Inhalation Injury Workup

Various studies are indicated in patients with inhalation injury.

Pulse oximetry

Cutaneous pulse oximetry uses a 2-wavelength technique of light refractance to measure hemoglobin saturation that is falsely elevated by CO-bound hemoglobin. Obtain direct measures of carboxyhemoglobin and oxyhemoglobin.

To monitor oxygen saturation, recognizing that cutaneous pulse oximetry is falsely elevated by CO is imperative. One must not rely on pulse oximetry until the carboxyhemoglobin level has reached the reference range.

Cooximetry (arterial blood) uses a 4-wavelength technique of light refractance to accurately measure carboxyhemoglobin and oxyhemoglobin, in addition to deoxyhemoglobin and methemoglobin. The percent oxyhemoglobin measured by cooximetry is an accurate measure of the arterial oxygen saturation.

Arterial blood gas

Arterial oxygen tension (partial pressure arterial oxygen [PaO₂]) also does not accurately reflect the degree of CO poisoning or cellular hypoxia. The PaO₂ level reflects the oxygen dissolved in blood that is not altered by the hemoglobin-bound CO. Because dissolved oxygen makes up only a small fraction of arterial oxygen content, a PaO₂ level within the reference range may lead to serious underestimation of the decrement in tissue oxygen delivery and the degree of hypoxia at the cellular level that occurs when CO blocks the delivery of oxygen to the tissues. With most blood gas machines, the oxygen saturation is calculated, based on the PaO₂ level. Thus, such a reading does not give an accurate determination of oxygen saturation, which must come from cooximatry.

ABG measurements are nonetheless useful to assess the adequacy of pulmonary gas exchange. Although the presence of a PaO₂ level that is within the reference range may not exclude significant tissue hypoxia due to the effects of CO, the presence of a low PaO₂ (< 60 mm Hg in room air) or hypercarbia (alveolar [arterial] carbon dioxide pressure [PaCO₂] level of 55 mm Hg) are indicative of significant respiratory insufficiency. Metabolic acidosis suggests inadequate oxygen delivery to the tissues.

The alveolar gas equation can be used to estimate the efficiency of pulmonary oxygen delivery to the arterial circulation in the presence of supplemental oxygen administration. This formula determines the alveolar oxygen pressure.

The difference between the partial pressure of oxygen in the alveolus and that measured on an ABG is the alveolar-arterial (A-a) gradient. This value, usually less than 5-10 mm Hg, may be several hundred mm Hg in the setting of significant pulmonary injury and can be used to assess improvement or deterioration in lung function when measured at a stable fraction of inspired oxygen (FiO₂).

The alveolar gas equation can be used to estimate the efficiency of pulmonary oxygen delivery to the arterial circulation in the presence of supplemental oxygen administration.

The formula is as follows: PaO₂ = (FiO₂)(PB - PH₂ O) - (PaCO₂/RQ). PB represents barometric pressure, PH₂ 0 represents partial pressure of water vapor (47 mm Hg at body temperature, ambient pressure), and RQ represents respiratory quotient (estimated at 0.8).

Carboxyhemoglobin level

Provide supplemental oxygen therapy to all patients with suspected CO intoxication. Smokers may have baseline levels up to 5-10% and may experience more significant CO poisoning for the same level of exposure as nonsmokers.

However, blood carboxyhemoglobin levels may underestimate the degree of CO intoxication because of oxygen administration before arrival to the hospital. The use of nomograms to extrapolate levels to the time of rescue has been shown to have greater prognostic value. Symptoms vary with peak carboxyhemoglobin levels, but correlation between carboxyhemoglobin levels and eventual neurologic outcome is poor. The symptoms in relation to carboxyhemoglobin in the blood are as follows:^[2]

• Carboxyhemoglobin level of 0-10% - Usually no symptoms
• Carboxyhemoglobin level of 10-20% - Mild headache, atypical dyspnea
• Carboxyhemoglobin level of 20-30% - Throbbing headache, impaired concentration
• Carboxyhemoglobin level of 30-40% - Severe headache, impaired thinking
• Carboxyhemoglobin level of 40-50% - Confusion, lethargy, syncope
• Carboxyhemoglobin level of 50-60% - Respiratory failure, seizures
• Carboxyhemoglobin level of more than 70% - Coma, death

