Inhalation injury

Overview

Inhalation injury remains one of the strongest independent predictors of mortality in burn patients. The injury produces three distinct pathophysiologic patterns: supraglottic thermal injury causing rapid airway edema, subglottic chemical injury from combustion products leading to cast formation and mucus plugging, and systemic toxicity from carbon monoxide and cyanide absorption ^[5][12]. The tracheobronchial tree is injured by steam and toxic chemicals, producing bronchoconstriction, mucosal sloughing, and inflammatory mediator release. Lung parenchyma sustains secondary damage through proteolytic elastase release, increased transvascular fluid flux, surfactant depletion, and progressive pulmonary edema and atelectasis ^[12].

The airway challenge is biphasic. Supraglottic edema can obstruct the airway within hours of injury, while subglottic chemical injury evolves over days with progressive respiratory failure. Missing the window for early intubation in the setting of progressive edema converts a manageable airway into a surgical emergency ^[3][6]. Inhalation injury also increases acute fluid resuscitation requirements. Endorf and Gamelli demonstrated that patients with a P:F ratio less than 350 at presentation required significantly more resuscitation fluid (ml/kg/%TBSA) than those with preserved gas exchange, independent of bronchoscopic grade ^[9].

Classification

Inhalation injury is classified into three subtypes based on the anatomic level of injury ^[5]:

• Supraglottic (upper airway): Thermal injury to structures above the vocal cords, producing edema that can cause rapid airway obstruction.
• Subglottic (tracheobronchial): Chemical injury from inhaled combustion products below the vocal cords, leading to mucosal sloughing, cast formation, and airway obstruction.
• Systemic: Carbon monoxide and cyanide poisoning from inhaled gases, affecting oxygen delivery and cellular respiration.

Each subtype requires different diagnostic and management approaches. No globally accepted grading system for inhalation injury exists, though bronchoscopic grading is the most widely used method for subglottic injury ^[3][4].

Assessment

Clinical assessment

Clinical signs warranting early intubation include stridor, shortness of breath, facial burns, singed nasal hairs, cough, soot in the oral cavity, and history of enclosed-space fire exposure ^[6]. The absence of these signs does not exclude inhalation injury. A high index of suspicion is required for any patient with enclosed-space exposure, even without visible burns ^[12][13].

Bronchoscopic grading

Bronchoscopy provides definitive diagnosis and grading of subglottic inhalation injury and is recommended for all patients with suspected lower airway involvement ^[3][4]. Endorf and Gamelli used a five-grade system (0-4) based on Abbreviated Injury Score (AIS) criteria ^[9]:

• Grade 0: No injury. Normal-appearing airway.
• Grade 1: Mild. Minor erythema or edema of the mucosa.
• Grade 2: Moderate. Moderate erythema, carbonaceous deposits, bronchorrhea, with or without bronchial compromise.
• Grade 3: Severe. Severe inflammation with friability, copious carbonaceous deposits, obliteration of airway detail.
• Grade 4: Massive. Mucosal sloughing, necrosis, endoluminal obliteration.

Grades 2-4 carry significantly worse survival than grades 0-1 ^[9]. An international RAND/UCLA expert panel rated initial bronchoscopic lavage and serial lavage for severe injury as appropriate interventions ^[10]. The panel also noted that bronchoscopic grading accuracy remains an area of uncertainty requiring further study ^[10].

Laboratory and imaging markers

• Carboxyhemoglobin (COHb): Levels above 10% support CO exposure; levels above 20-25% indicate significant poisoning. Levels may be misleadingly low if high-flow oxygen was administered in the field ^[5][13].
• Lactate: Elevated lactate in the setting of fire exposure raises suspicion for cyanide toxicity or CO-mediated impaired oxygen delivery ^[12].
• P:F ratio: An initial P:F ratio less than 350 predicts increased acute fluid requirements and may be a more useful predictor of resuscitation needs than bronchoscopic grade alone ^[9].
• CT chest: May demonstrate airway edema, ground-glass opacities, or bronchial wall thickening but is not routinely obtained in the acute setting. Its role is complementary rather than primary ^[4][13].

Management

Airway management

• Supraglottic injury: Early intubation when clinical signs of upper airway involvement are present. Do not wait for deterioration ^[6][7]. The RAND/UCLA expert panel rated intubation with an endotracheal tube of 8.0 mm or greater internal diameter as appropriate to allow therapeutic bronchoscopy ^[10].
• Timing: Airway edema peaks 12-24 hours after injury. The intubation decision is made in the first hours; if clinical signs are equivocal, serial reassessment every 1-2 hours is mandatory ^[6][13].

Nebulized therapy protocol

Multimodal nebulized therapy targeting fibrin cast formation and mucus plugging is the standard subglottic treatment ^[3][8][10]:

• Nebulized heparin (5,000 units) every 4 hours for the first 7 days: Targets fibrin cast formation. Desai et al. demonstrated significant reduction in reintubation rates, atelectasis, and mortality in pediatric patients treated with aerosolized heparin and NAC compared to controls ^[8].
• Nebulized N-acetylcysteine (3 ml of 20% solution) every 4 hours: Mucolytic agent targeting mucus casts ^[8].
• Nebulized bronchodilators (albuterol): For bronchospasm ^[3][10].

The RAND/UCLA expert panel rated nebulized heparin and albuterol as appropriate for moderate-severe injury, and NAC as appropriate for moderate injury. For mild injury, expert opinion was divided ^[10].

