Pathophysiology

Inhalation injury can be both thermal and chemical. Thermal injury is usually limited to the upper airway because of the low thermal capacity of heated air and the efficient heat exchange of the oropharynx. However, direct thermal injury to the trachea and bronchi can result following exposure to steam which can overwhelm the upper airway because of its higher thermal capacity. Chemical injury of the airway can be caused by irritants or cytotoxic chemicals that either adhere to the fine particles in smokes or become aerosolized as mists. The type and volume of these irritants generated by combustion can vary depending on the material burned, the temperature of the fire, and the amount of oxygen present in the fire environment. Injury can occur at all levels of the airway and is typically heterogeneous in nature, with some airways sustaining severe damage and others sustaining mild or no injury.

Even a short duration of exposure to highly reactive irritants can result in loss of cilia and superficial epithelial erosions of the tracheobronchial tree. Injury activates the inflammatory cascade resulting in histologically evident inflammatory changes of the respiratory mucosa within 2 h of injury. More prolonged exposure results in epithelial necrosis and sloughing, exposing areas of bronchial and alveolar basement membrane. Such tissue injury evokes release of arachidonic acid metabolites and other vasoactive substances which may produce pulmonary hypertension and bronchospasm and cause mismatching of airflow and blood flow within the lung. Cast formation may occur with subsequent obstruction of bronchioles and atelectasis, which decrease compliance and impair oxygenation and ventilation.

A significant determinant of inhalation injury's effect on outcome is the extent of the accompanying burn. Smoke inhalation with minimal or no cutaneous burn is often limited to a chemical tracheobronchitis and does not significantly add to the morbidity and mortality of thermal injury. Patients with inhalation injury and a medium-sized burn have an increased fluid requirement during the resuscitative phase and often develop significant sequelae of their pulmonary injury. In these medium-sized burns inhalation injury can add up to 20 per cent to the expected mortality (Shirani.. ef,a/ 1987). However, inhalation injury produces little appreciable increase in the already discouragingly high mortality of patients with extensive thermal injury.

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