenzae (gram negative)
• Streptococcus pneumoniae (gram positive)
2. Late Onset: The infection occurs 96 hours after intubation, and is generally associated with bacteria that are tough to kill, such as:
• Methicillin Resistant Staphylococcus aureus (MRSA)
• Acinetobacter or Enterobacter 
• Pseudomonas aeruginosa (1, page 3)(2)

In most cases, VAP is associated with more than one causative agent.

The various bacterial agents known to colonize in the lungs of ventilator patients have been known to come from various sources within the body, including: (2)

• Oropharynx (mouth and throat)
• Sinus Cavities (nasal drainage)
• Nares
• Dental Plaque
• Gastrointestinal tract
• Patient to patient (poor hand washing by clinicians)
• Ventilator circuit (2)

It's not necessarily the ventilator itself that raises the risk for pneumonia, but the cuffed endotracheal tube (ETT).  Secretions containing bacteria pool above the cuff, and may be forced into the lower airway during the various activities performed by caregivers, such as:  (2) (1, page 4)(3, page 1583)

• Ventilator induced breaths 
• Instillation of saline into the ETT
• Suctioning
• Coughing
• Repositioning the ETT   (1, page 4)

Complicating this problem is that the ETT prevents normal physiological functions meant to keep the lungs sterile. The resulting factor is an increased risk for developing pneumonia while intubated. (1, page 4)

Other reasons the ETT may cause VAP: (1, page 4)

• It prevents a natural cough
• It prevents humidification of upper airway (causing dry mouth)
• Inhibits upper airway reflexes, such as cough and sneeze
• Inhibits cilliary transport of germs to the upper airway
• Allows germs to bypass the upper airway, allowing them direct access to the lungs
• Act as a reservoir for pathogens by providing a place for biofilm to form
• Having a cuff provides a place for secretions to pool (as noted above) (1, page 4)

There are also certain factors associated with an increased risk: (1, page 4)

• Over age 65
• Underlying chronic illness (COPD, asthma, GERD)
• Immunosuppression (AIDS)
• Depressed consciousness (Sedated, paralyzed)
• Thoracic or abdominal surgery
• Previous antibiotic therapy
• Previous pneumonia or remote infection
• Nasogastric tube placement
• Bolus enteral feeding 
• Gastric over-distension
• Stress ulcer treatment
• Supine patient position (lying flat on back)
• Nasal intubation route
• Instillation of normal saline
• Understaffing 
• Non-conformance to handwashing protocol
• Indiscriminate use of antibiotics
• Lack of training in VAP prevention (1, page 4)

So, you can see here how patients who require intubation and mechanical ventilation are at an increased risk for developing VAP.  Once this information made its way to the mainstream guidelines, protocols, and order sets (Ventilator Bundles) were created in order to attempt to reduce the incidence thereof.

References:

1. Van Hooser, Theron, "Ventilator Associated Pneumonia: Best Practice Strategies for Caregivers," 2002, http://en.haiwatch.com/data/upload/tools/VAP_CEU_Booklet_Z0406.pdf, Kimberly Clark Co., accessed 4/21/14
2. Augustyn, Beth, "Ventilator Associated Pneumonia: Risk Factors and Prevention,"  Critical Care Nurse, August, 2007, volume 27, number 4, pages 32-29
3. Bowton, David L, R. Duncan Hite, Shayne Martin, and Robert Sheretz, "The Impace of Hospital-Wide Use of a Tapered-Cuff Endotracheal Tube on the Incidence of Ventilator-Associated-Pneumonia," Respiratory Care, October, 2013, volume 58, number 10, pages 1582-1587