Why is N-acetylcysteine (NAC) given in airway burns?

N-acetylcysteine in Airway Burns: Mechanism and Clinical Application

N-acetylcysteine (NAC) is primarily used in airway burns for its mucolytic properties, breaking disulfide bonds in mucus to decrease viscosity and improve airway clearance, though evidence for improved clinical outcomes in burn patients is limited.

Mechanism of Action

• Mucolytic Effect: NAC contains a sulfhydryl group that "opens" disulfide linkages in mucoproteins, effectively reducing mucus viscosity 1
• This mucolytic action helps liquefy viscid or inspissated mucous secretions that can obstruct airways following thermal injury
• The mucolytic activity of NAC increases with increasing pH, with significant mucolysis occurring between pH 7 and 9 1
• NAC's action is unaltered by the presence of DNA in secretions, which increases with purulence 1

Clinical Indications in Airway Burns

NAC is indicated as adjunctive therapy for patients with:

• Abnormal, viscid, or inspissated mucous secretions following inhalation injury 1
• Post-traumatic chest conditions 1
• Atelectasis due to mucous obstruction, which can occur after smoke inhalation 1

Administration and Precautions

• NAC is typically administered via nebulization in burn patients with inhalation injuries
• Important warning: After proper administration, an increased volume of liquefied bronchial secretions may occur, requiring:
  • Maintenance of open airway through mechanical suction when cough is inadequate 1
  • Endotracheal aspiration with or without bronchoscopy when mechanical blockage occurs 1
• Bronchospasm risk: Patients may develop increased airways obstruction of unpredictable severity
  • Patients cannot be identified a priori as potential reactors 1
  • Even patients who previously tolerated NAC may react to subsequent treatments 1
  • Most bronchospasm cases are quickly relieved with nebulized bronchodilators 1
  • Treatment should be discontinued immediately if bronchospasm progresses 1

Evidence for Use in Burn Patients

Despite its theoretical benefits, recent evidence questions NAC's efficacy in burn patients:

• A recent study of 40 patients with grade II-III inhalation injuries found that a protocol using nebulized heparin, NAC, and albuterol did not:
  • Shorten ventilator days (5 vs 6 days, p=0.76)
  • Improve survival rates (57.9% vs 52.4%, p=0.73) 2

Additional Properties of NAC

Beyond its mucolytic effects, NAC also provides:

• Antioxidant properties - acts as a free radical scavenger 3, 4
• Anti-inflammatory effects - may help reduce inflammatory markers in acute lung injury 3
• Low toxicity profile with rare side effects 3

Clinical Considerations

• NAC is generally well-tolerated but may cause gastrointestinal side effects (nausea, vomiting, diarrhea) 5
• Proper cleaning of nebulizer equipment is essential to prevent contamination 6
• NAC should be used with caution in respiratory conditions, and patients should be monitored for bronchospasm 6, 1
• When used for airway burns, NAC should be combined with appropriate respiratory support and monitoring

Conclusion

While NAC continues to be used in airway burn management primarily for its mucolytic properties, clinicians should be aware of its limitations and the need for larger, randomized controlled trials to definitively establish its efficacy in this specific patient population.