Cyanide levels

Levels correlate closely with the level of exposure and toxicity but may not be readily available. Many hospitals send out tests for cyanide levels; therefore, laboratory confirmation may take several days to a week. Persistent neurologic dysfunction unresponsive to use of supplemental oxygen, cardiac dysfunction, and severe lactic acidosis, particularly in the presence of high mixed venous oxygen saturation, are indicative of cyanide intoxication. In a setting consistent with potential cyanide exposure, institute specific empiric therapy while waiting for laboratory confirmation of the diagnosis.

Electrolyte levels

Obtain tests at regular and frequent intervals to monitor for electrolyte abnormalities that result from large-volume fluid resuscitation. Use results to adjust both fluid and electrolyte replacement.

CBC count, type, and cross

Hemoconcentration resulting from fluid losses is common immediately following injury. Adequate restoration of intravascular volume results in a progressive fall in hematocrit. Severe anemia may require blood replacement, particularly in the presence of significant hypoxia or hemodynamic instability. A baseline WBC count can also be used for comparison when concerns arise about infection.

C-spine radiography is indicated to investigate neck injury in all unconscious patients and in those in whom a potential mechanism of injury cannot be excluded (eg, jumped from window to escape fire, fell down stairs).

Roentgenographic abnormalities are frequently delayed and may not manifest on the initial chest radiograph. Radiographic evidence of pulmonary injury typically appears 24-36 hours after the inhalation. Obtain a chest radiograph at the baseline examination for subsequent comparison in cases of significant injury. Radiographic studies are also useful to establish correct placement of the endotracheal tube and central venous catheters.

Direct laryngoscopy and fiberoptic bronchoscopy

Both have diagnostic and therapeutic use. Visualization of erythema, edema, ulceration, and soot deposition make bronchoscopy useful in evaluating the extent of injury to the tracheobronchial tree, although severe vasoconstriction from hypovolemia may mask significant injury.

Fiberoptic bronchoscopy can also be used to facilitate endotracheal tube placement, even in the technically difficult airway.

Bronchoscopy is more sensitive and accurate than clinical examination alone in diagnosing inhalation injury and is, therefore, particularly useful in cases in which the decision to perform endotracheal intubation is unclear.

Serial bronchoscopy can help remove debris and necrotic cells in cases with aggressive pulmonary toilet or when suctioning and positive pressure ventilation are insufficient

The use of bronchoscopy among patients with inhalation injury complicated by pneumonia is associated with a decreased duration of mechanical ventilation, length of intensive care unit stay, and decreased overall hospital cost.^[4]

Because of their small size, a child's airway can only accommodate a relatively small diameter bronchoscope. Extremely small diameter fiberoptic bronchoscopes with a suction port (capable of entering an endotracheal tube sized for a small toddler or infant) are only recently available, and whether these limit the ability to remove heavy particulate matter is unclear.

Radionucleotide scintigraphy

Delayed or inhomogeneous clearance of 133Xenon can be used to detect small airway parenchymal injury but adds little to the clinical management and is not known to have any particular therapeutic advantage.^[5]

Likewise, increased clearance of aerosolized technetium-99m–labeled diethylenetriaminepentaacetate (99mTcDTPA) is a sensitive indicator of injury to the alveolar capillary membrane; however, its clinical use is not yet established.

Pulmonary function tests

With an inhalation injury, a decrease in pulmonary compliance, vital capacity, and functional residual capacity occurs. Airway obstruction causes a decrease in FEV₁ and peak flow. Although this helps determine lower airway disease and injury, similar to radionucleotide scintigraphy, it has little clinical use in the initial stages of treatment.

In patients with cutaneous burns, the reduction in vital capacity and FEV₁ correlates closely with the extent of surface burns. Full resolution of pulmonary function test result abnormalities may take several months.

Some agents, particularly chlorine gas, may result in reactive airways syndrome, with subsequent development of airflow obstruction.