Thermal Airway Injury

Thermal airway injury is damage to the upper airway (and rarely the lower airway) from inhaled hot gas, steam or flame. The upper airway absorbs heat, so injury is mostly over the vocal cords in the supraglottic airway, but steam can injure the lower airway. It often coexists with smoke inhalation and carbon monoxide or cyanide poisoning.

Epidemiology. Inhalation injury occurs in about 10 to 20% of burn admissions and raises mortality significantly. It is common in enclosed space fires.

Causes and associations. Enclosed space fires, explosions, steam, hot liquids, flash burns and industrial accidents.

Pathophysiology. Heat injury causes mucosal oedema peaking at 12 to 36 hours, erythema, ulceration and sloughing. Airway obstruction can develop quickly, made worse by fluid resuscitation. Smoke inhalation adds chemical injury, bronchospasm, sloughing, small airway obstruction, infection and ARDS.

Presentation includes facial burns, singed nasal hair or eyebrows, soot in the mouth, nose or sputum, hoarseness, cough, stridor, wheeze, dyspnoea, drooling, dysphagia, oropharyngeal blistering and altered consciousness.

Red flags.

• Stridor, hoarse voice, drooling or inability to swallow.
• Burns involving the face, neck or mouth.
• Deep burns of the neck with circumferential constriction.
• Enclosed space exposure, altered consciousness or carboxyhaemoglobin raised.

Direct laryngoscopy or flexible nasendoscopy shows the airway oedema, erythema and soot. Bronchoscopy grades lower airway injury. Measure carboxyhaemoglobin by co-oximetry (oximetry readings are falsely normal), lactate (over 10 mmol/L suggests cyanide poisoning), arterial gas and chest radiograph (often normal early). Calculate burn area and depth.

Differential diagnosis. Chemical inhalation injury, anaphylaxis, epiglottitis, foreign body and carbon monoxide poisoning alone.

Airway. Intubate early, before swelling, for any of: stridor, hoarseness, deep facial or neck burns, oropharyngeal burns with soot, respiratory distress, GCS 8 or less, or major burn needing large fluid volumes with airway signs. Use the largest tube possible (7.5 mm or more for adults to permit bronchoscopy), secure it without ties that compress swollen skin, and have difficult airway equipment ready. Extubate only when the cuff leak returns and swelling has settled.

Oxygen and toxins.

• 100% oxygen by non-rebreather mask or ventilator for suspected carbon monoxide poisoning until carboxyhaemoglobin is under 5% (hyperbaric oxygen is considered for severe cases, pregnancy or neurological signs).
• Hydroxocobalamin 5 g IV over 15 minutes for suspected cyanide poisoning (lactate over 8 to 10 mmol/L, coma or shock), repeat 5 g once if needed. Children: 70 mg/kg. Can interfere with co-oximetry and cause red skin and urine.

Ventilation. Lung protective ventilation (6 mL/kg predicted body weight, plateau under 30 cmH2O), bronchoscopic removal of casts and secretions, and bronchodilators (salbutamol 2.5 to 5 mg nebulised every 4 hours).

Nebulised therapy (specialist practice). Heparin 5000 units nebulised every 4 hours with acetylcysteine 3 mL of 20% nebulised every 4 hours for up to 7 days, to reduce cast formation, with monitoring of coagulation.

Fluids for burns. In burns over 15 to 20% of body surface area, give compound sodium lactate solution at 2 to 4 mL/kg for each percent of burn in the first 24 hours, half over the first 8 hours, and titrate to urine output of 0.5 mL/kg/h (1 mL/kg/h in children under 30 kg). Inhalation injury raises requirements, but avoid over resuscitation.

Not advised. Prophylactic antibiotics and corticosteroids. Treat tracheobronchitis and pneumonia on culture and clinical evidence.

Escharotomy. For circumferential neck or chest burns with restricted ventilation.

Monitoring. Airway patency and cuff leak, oxygen saturation, arterial gas, carboxyhaemoglobin, lactate, urine output and bronchoscopy findings.

Referral and escalation. Transfer to a burns unit for inhalation injury, facial burns, burns over 10% of body surface area and children. Involve ENT and anaesthesia early.

Special groups. In children, the airway is narrow, so intubate early and use a lower threshold. In pregnancy, give oxygen at high concentration for carbon monoxide, since foetal haemoglobin binds carbon monoxide strongly.