Diagnosing Ventilator Acquired Pneumonia

A 65 year old woman developed a left lower lobe hospital acquired pneumonia following a elective laparoscopic procedure for which she was ventilated for 4 days. Twenty four hours post extubation, she developed hypoxic respiratory failure with bilateral patchy shadowing on chest X-ray. She was reintubated and subsequently grew Pseudomonas aeruginosa from tracheal aspirate.

How do we diagnose Ventilator Associated Pneumonia (VAP)?

Adrian Wong

Healthcare-associated infections (HAIs) including VAP are becoming more of a public issue. 10% of hospital patients will be affected by a HAI and up to a third of these are thought to be preventable. NICE defines VAP as an inflammation of lung parenchyma occurring after 48hrs following intubation of the trachea, and occurs due to organisms not present or incubating at the time mechanical ventilation was commenced (1). VAP is the most significant HAI in the ITU with a mortality of up to 30%.

Considering the significant clinical and financial implications, the Department of Health (DoH) is increasingly encouraging ICUs to declare VAP rates as a measure of not just individual unit’s performance but also as a comparison between units. Experience from the continuous drive in quantifying the rates of catheter-related blood stream infection as part of the Matching Michigan campaign as been shown to reduce their rate and thus has proven benefits to both patients and healthcare providers. However, there is no consensus guidelines published for the diagnostic criteria for VAP. This has led to significant variation in reported incidence (9-28%)(2-3).

The use of clinical criteria in defining VAP is limited by the fact that 1) wide range of differential diagnosis that may mimic VAP such as ARDS, pulmonary oedema, pulmonary contusion, etc and 2) clinical criteria are subjective and hence open to inter- and intra-observer variation.

Scoring systems currently available use a combination of clinical, microbiological and radiological criteria. They have the advantage of providing an easy method of pooling data to determine the likelihood of a positive or negative result. The Clinical Pulmonary Infection Score (CPIS)(4), Centre for Disease Control National Healthcare Safety Network (CDC NHSN) Definition(5) and the Hospitals in Europe Link for Infection Control through Surveillance (HELICS)(6) criteria are all recognised and utilised systems in the UK. The CPIS, which incorporates only qualitative microbiological analysis, is the most commonly utilised scoring system in UK with a reported sensitivity of 72-89% and a specificity of 17-85%(4). A criticism of the CPIS is that antimicrobial therapy would be delayed if reliant on the tracheal aspirate culture for an overall score to be considered positive. The CDC NHSN, which was not designed specifically for VAP, utilises a combination of radiological and clinical signs. HELICS is widely used in Europe and relies on a combination of clinical, radiological and microbiological criteria.

References

NICE Guidelines PSG002. Technical patient safety solutions for prevention of ventilator-associated pneumonia in adults: guidance. 2008.
B Chandler, J Hunter. Ventilator-associated pneumonia: a concise review. JICS 2009;10: 29-33.
Masterton R, Galloway A, French G et al. Guidelines for the management of hospital-acquired pneumonia in the UK. J of Antimicrobial Chemotherapy 2008; 62: 5-34.
Pugin J et al. Diagnosis of ventilator-associated pneumonia by bacteriologic analysis of bronchoscopic and nonbronchoscopic ‘blind’ bronchoalveolar lavae fluid. Am Rev Resp Dis 1991; 1121-1129.
Horan T, Andrus M and Dudect M. CDC/NHSN surveillance definition of health care-associated infection criteria for specific types of infections in the acute care setting. Am J Infect Control 2008; 36: 309-332.
Hospitals in Europe Link for Infection Control through Surveillance. Surveillance of Nosocomial Infections in Intensive Care Units. http://helics.univ-lyon1.fr/helicshome.htm
The Health and Social Care Act 2008: Code of Practice for the NHS on the prevention and control of healthcare associated infections and related guidance.