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Smoke Inhalation: Differential Diagnoses & Workup

Author: Keith A Lafferty, MD, Adjunct Assistant Professor of Emergency Medicine, Temple University; Consulting Staff, Department of Emergency Medicine, South West Regional Medical Center
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Updated: Jul 7, 2008

Differential Diagnoses

Workup

Laboratory Studies

• Electrolyte testing can identify an anion gap acidosis.
• Elevated lactate levels may be a source of metabolic acidosis secondary to CN, methemoglobinemia, CO, or hypoxia. Lactate levels higher than 10 mmol/L are a sensitive indicator of CN levels higher than 1 mg/mg; therefore, they should be treated as such. Note that in most institutions, CN levels can take hours to days for results; therefore, one must rely on clinical and indirect laboratory data.
• BUN and creatinine levels should be obtained for baseline renal function in patients in shock or rhabdomyolysis. Patients with large cutaneous burns, crush injuries, or prolonged immobilization should have their serum creatine kinase (CK) checked and, if appropriate, urine myoglobin.
• Thermal degradation products of various compounds, including phosphorous-based fire retardants, are capable of impairing cholinesterase activity. A prospective study measured serum erythrocyte cholinesterase activity at the scene of residential fires for 49 victims. A significant lower level of cholinesterase activity was noted in these patients as compared to controls. Obviously, further investigation into the clinical significance of this lower enzymatic activity is needed before it can be used clinically.
• The pulse oximeter can be misleading in the setting of CO exposure or methemoglobinemia because it uses only 2 wavelengths of light (the red and the infrared spectrum), which detect oxygenated and deoxygenated hemoglobin (Hb) only and not any other form of Hb. Cooximeters transmit 4 wavelengths of light through a blood sample and are capable of detecting methemoglobin and Hb-CO (in addition to Hb and oxyhemoglobin [HbO₂]).
  • Be aware that, on routine blood gas analysis, the percent saturation of Hb is calculated from the alveolar-arterial difference in partial pressure of oxygen (PaO₂), which can give a falsely elevated saturation. The difference between saturations obtained by cooximetry and calculated figures is known as the saturation gap and is an indicator that a dyshemoglobinemia is present.
  • Finally, light reflection in methemoglobinemia is similar to that in reduced Hb, and a depressed saturation may be shown on pulse oximetry, but the decrease does not accurately reflect the level of methemoglobinemia. In fact, as levels reach 30% or higher, the pulse oximeter does not go below 85%.
• Lead-containing paint is common in structures built before 1977, and this element can become aerosolized and absorbed directly into the bloodstream from the lungs. While it is true that severe smoke inhalation has been shown to increase serum lead levels more than 2-fold, no evidence suggests that these elevations are clinically relevant.

Imaging Studies

• Chest radiography
  • Obtain chest x-ray films (CXRs) in patients with a history of significant exposure or pulmonary symptoms.
  • Most x-ray film findings are normal after smoke inhalation; initial CXR is only 8% sensitive for smoke inhalation.
  • Findings may include atelectasis, pulmonary edema, and acute respiratory distress syndrome (ARDS).
  • Insensitivity of the CXR and lack of reliability of clinical signs of inhalation injury may necessitate use of other diagnostic techniques.
• Xenon ventilation-perfusion scan
  • As even bronchoscopic examination may fail to detect injury caused by inhalation of fine particulate aerosol material that may reach terminal bronchioles, consider xenon ventilation-perfusion scans in any patient suspected of having an inhalation injury even if bronchoscopic examination has been negative. This is because bronchoscopy does not evaluate the lower airways and, although 90% of particles measuring 5-10 microns in diameter impact in the upper airways, those measuring 0.5-3 microns reach the terminal bronchioles. In fact, particles this size may escape some filtration devices worn by firefighters.
  • Unequal lung field radiation density and/or retention of the radiolabeled gas in the lung field for longer than 90 seconds constitutes a positive scan.
  • Although the accuracy is reported as 87%, xenon ventilation-perfusion scan lacks specificity in patients with preexisting pulmonary disease.
  • This test may be more appropriate for use in a burn unit or intensive care unit rather than the ED.
• CT of the brain
  • CT of the brain may show signs of cerebral infarction due to hypoxia, ischemia, and hypotension.
  • An interesting and well-reported finding for severe CO toxicity is bilateral globus pallidus low-density lesions. These lesions may be delayed for up to several days.
  • This finding is highly specific for CO insult unlike focal cortical hypoperfusion, which is nonspecific.

Other Tests

• Perform electrocardiogram (ECG) in any patient presenting with smoke inhalation. Potential for decreased oxygen delivery from asphyxiation, dyshemoglobinemia, and cessation of electron transport system can result in myocardial ischemia.
• Pulmonary function test results are abnormal soon after inhalation injuries.
  • In atelectasis, consolidation, and ARDS, vital capacity, pulmonary compliance, and functional residual capacity are reduced.
  • In patients with bronchospasm, forced expiratory volume in 1 second (FEV₁), peak flow, and midexpiratory flow rates are reduced.
  • Diagnostic accuracy is 91%.

Procedures

• Bronchoscopy can be diagnostic as well as therapeutic, particularly when lobar atelectasis is present.
  • This procedure examines the airways from the oropharynx to the lobar bronchi.
  • Although it may be performed in the ED, the burn unit may be a more appropriate setting, especially in patients who are intubated.
  • Erythema, charring, deposition of soot, edema, and/or mucosal ulceration may be present.
  • Impending airway obstruction may be inferred and intubation may be facilitated by this technique.
  • Diagnostic accuracy is reported to be 86%.
  • Studies have shown up to a 96% correlation between bronchoscopic findings and the triad of closed-space smoke exposure, HbCO levels of 10% or greater, and carbonaceous sputum.
  • Another study reports that serial bronchoscopy was twice as sensitive for diagnosing inhalation injury as clinical findings alone.

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