Anatomic Alterations of the Lungs

The inhalation of smoke and hot gases and body surface burns—in any combination—continue to be a major cause of morbidity and mortality among fire victims and firefighters. In general, fire-related pulmonary injuries can be divided into thermal and smoke (toxic gases) injuries.

Thermal Injury

Thermal injury refers to injury caused by the inhalation of hot gases. Thermal injuries are usually confined to the upper airway—the nasal cavity, oral cavity, nasopharynx, oropharynx, and larynx. The distal airways and the alveoli are usually spared serious injury because of (1) the remarkable ability of the upper airways to cool hot gases, (2) reflex laryngospasm, and (3) glottic closure. The upper airway is an extremely efficient “heat sink.” In fact, in 1945, Moritz and associates demonstrated that the inhalation of hot gases alone did not produce significant damage to the lung. Anesthetized dogs were forced to breathe air heated to 500° C through an insulated endotracheal tube. The researchers’ results showed that the air temperature dropped to 50° C by the time it reached the level of the carina. No histologic damage was noticed in the lower trachea or lungs.

Even though thermal injury may occur with or without surface burns, the presence of facial burns is a classic predictor of thermal injury. Thermal injury to the upper airway results in blistering, mucosal edema, vascular congestion, epithelial sloughing, and accumulation of thick secretions. An acute upper airway obstruction (UAO) occurs in about 20% to 30% of hospitalized patients with thermal injury and is usually most marked in the supraglottic structures. When body surface burns require the rapid administration of resuscitative fluids, a UAO may develop rapidly (see Figure 41-1).

It should be noted that the inhalation of steam at 100° C or greater usually results in severe damage at all levels of the respiratory tract. This damage occurs because steam has about 500 times the heat energy content of dry gas at the same temperature. Thermal injury to the distal airways results in mucosal edema, vascular congestion, epithelial sloughing, cryptogenic organizing pneumonia (COP)—also known as bronchiolitis obliterans organizing pneumonia (BOOP)—atelectasis, and pulmonary edema.

Therefore, direct thermal injuries usually do not occur below the level of the larynx, except in the rare instance of steam inhalation. Damage to the distal airways is mostly caused by a variety of harmful products found in smoke.

Etiology and Epidemiology

According to the National Fire Protection Association (NFPA), public fire departments responded to an estimated 1,557,500 fires in the United States in 2007. There were about 530,500 structure fires (85% were residential fires), 258,000 vehicle fires, and 769,000 outside and other fires. This means that every 20 seconds a fire department responded to a fire somewhere in the nation. A fire occurs in a structure at the rate of once every 59 seconds, and in particular, a residential fire occurs every 76 seconds. Fires occur in vehicles at the rate of 1 every 122 seconds, and there is a fire in an outside property every 41 seconds. In addition, there were 3430 civilian fire deaths (1 every 153 minutes) and 17,675 civilian injuries (1 every 30 minutes) according to the NFPA in 2007. The NFPA estimated that more than 14 billion dollars in property damage occurred as a result of fires in 2007.

The prognosis of fire victims usually is determined by the (1) extent and duration of smoke exposure, (2) chemical composition of the smoke, (3) size and depth of body surface burns, (4) temperature of gases inhaled, (5) age (the prognosis worsens in the very young or old), and (6) preexisting health status. When smoke inhalation injury is accompanied by a full-thickness or third-degree skin burn, the mortality rate almost doubles.

Smoke can result from either pyrolysis (smoldering in a low-oxygen environment) or combustion (burning, with visible flame, in an adequate-oxygen environment). Smoke is composed of a complex mixture of particulates, toxic gases, and vapors. The composition of smoke varies according to the chemical makeup of the material that is burning and the amount of oxygen being consumed by the fire. Table 41-1 lists some of the more common toxic substances produced by burning products that frequently are found in office, industrial, and residential buildings.

Although in some instances the toxic components of the smoke may be obvious, in most cases the precise identification of the inhaled toxins is not feasible. In general, the inhalation of smoke with toxic agents that have high water solubility (e.g., ammonia, sulfur dioxide, and hydrogen fluoride) affects the structures of the upper airway. In contrast, the inhalation of toxic agents that have a low water solubility (e.g., hydrogen chloride, chlorine, phosgene, and oxides of nitrogen) affects the distal airways and alveoli. Many of the substances in smoke are caustic and can cause significant injury to the tracheobronchial tree (e.g., aldehydes [especially acrolein], hydrochloride, and oxides of sulfur).

Body Surface Burns

Because the amount and severity of body surface burns play a major role in the patient’s risk of mortality and morbidity, an approximate estimate of the percentage of the body surface area burned is important. Table 41-2 lists the approximate percentage of surface area for various body regions of adults and infants. The severity and depth of burns usually are defined as follows: