The Outcomes of Inhalation Injuries in Lesser Burns: Still a Deadly Injury

In the United States, it is estimated that more than 350 000 fires have resulted in over 2000 deaths, 13 000 injuries and billions of U.S. dollars in property loss. The advent of specialized burn centers instituting effective fluid resuscitation and early surgical excision of burned tissue has significantly improved the survivability from burn injuries; however, mortality still remains in the top 15 causes of disease burden worldwide.^1-4 Clear determinants of mortality and morbidity in burn injuries include increased age, mechanism of burn, severity or extension of injury, race, sex, and the presence of inhalation injury with or without cutaneous compromise.^5,6

Inhalation injuries can compromise the upper airway, tracheobronchial airway, and lung parenchyma, or even cause systemic toxicity secondary to carbon monoxide or hydrogen cyanide inhalation. Smaller or lower molecular weight particles are more likely to produce direct damage to distal airways and alveoli.^7,8 There is compromise of the mucociliary clearance, which predisposes to distal airway obstruction, atelectasis, bacterial proliferation, and impairment of gas exchange.^8-11 Moreover, smoke inhalation stimulates nerves that release neuropeptides at the tracheobronchial tree producing bronchoconstriction, vasodilation, increased vascular permeability, induction of nitric oxide synthase (NOS) and generation of reactive oxygen species (ROS).^8,10-14 These series of events may lead to not only cellular damage, but also to an increase in the transvascular shift of proteins that produce exudate and cast formation due to excessive fibrin deposition. Furthermore, there is a decrease in surfactant and fibrinolytic activity that results in a severe imbalance of alveolar homeostasis.^8,14,15

Inhalation injury adds a negative effect on burn patients, increasing morbidity and mortality as described by Thompson et al.¹⁶ In their study of 1018 patients, they determined that the presence of inhalation injury increases mortality in every age and burn size category.¹⁶ Masaru et al found in their study of 5560 patients admitted to 13 burn facilities of the Tokyo Burn Unit Association, that inhalation injury was an independent predictor of outcomes and the most important predictor of overall mortality among burn patients.¹⁷

Inhalation injury, even in isolation, increases the requirements of fluid resuscitation and the incidence of pulmonary complications.¹⁸ While most of the advancements made have been in cutaneous burn management, there has been poor improvement for inhalation injury-specific therapies. Cutaneous burns can be replaced with skin grafts, but secondary pulmonary damage from inhalation injury can only be treated with supportive and protective measures in the ICU.

It has been widely proposed that inhalation injuries worsen burn outcomes, yet no large-scale study has shown the exact relationship between the two.  This study proposes inhalation injury as an independent risk factor that worsens burn outcomes.

This study is a review of retrospectively collected data from the American Burn Association Burn Registry from 2002 to 2011. The main goal was to compare the outcomes of burn patients with inhalation injury vs burn patients with no inhalation injury. Inclusion criteria included patients with a TBSA of less than 15% and adequate data recording inhalation injury within the registry. A TBSA of 15% was the cutoff for the study to minimize the cutaneous burn injuries as a confounder. Visual findings of mucosal erythema, charring, blistering, edema, ulceration, or fibrin casts on bronchoscopy confirmed the presence of inhalation injury; however, the degree of injury was not consistently reported. For this reason, it was not included in the analysis.

Patients were stratified into 2 groups. Group 1 included patients  with proven inhalation injuries and group 2 included burn patients that presented with no inhalation injuries on admission. Demographic characteristics and outcome variables were collected and compared between each group.

Outcome measures included in-hospital mortality rate, hospital LOS, ICU LOS, and days on a ventilator. Chi-squared and t-test analyses were used with significance defined as P < 0.05.

A total of 93 781 burn patients met the inclusion criteria. There were 4204 patients with inhalation injuries (group 1) and 89 577 patients with no inhalation injuries (group 2). The average age of patients in group 1 was 44.8 years while the average age in group 2 was 31.2 years. There was no statistically significant difference between the 2 groups in terms of TBSA (P =0.24, t-test) as shown in Table 1. In group 1, the percent TBSA burn was on average 3.50% compared to 3.58% in group 2.

There was a significantly higher ICU LOS in the inhalation injury group compared to those with non-inhalation injury. Group 1 had an average of 8.55 days compared to 6.27 days in group 2 (P = 0.0001, χ2). There was also a significantly higher LOS in group 1 compared to group 2, with an average of 11.48 days compared to 6.27 days, respectively. (P = 0.0001, χ2). Average days on ventilatory support was also significantly increased for patients in group 1 at 6.07 days compared to 0.67 days for patients in group 2(P = 0.0001, χ2). The in-hospital mortality was significantly higher in group 1 at 8.54% vs group 2 at 1.42% (P = 0.0001, χ2).

