# Inhalation Injury in Burns — ABA 2016

## Overview

Inhalation injury is present in approximately **10-20%** of patients admitted to burn centers and is a major independent predictor of mortality. The combination of inhalation injury and cutaneous burns significantly increases mortality compared to either injury alone.

## Diagnosis

### Clinical Suspicion

Suspect inhalation injury in any patient with:

- **History of closed-space fire exposure** (house fire, vehicle fire, enclosed space with smoke)
- **Facial burns** (especially perioral, perinasal)
- **Singed nasal hairs, eyebrows, or facial hair**
- **Soot in the oropharynx, nares, or sputum** (carbonaceous sputum)
- **Hoarseness, stridor, or voice changes**
- **Wheezing or respiratory distress**
- **Altered mental status** (may indicate carbon monoxide or cyanide poisoning)

### Diagnostic Evaluation

| Test | Role | Key Thresholds |
|---|---|---|
| **Carboxyhemoglobin (COHb) level** | Assess CO exposure severity | Normal < 3% (non-smokers), < 10% (smokers); symptomatic > 15%; severe > 25%; lethal risk > 50-60% |
| **Arterial blood gas (ABG)** | Assess oxygenation, ventilation, acid-base status | Note: PaO2 and SpO2 may be falsely normal with CO poisoning |
| **Fiberoptic bronchoscopy** | Gold standard for diagnosis of airway injury below the glottis | Findings: edema, erythema, carbonaceous deposits, mucosal sloughing, ulceration |
| **Chest radiograph** | Baseline; typically normal initially | Abnormalities (infiltrates, ARDS pattern) develop over 24-48 hours |
| **Lactate** | Assess for cyanide toxicity (concurrent with CO) | Elevated lactate with metabolic acidosis suggests cyanide poisoning |

### Diagnostic Decision Algorithm

```
Patient with suspected inhalation injury
  -> Immediate: Check COHb level, ABG, lactate
  -> Place on 100% FiO2 via non-rebreather mask
  -> Assess airway patency
      -> Stridor, severe hoarseness, visible pharyngeal edema?
          -> Immediate endotracheal intubation (do not delay)
      -> No acute airway compromise?
          -> Perform fiberoptic bronchoscopy (when available and patient stable)
          -> Observe closely for 12-24 hours for delayed airway edema
  -> COHb > 15%?
      -> Treat for carbon monoxide poisoning (see below)
  -> Elevated lactate + metabolic acidosis with smoke exposure?
      -> Consider empiric cyanide antidote treatment
```

## Airway Management

### Indications for Intubation

Intubation should be performed when airway patency is jeopardized. In the context of burn/inhalation injury, specific indications include:

- **Stridor or progressive hoarseness**
- **Visible oropharyngeal or supraglottic edema** on examination or bronchoscopy
- **Large TBSA burns (> 40%)** with anticipated massive facial/airway edema from fluid resuscitation
- **Altered mental status** (GCS < 8)
- **Respiratory failure** (PaO2 < 60 mmHg, PaCO2 > 50 mmHg on supplemental oxygen)
- **Deep circumferential neck burns** with impending airway compromise

### Intubation Principles

- **Intubate early** when in doubt -- delayed intubation in a patient with progressive airway edema is far more dangerous than early intubation
- Use the **largest endotracheal tube** that can be safely placed (facilitates subsequent bronchoscopy and pulmonary toilet)
- Secure the ETT with **tape or cotton ties** (standard ETT holders may not adhere to burned skin)
- **Avoid nasotracheal intubation** in patients with facial burns (risk of sinusitis, necrosis)

### Observation Protocol (No Immediate Intubation)

If the airway is patent and the patient does not meet intubation criteria:

- Observe for a **minimum of 12-24 hours** after smoke exposure
- Serial assessments of **voice quality, respiratory effort, oxygen saturation**
- Keep intubation equipment at bedside
- Be aware that airway edema typically **peaks at 12-24 hours** post-injury and may worsen with fluid resuscitation

## Carbon Monoxide Poisoning

Carbon monoxide (CO) has an affinity for hemoglobin approximately **200-250 times** greater than oxygen, forming carboxyhemoglobin (COHb) and causing tissue hypoxia.

