Severity of smoke inhalation injury and risk of ventilator-associated pneumonia: a retrospective cohort study

Abstract

Introduction:Inhalation injury is present in up to one-third of severe burn injuries and is a major contributor to burn-related mortality. Bronchoscopy is commonly performed within 48 hours of presentation to grade inhalation injury severity. Whether higher grade inhalation injury is associated with increased risk of ventilator-associated pneumonia (VAP), an important complication among patients with burn injury, is unknown. Methods: We performed a retrospective cohort study of patients with suspected inhalation injury at a large American Burn Association-verified burn center from 2011-2022 who underwent diagnostic bronchoscopy within 48 hours of admission. Abbreviated injury severity score (AIS) was used to grade inhalation injury. VAP was defined using CDC PNU2 clinical criteria and isolation of a pathogenic organism from bronchoalveolar lavage fluid >10,000 CFU/ml after ≥48 hours of mechanical ventilation. Competing risk analysis was performed to estimate the association of high-grade (grades 3-4) inhalation injury versus low-grade (grades 1-2) inhalation injury with VAP. Death and extubation were considered competing risks. Poisson regression was performed to test for associations between inhalation injury severity, ventilator-free days, and mortality. Results: 231 patients were analyzed. Fifty-one (21%) had high-grade injury and 181 (74%) had low-grade injury. VAP occurred in 10/51 (18%) patients with high-grade injury and 21/181 (11%) with low-grade injury. After excluding patients who received <48 hours of mechanical ventilation, high-grade injury was not associated with increased incidence of VAP in competing risk analysis after adjusting for age, burn size, and pre-injury lung disease (subdistribution hazard ratio: 1.59, 95%CI 0.66-3.87, p = 0.30; cause-specific hazard ratio: 1.03, 95%CI 0.43-2.48, p =0.94). High-grade injury compared with low-grade injury was associated with fewer ventilator-free days (RR 0.72, 95%CI 0.57-0.9, p=0.01). Burn size modified the association of inhalation injury severity and hospital mortality (interaction term p-value = 0.01). Among patients with burn size <20% total body surface area (TBSA), higher inhalation injury severity exhibited a strong association with mortality (RR 3.83; 95%CI 1.71–8.58; p = 0.001), but at burn size >20% TBSA the association was attenuated (RR 0.75; 95% CI 0.35–1.62); p = 0.47). Conclusion: Among patients with inhalation injury requiring mechanical ventilation for > 48 hours, higher severity inhalation injury was not associated with increased incidence of VAP in competing risk analysis, though low-grade patients were more likely to be extubated within 48 hours which may explain these null findings. High-grade injury was associated with fewer ventilator-free days, and higher hospital mortality among those with TBSA <20%. Additional research is needed to disentangle the relationship of inhalation injury and VAP in this high-risk population.