Inhalation injury refers to the direct thermal and chemical damage caused to the airways and lung parenchyma by heat, irritants, and smoke from pyrolysis.⁸ Many of the consequences of smoke inhalation result from an inflammatory response involving mediators following the interaction of irritant substances with lung parenchyma. This inflammatory response leads to pulmonary edema, cast formation, airway obstruction, loss of hypoxic pulmonary vasoconstriction, and ventilation/perfusion mismatch.¹⁹

Smoke inhalation should be suspected in any patient with a history of flame, smoke, or chemical exposure in confined spaces as well as prolong exposure.^14,20 A thorough history including place of occurrence, duration of exposure, and history of syncope during the event are important factors in diagnosing inhalation injuries. Clinical presentation can be characterized by dyspnea, nausea, vomiting, dizziness, and cough. Signs of external burns include singed nasal hair and  eyebrows, blistering around the mouth, soot in upper airways, and carbonaceous sputum.^8,14,18 Standard laboratory studies of complete blood count, basic metabolic panel, toxicology, arterial blood gases, co-oximetry and blood sample for carboxyhemoglobin (COHb) level (to determine COHb and methemoglobin concentration) are included in the primary workup of patients with a suspected smoke inhalation injury.^21,22

After the initial stabilization of a burn patient, visual inspection of the airway with bronchoscopy is indicated to diagnose inhalation injury in the setting of smoke exposure. Visual findings of mucosal erythema or charring, blistering, edema, ulceration, or fibrin casts are diagnostic for inhalation injury.^14,23,24 The degree of the inhalation injury can be graded by bronchoscopy with the Abbreviated Injury Score, which goes from 0 (no injury) to 4 (massive injury).^14,25,26 The higher the score, the higher the need for supportive treatment with bronchodilators and airway clearance measures. The objective of treatment is to prevent further pulmonary complications by minimizing bronchospasm, managing secretions, and clearing fibrin casts and necrotic material from the airway.^13,27

Lung-protective strategy should be employed when mechanically ventilating patients with inhalation injury.^28,29  Providing improved oxygenation and ventilation at lower pressures than conventional ventilators aids in secretion clearance and reversal of inhalation-injury-induced small airway obstruction and atelectasis.³⁰ In this study, burn patients with inhalation injury were found to have a significant increase in days on ventilation compared to patients with no inhalation injury, indicating that ventilatory support is increased in patients with inhalation injuries.

Patients with inhalation injury require close monitoring in the ICU as these injuries can lead to airway inflammation, systemic toxicity (carbon monoxide, hydrogen cyanide poisoning), pneumonia, fluid imbalance, hyper-metabolic state, and possible acute respiratory distress syndrome, all of which can be minimized with close ICU monitoring.⁷ These statements correlate with some of the findings in this study. Patients with inhalation injury had a significantly higher ICU LOS compared to burn patients with no inhalation injury, revealing how these injuries increase the need for more close monitoring and a higher level of care.

Improvements in mortality from inhalation injury are mostly due to widespread improvements in critical care rather than focused interventions for smoke inhalation as shown by the work of Dries et al.¹⁸ The current management of these injuries at this point is primarily supportive as stated by Cancio et al in his review.³⁰

The purpose of this study was to determine the relationship of inhalation injury in burn patients and its associated adverse effects. Inhalation injuries are associated with increased mortality in burn patients and often included in multivariate predictors of burn mortality, along with factors such as burn size and age.^31,32 In the present study, in-hospital mortality was found to be significantly higher in patients with inhalation injury compared to burn patients with no inhalation injury. The findings correlate with multiple studies that indicated inhalation injury as an independent risk factor for increase mortality.^16,33-36

Given the retrospective nature of this study, there was a lack of severity grading for the inhalation injuries. The authors used a TBSA of 15% as a cutoff to show that inhalation injuries continue to be a significant predictor of worsening outcomes, including mortality even in limited cutaneous burn injuries, and this study supported this hypothesis.

Inhalation injuries are commonly used to predict the clinical outcome in burn patients. Based on the results of this study, the presence of inhalation injury in burn patients is associated with an increased ICU LOS, hospital LOS, average days on a ventilator, and in-hospital mortality. The results suggest worse outcomes and an increased need for resources in the management of these patients.

Affiliations: ^aDepartment of Surgery, Kendall Regional Medical Center, Miami, Florida; ^bBurn and Reconstructive Centers of Florida, Miami, Florida; ^cDepartment of Surgery, University of South Florida, Tampa, Florida; ^dDepartment of Medicine, Universidad del Norte, Barranquilla, Colombia; ^eDepartment of Plastic Surgery, Tulane University, New Orleans, Louisiana

Correspondence: Salomon Puyana, MD, MS, 11750 SW 40th Ave, Miami, FL 33131; salomonpuyana@gmail.com

Disclosure: The authors disclose no financial or other conflicts of interest.