### Treatment

| COHb Level | Symptoms | Treatment |
|---|---|---|
| **< 15%** | Headache, nausea | 100% O2 via NRB mask; monitor |
| **15-25%** | Confusion, dizziness, visual disturbances | 100% O2 via NRB mask; consider intubation if altered |
| **25-40%** | Ataxia, syncope, tachycardia | 100% O2; strongly consider intubation and mechanical ventilation |
| **> 40%** | Seizures, coma, cardiovascular collapse | 100% O2 via ETT; consider hyperbaric oxygen therapy (HBO) |

### Key Principles

- **100% FiO2** is the immediate treatment for all patients with suspected CO poisoning
- CO half-life: **~320 minutes** on room air, **~80 minutes** on 100% FiO2, **~23 minutes** with hyperbaric oxygen (HBO) at 3 ATA
- Continue 100% FiO2 until COHb level **falls below 5%**
- **Standard pulse oximetry (SpO2) is unreliable** in CO poisoning -- it cannot distinguish between oxyhemoglobin and carboxyhemoglobin. Use **co-oximetry** for accurate COHb measurement.

### Hyperbaric Oxygen Therapy (HBO)

Consider HBO for:
- COHb > 25%
- Loss of consciousness at any time
- Neurological symptoms (confusion, ataxia, seizures)
- Myocardial ischemia or arrhythmias
- Pregnant patients with COHb > 15%

HBO should not delay other critical burn care interventions (fluid resuscitation, airway management).

## Cyanide Poisoning

Cyanide (HCN) is produced from combustion of synthetic materials (plastics, synthetic fabrics, polyurethane). Concurrent cyanide and CO poisoning is common in enclosed-space fires.

### Clinical Suspicion

- Closed-space fire exposure with **altered mental status disproportionate to COHb level**
- **Persistent metabolic acidosis** (lactic acidosis) despite adequate oxygenation and resuscitation
- **Elevated lactate > 10 mmol/L** in the setting of smoke inhalation

### Treatment

| Agent | Dose | Notes |
|---|---|---|
| **Hydroxocobalamin (Cyanokit)** | 5 g IV over 15 minutes (adults); may repeat once | Preferred antidote; binds cyanide to form cyanocobalamin (vitamin B12); does not impair oxygen carrying capacity |
| **Sodium thiosulfate** | 12.5 g IV | Alternative; slower onset; may be combined with hydroxocobalamin |

- **Hydroxocobalamin is the preferred antidote** in the setting of concurrent CO poisoning because it does not worsen oxygen delivery (unlike the methemoglobin-forming cyanide antidote kit)
- **Empiric treatment** is recommended when clinical suspicion is high; do not wait for confirmatory cyanide levels (results take too long for clinical utility)

## Pulmonary Management

### Mechanical Ventilation

For intubated patients with inhalation injury:

- **Lung-protective ventilation:** Tidal volume 6-8 mL/kg predicted body weight
- **PEEP:** 5-10 cmH2O; titrate to oxygenation (avoid atelectasis)
- **Permissive hypercapnia** is acceptable (target pH > 7.20) to minimize ventilator-induced lung injury
- **Plateau pressure:** Target < 30 cmH2O

### Nebulized Therapies

| Agent | Dose/Frequency | Purpose |
|---|---|---|
| **Nebulized heparin** | 5,000-10,000 units q4h | Reduces airway cast formation; improves airway patency |
| **N-acetylcysteine (NAC)** | 3 mL of 20% solution q4h | Mucolytic; reduces airway plugging; antioxidant |
| **Nebulized albuterol (salbutamol)** | 2.5 mg q4h or PRN | Bronchodilator for bronchospasm |

- These agents are often administered in an **alternating protocol** (heparin and NAC alternating every 2 hours)
- Aggressive **pulmonary toilet** (suctioning, chest physiotherapy) is essential

### Monitoring

- Serial **chest radiographs** (initial may be normal; changes develop over 24-72 hours)
- Serial **bronchoscopy** may be performed to assess injury progression, clear casts, and guide management
- Monitor for development of **ARDS** (PaO2/FiO2 < 300), which may occur 24-72 hours post-injury

## Special Considerations

### Increased Fluid Requirements

Inhalation injury increases resuscitation fluid requirements by approximately **30-40%** above predicted volumes based on TBSA alone. Monitor closely for both under-resuscitation and over-resuscitation.

### Tracheostomy

- Consider **early tracheostomy** in patients expected to require prolonged mechanical ventilation (> 14 days)
- Tracheostomy through burned skin is acceptable when necessary but increases infection risk; meticulous wound care at the stoma site is essential

## Limitations

- Bronchoscopic grading of inhalation injury severity lacks a universally accepted standardized scoring system, limiting inter-institutional comparability.
- The optimal role and timing of hyperbaric oxygen therapy for CO poisoning remains debated; access to HBO facilities may be limited.
- Nebulized heparin/NAC protocols are based on limited randomized controlled trial data; most evidence is from observational studies and expert consensus.
- Cyanide blood levels are not available in real-time at most institutions; treatment must be based on clinical suspicion rather than confirmatory testing.
