Resistant Organisms May Be a Major Cause of Severe Pneumonia

PHILADELPHIA—Traditional community-acquired pneumonia (CAP) pathogens are infrequent in patients who present to the hospital with pneumonia and require mechanical ventilation, according to research presented at the American College of Chest Physicians 2008 Annual International Assembly. Shifts in the microbiology of this population could indicate that CAP is being supplanted by health care–associated pneumonia (HCAP) in the ICU, the study findings further suggested.

According to Matthew P. Schreiber, MD, of the Washington Hospital Center in Washington, DC, there may be significant microbiologic resistance in patients with severe pneumonia who also require mechanical ventilation. The current criteria for HCAP could exclude many of these patients, because of a failure to capture resistant organisms, he further hypothesized. “There is a real need to understand the microbiology and the trend of that microbiology, because it is key to appropriate antibiotic prescribing.”

BRONCHOSCOPIC INTERVENTIONS

In a three-year (2005 to 2007) retrospective study, Dr. Schreiber and colleagues identified all patients who were admitted to the hospital for pneumonia, required mechanical ventilation on presentation, and had a positive culture. The study population originated from a large, inner-city tertiary facility, at which about two-thirds of the 1,100 annual ICU admissions require mechanical support and approximately 25% of these patients die during their hospital stay.

Identification of the culprit organism served as the primary end point, and pneumonia was diagnosed based on traditional clinical criteria. The investigators adapted American Thoracic Society criteria to identify HCAP, so that patients who met any one of five criteria were diagnosed with HCAP, while those who met no criteria were diagnosed with CAP. Microbiologic diagnoses were based on invasive lower airway cultures, blood cultures, and urinary antigens. Pathogens were grouped into traditional CAP organisms versus antibiotic-resistant pathogens; temporal changes in microbiology also were studied.

In 266 patients, the most frequently identified organisms included Staphylococcus pneumoniae (18.6%), methicillin-resistant Staphylococcus aureus (MRSA; 15.5%), Streptococcus viridans (15.9%), Pseudomonas aeruginosa (11.6%), and methicillin-sensitive S aureus (MSSA; 11.3%). Traditional pneumonia pathogens such as Legionella species and Haemophilus influenzae were rarely involved, with an aggregate proportion of 14% for all such organisms. During the study period, the proportion of patients with severe pneumonia who needed mechanical ventilation because of MRSA remained unchanged, and the proportion of pneumonias due to P aeruginosa dropped from 12% to 6%. The rates of MRSA, MSSA, and S viridans achieved statistical significance.

SEVERITY-BASED PROTOCOL NEEDED

In aggregating the results, the investigators found—as expected—that there was more resistance in patients classified as having HCAP. However, the number of resistant organisms identified in CAP patients with severe illness requiring mechanical ventilation was “eye-opening,” Dr. Schreiber described, as approximately one in five patients admitted to the emergency department who did not meet the definition for HCAP actually had a resistant organism. Equally revealing was the fact that two-thirds of the study patients had high mortality rates, which was not due to a sudden spike in HCAP admission, the researchers determined.

Dr. Schreiber recognized that these findings may not be generalizable, that the investigators did not capture a significant number of pneumonias without an identifiable organism, and that the definition of pneumonia used differed from that of the American Thoracic Society.

In considering whether current HCAP criteria can be used as a screening tool to detect resistant organisms, Dr. Schreiber acknowledged, “it’s only doing as well as betting red or black on the roulette wheel.” Treatment strategies should be revised to reflect the current microbiologic epidemiology, he asserted.

“Perhaps a severity-based treatment protocol should be used instead of an epidemiology-based treatment protocol,” said Dr. Schreiber. Physicians must take this into account when deciding on initial antibiotic therapy protocols for critically ill, mechanically ventilated patients who are hospitalized with pneumonia, he added.

—Mary Brady Service

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