Abbreviated Injury Score (AIS) for Inhalation Injury

Selection of the appropriate findings:

The AIS for inhalation injury was first proposed by Endorf and Gamelli in 2007. The purpose of their study was to identify whether severity of inhalation injury correlated better with pulmonary parameters (e.g. lung compliance, PaO₂:FiO₂ ratio) and acute fluid resuscitation requirements than bronchoscopically assessed inhalation injury severity. They retrospectively reviewed 80 adult patients requiring intubation, mechanical ventilation, and flexible bronchoscopy for suspected inhalation injury within the first 24 hours of admission. They used the AIS classification to separate their patients into two groups of bronchoscopic inhalation injury: one including grades 0 and 1 and another including grades 2, 3, and 4. They compared characteristics, such as fluid resuscitation requirements, initial oxygenation, lung compliance, and duration of mechanical ventilation; only bronchoscopic severity correlated with decreased survival (p = 0.03).

Hassan et al also found a significant (p <0.01) increase in mortality with bronchoscopic severity. They did not use the AIS.

Since 2007, a number of studies have used AIS in order to establish a clear relationship between bronchoscopic grade of injury and a range of clinical outcomes; however, the results have been varied. For instance, in contrast to Endorf and Gamelli’s study, Albright et al (2012), Mosier et al (2012), and Spano et al (2016) all found that increasing AIS grade did not have a significant effect on mortality, with p-values of 0.21, 0.10, and 0.15, respectively.

Albright et al did show that increasing severity was associated with longer ventilator days (p = 0.036) and intensive care unit (ICU) stay (p = 0.040). Additionally, Mosier et al noted a significant association between AIS grade severity and development of acute respiratory distress syndrome (ARDS) at 24 hours (p <0.01).

Interpretation:

*From Endorf and Gamelli.

• No universal consensus exists on grading for inhalation injury. The AIS has been widely utilized as the predominant bronchoscopic inhalation injury severity score in the literature (but have not been compared head to head with other bronchoscopic criteria).
• Of note, a multicenter prospective cohort study by the American Burn Association is currently underway to develop and validate a scoring system for inhalation injury based on clinical, radiographic, bronchoscopic, and biochemical parameters.

Adult patients with suspected inhalation injury undergoing flexible bronchoscopy.

• May predict development of ARDS, length of time on mechanical ventilation, and prolonged ICU stay.
• Some studies have found a non-statistically significant trend towards worse outcomes with higher AIS.
• AIS severity has not been consistently associated with mortality (Sheridan 2016).
• Cannot reliably predict need for high fluid resuscitation requirements.
• A typical flexible bronchoscope is on average 5 mm in diameter. Hence, bronchoscopy can only identify proximal airway changes and not the narrower distal and peripheral airway changes. Thus, bronchoscopic findings cannot be fully relied upon to accurately reflect the overall severity of airway inhalation injury.

Macroscopic manifestations of airway abnormalities may be delayed and hence, falsely reassure the clinician that inhalation injury has not occurred (Hunt 1975).

• Supportive treatment is the primary means of inhalation injury management, as there is very little in the way of pharmacologic treatment once the inhalation injury has occurred.
• Bronchoscopy can play a therapeutic role in airway clearance as necrotic tissue and eschar can form pseudomembranes, sloughing of mucosa, and bronchial obstruction.
• Other measures include intensive bronchial hygiene, including bronchodilators (e.g. inhaled β2 agonists), frequent chest physiotherapy, and early patient ambulation).
• Upper airway edema can progress, particularly over the first 24 hours after injury, necessitating intubation. If mechanical ventilation is required, a high frequency percussive mode of ventilation can be considered, as some studies have shown benefit to this patient population. A lung-protective, low tidal volume ventilation strategy (6-8 cc/kg of predicted body weight) is preferred in adults.
• Other supportive measures have been used with varied success, including prone positioning, extracorporeal membrane oxygenation (ECMO), inhaled anticoagulants (e.g. heparin, antithrombin), and inhaled N-acetylcysteine (NAC).
• Additionally, referral to a designated burn center should be considered if any inhalation injury is present, according to American Burn Association guidelines.

High AIS severity alone should not dictate management decisions, which should as always be made in conjunction with history, physical exam, and laboratory findings.