Ventilator strategies

Lung-protective ventilation is the standard approach ^[10][11]:

• Tidal volume: 6-8 ml/kg predicted body weight. A systematic review by Glas et al. showed tidal volumes in burn patients have declined from 14 ml/kg in older studies to approximately 8 ml/kg in studies after 2006, mirroring practice changes in non-burn critical care ^[11].
• Plateau pressure: Limit to 30 cmH2O or less. Peak inspiratory pressures above 35 cmH2O were historically common in burn patients and associated with higher barotrauma rates ^[11].
• PEEP: Titrate to oxygenation. Low PEEP levels (below 10 cmH2O) were used in 70% of studies in the systematic review ^[11].
• The RAND/UCLA expert panel rated non-protective ventilatory strategies, high-frequency oscillatory ventilation (HFOV), and high-frequency percussive ventilation (HFPV) as inappropriate ^[10]. Airway pressure release ventilation (APRV) was rated uncertain with expert disagreement ^[10].

Inhalation injury presents specific ventilator challenges distinct from generic ARDS management: cast formation causes acute large-airway obstruction requiring bronchoscopic clearance rather than ventilator adjustments, and airway resistance changes can be abrupt and position-dependent.

Pulmonary toilet

Aggressive pulmonary toilet is essential and includes therapeutic coughing, chest physiotherapy, deep breathing exercises, frequent suctioning, and early ambulation ^[12]. Serial therapeutic bronchoscopy for cast removal should be considered in moderate-severe injury ^[10].

Systemic toxicity

• Carbon monoxide poisoning: 100% oxygen administration. CO poisoning is the most common cause of death in inhalation injury ^[5]. Half-life of COHb decreases from approximately 4-5 hours on room air to 40-80 minutes on 100% FiO2 ^[12].
• Cyanide toxicity: Hydroxocobalamin administration. A retrospective cohort study of 68 patients found no significant differences in methemoglobin levels, lactate, mortality, or kidney function between treated and untreated groups, supporting its safety profile ^[2]. The RAND/UCLA expert panel rated prophylactic systemic antibiotics and corticosteroids as inappropriate ^[10].

Resuscitation considerations

Inhalation injury increases acute fluid requirements beyond what burn size alone predicts. Endorf and Gamelli showed that an initial P:F ratio below 350 was a better predictor of increased fluid needs than bronchoscopic grade ^[9]. Clinicians should anticipate exceeding Parkland formula estimates in patients with concomitant inhalation injury and titrate to urine output.

Outcomes

Inhalation injury patients have significantly higher rates of outpatient pulmonary sequelae compared to nonventilated controls (21% vs 4%, P = .023) ^[1]. These patients develop pulmonary complaints significantly earlier after discharge than ventilated controls without inhalation injury (mean 162 days vs 513 days, P = .024) ^[1]. These findings support structured outpatient pulmonary follow-up in this population.

Controversies and Evidence Gaps

Management data are predominantly observational. No globally accepted grading system or standardized treatment protocol for inhalation injury exists. The RAND/UCLA expert panel found consensus on many diagnostic and management strategies but identified key areas of uncertainty: bronchoscopic grading accuracy, the value of lavage across severity groups, and the effectiveness of nebulized therapies in mild injury ^[10]. Key clinical controversies include:

• Prophylactic intubation vs. watchful waiting: No randomized data guide this decision. Practice varies by center.
• Bronchoscopic grading driving clinical decisions: Endorf and Gamelli showed that P:F ratio may be more clinically useful than bronchoscopic grade for predicting fluid requirements ^[9], questioning whether grading alone should drive management intensity.
• Nebulized therapy in mild injury: Expert opinion is divided on whether the heparin/NAC protocol is indicated for grade 1 injury ^[10].
• Ventilator mode selection: APRV remains contentious, with expert disagreement ^[10].
• Long-term follow-up protocols: The long-term pulmonary outcomes study was retrospective and single-center with a modest sample (33 inhalation injury patients), limiting generalizability ^[1]. Optimal screening and follow-up intervals are undefined.
• Hydroxocobalamin evidence: The hydroxocobalamin study was also retrospective with a small sample, and the rarity of confirmed cyanide toxicity cases limits statistical power ^[2].

References

[1] Wier et al. (2023). Long-Term Pulmonary Sequelae After Inhalation Injury: A Retrospective Case-Control Study. PMID: 34216469 [2] Stanton et al. (2024). Hydroxocobalamin is not associated with methemoglobinemia in patients with inhalation injury and suspected cyanide toxicity and a proposed algorithm for hydroxocobalamin administration. PMID: 38760187 [3] Foncerrada et al. (2018). Inhalation Injury in the Burned Patient. PMID: 29461292 [4] Putz et al. (2016). Tracheal damage. PMID: 29558579 [5] Heimbach et al. (1988). Inhalation injuries. PMID: 3057948 [6] Vivo et al. (2016). Initial evaluation and management of the critical burn patient. PMID: 26724246 [7] Harvey et al. (1984). Emergent burn care. PMID: 6367073 [8] Desai et al. (1998). Reduction in mortality in pediatric patients with inhalation injury with aerosolized heparin/N-acetylcysteine therapy. PMID: 9622463 [9] Endorf & Gamelli (2007). Inhalation injury, pulmonary perturbations, and fluid resuscitation. PMID: 17211205 [10] Milton-Jones, Singh et al. (2023). An international RAND/UCLA expert panel to determine the optimal diagnosis and management of burn inhalation injury. PMID: 38012797 [11] Glas et al. (2019). Changes in ventilator settings and ventilation-induced lung injury in burn patients: a systematic review. PMID: 31202528 [12] Gupta et al. (2018). Smoke Inhalation Injury: Etiopathogenesis, Diagnosis, and Management. PMID: 29657376 [13] Haruta & Mandell (2023). Assessment and Management of Acute Burn Injuries. PMID: 37806692