Data showing an increase in prevalence of MRSA and other multi-drug resistant organism (MDRO) infections in ICU patients suggests that antimicrobial resistance continues to increase in this population As a measure of anti-icrobial stewardship, emphasis on identification of patients who are appropriate for empiric anti-MRSA therapy is needed to reduce unnecessary antibiotic exposure. For a study published in the Journal of Infectious Diseases and Epidemiology, my colleagues and I aimed to identify the incidence of, and risk factors for, MRSA ventilator-associated pneumonia (VAP) in surgical ICU (SICU) patients to better identify those who benefit from empiric MRSA coverage.

MRSA vs non-MRSA VAP

This was a single-center, retrospective risk factor analysis of adult SICU patients diagnosed with VAP between 2013 and 2016. VAP was defined as a lower respiratory culture isolating more than 10,000 colony-forming units/ml of pathogenic bacterial organisms collected more than 48 hours after endotracheal intubation. Respiratory cultures used for diagnosis of VAP were obtained either by bronchoscopic bronchial alveolar lavage (BAL) or non-bronchoscopic BAL (mini or blind BAL). In the case of multiple VAPs during admission, only the patient’s first episode of VAP was included in the analysis. The causative pathogens of that first episode dictated group allocation (MRSA-VAP group vs. non-MRSA VAP group).

Among participants, 22.1% were in the MRSA group and 78.9% in the non-MRSA group. The most prevalent organisms in the non- MRSA VAP group were Enterobacteriaceae (32.1%) and Pseudomonas aeruginosa (29%). Significantly more females had MRSA VAP (54.8% vs. 32.1%), as did a greater proportion of patients with a history of MRSA or MDRO infection (16.1% vs. 4.6% and 22.6% vs. 9.2%, respectively). Fewer patients in the MRSA VAP group had antibiotic exposure prior to their first VAP (58.1% vs. 78.9%). The MRSA VAP group had lower sequential organ failure assessment scores at the time of VAP diagnosis, driven by fewer patients with elevated bilirubin (3.2% vs. 22%) and hemodynamic instability (16.1% vs. 34.9%). A lower proportion of patients with MRSA VAP met the criteria for septic shock at the time of VAP diagnosis (6.5% vs. 23.9%). The MRSA VAP group had lower all-cause mortality than the non-MRSA VAP group (12.5% vs. 28.7%), though this did not reach statistical significance. After multivariable logistic regression modeling, MRSA risk was 2.3-fold higher in females than males and 4.2-fold higher among those with a history of MRSA than those with no such history (Table). Patients with prior antibiotic exposure were 60% less likely to develop MRSA VAP than those without such exposure.

What the Results Suggest

This study identified an incidence of MRSA VAP that was twice what had been described in previous literature examining risk factors in a SICU cohort. Our results suggest a lower severity of illness at the time of diagnosis in the MRSA VAP group when compared with the non-MRSA VAP group, that selection of appropriate empiric antibiotics for VAP must be balanced with antimicrobial stewardship, and that empiric MRSA coverage should be considered in patients with a history of a MRSA infection and/or no prior antibiotic exposure during their admission, particularly if the VAP has developed later in the ICU stay.

Other modalities, such as MRSA nasal swabs on admission, may be useful tools to guide anti- MRSA therapy. Gram-negative pathogens, particularly MDROs, appear to be more prevalent in this patient population and conferred a greater severity of illness. Unaccounted for in our study, a patient’s microbiota may play a role in the risk of VAP. Antibiotic exposure as an individual stressor has been associated with an increase in the presence of genes related to antibiotic resistance in the gut microbiome, which was demonstrated by an increased prevalence of MDR gram-negative organisms in the non-MRSA VAP population. Our results lend additional guidance to selection of empiric antibiotic therapy for VAP in SICU patients; however, further research with risk factor studies accounting for individual factors and dysbiosis are needed.

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