“So the patient has ventilator-associated pneumonia!” the junior doctor declared reaching for the drug chart. The Critical Care Consultant and I exchanged a glance…
So what is ventilator-associated pneumonia (VAP)
Ventilator-associated pneumonia is a lower respiratory tract infection that develops after 48 hours of mechanical ventilation. Early diagnosis and management are essential to prevent the 25-50% mortality.
VAP is suspected when the patient develops new or progressive infiltrates on a chest x-ray associated with a fever, increased purulent secretions and raised white blood cell count (WBC). Clinically the patient has an increased respiratory rate and reduced oxygenation as well as increasing ventilator requirements, just like our patient above. However, the chest x-ray findings alone are insufficient for diagnosing VAP as they can occur in ventilated patients in the absence of pneumonia; about 40% of patients with chest x-ray findings suggestive of VAP do not actually have pneumonia. In order to diagnose VAP microbiological confirmation is required.
Once VAP is suspected microbiology specimens should be taken and then the antibiotics should be started. Try to wait for the specimens to be taken before starting antibiotics as once antibiotics have been started the microbiology cultures are likely to be negative; however if the patient is septic do not unnecessarily delay the antibiotics in order to gain specimens.
The best respiratory specimens to take in suspected VAP are from the lower respiratory tract. These include tracheobronchial aspiration or a bronchoalveolar lavage (BAL). BAL samples are better as they are less prone to contamination from the upper respiratory tract and may lead to more specific antibiotic therapy. All samples should ideally have a Gram film to look for the most likely type of bacteria and also to confirm the presence of white blood cells. Quantitative cultures to count the number of bacteria present in the BAL or tracheobronchial aspiration gives the best diagnostic accuracy: significant values are 1,000,000 cfu/ml for aspirates and 10,000 cfu/ml for BALs. However VAP can still be present at lower levels.
The most important reason for taking the microbiology samples is to try and identify the causative bacterium and guide specific antibiotic therapy.
Scoring systems for VAP
Some have tried to develop scoring systems to improve diagnostic accuracy in VAP, although these are not perfect they can be useful. The most popular is the Clinical Pulmonary Infection Score (CPIS) where a score of 6 or more points is suggestive of VAP:
Current diagnostic criteria for VAP:
- Patient mechanically ventilated for ≥ 48 hours PLUS
- New or progressive infiltrates on chest x-ray PLUS
- Raised WBC PLUS
- Growth of a pathogenic bacterium at significant levels from a lower respiratory tract sample (aspiration or BAL)
So what does all of this vague diagnostic rambling tell us? VAP is difficult to diagnose with any accuracy in many patients and a number of patients are over-treated in order that genuine infections are not missed.
Ventilator-associated event (VAE)
Because the previous VAP diagnostic strategies are so poor there has been a move away from specifically trying to diagnose VAP and talk instead about ventilator-associated events (VAE) in patients who have been mechanically ventilated for 48 hours. The VAE are classified in the table below. It should be noted that this is a surveillance tool which is not intended to be used in the acute setting and that the terminology remains ambiguous with terms like “possible” and “probable”! In fact as this new system is non-clinical and no more helpful for acute diagnosis, it might beg the question why change from VAP? One cynical explanation for developing a standardised surveillance tool is to allow for rates of VAP to be recorded, monitored, benchmarked and ultimately targets set, like with MRSA. After all, this is a hospital-acquired infection...even if it is a result of a life-saving intervention.
At the end of the day, patients who are ventilated and develop what looks like a pneumonia are still treated with empirical antibiotics. A decision is then made after 2-3 days, once results are available, about what the diagnosis is and whether to continue the antibiotics for a full 7 day course.
For the patient on our ward round we took microbiology specimens and started empirical treatment for VAP with IV Piptazobactam. Two days later the BAL was growing a Staphylococcus aureus, a causative microorganism of VAP, the antibiotics were narrowed down to IV Flucloxacillin and the patient made a full recovery.
On leaving the Critical Care unit I overheard the junior doctor saying to one of the nurses, “I told them the patient had a ventilator-associated pneumonia”. I just smiled and walked on… If it looks like a VAP, then it needs to be treated like a VAP...at least until the Microbiologist tells you it isn’t!