2.3 Pathophysiology of Supraglottic, Subglottic, and Inhalational Injury

2.3 Pathophysiology of Supraglottic, Subglottic, and Inhalational Injury

Core Principle: Inhalation injury is the single most significant independent predictor of mortality in thermal trauma, increasing the mortality of any given burn size by up to 20% to 60%. It comprises three distinct, anatomically and biologically divergent pathophysiological processes: (1) Supraglottic thermal injury, (2) Subglottic chemical tracheobronchitis, and (3) Systemic chemical asphyxiant toxicity.

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1. The Triad of Inhalation Injury

                                  ┌────────────────────────────────────────┐
                                  │      THE INHALATION INJURY TRIAD       │
                                  └───────────────────┬────────────────────┘
                                                      │
         ┌────────────────────────────────────────────┼────────────────────────────────────────────┐
         ▼                                            ▼                                            ▼
┌─────────────────────────────────┐          ┌─────────────────────────────────┐          ┌─────────────────────────────────┐
│ 1. Supraglottic Thermal Injury  │          │ 2. Subglottic Chemical Injury   │          │ 3. Systemic Asphyxiants         │
├─────────────────────────────────┤          ├─────────────────────────────────┤          ├─────────────────────────────────┤
│ • True thermal burn (dry air)   │          │ • Incomplete combustion toxins  │          │ • Carbon Monoxide (CO):         │
│ • Pharyngeal heat dissipation   │          │ • Acid / alkali mucosal lysis   │          │   - 200-250x Hb affinity        │
│ • Progressive laryngeal edema   │          │ • Mucociliary escalator loss    │          │   - Left shift oxy-Hb curve     │
│ • 2 to 24-hour occlusion risk   │          │ • Fibrin/cellular cast plugging │          │ • Hydrogen Cyanide (HCN):       │
│ • Worsened by fluid resuscitation│         │ • V/Q mismatch & high VAP risk  │          │   - Inhibits Cytochrome a3      │
└─────────────────────────────────┘          └─────────────────────────────────┘          └─────────────────────────────────┘

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2. Supraglottic Thermal Injury: Upper Airway Edema Mechanics

Dry air has a relatively low heat-carrying capacity (specific heat). As superheated air and flames are inhaled, the highly vascular structures of the nasopharynx, oropharynx, epiglottis, and supraglottic larynx act as an exceptionally efficient thermal heat sink, rapidly dissipating thermal energy before it can reach below the true vocal cords.

Pathophysiological Exceptions:

• Steam Inhalation: Steam has a heat-carrying capacity 4,000 times greater than dry air. Inhaled steam does not dissipate in the pharynx; it penetrates past the vocal cords into the subglottic space, causing direct, circumferential thermal burns throughout the tracheobronchial tree.
• Explosions / Flammable Gas Under Pressure: Rapid blast kinetics can force burning gases directly past the vocal cords before reflexive laryngeal closure occurs.

Evolution of Supraglottic Obstruction:

1. Thermal Denaturation: Direct burn causes blistering, erythema, and mucosal sloughing of the uvula, epiglottis, aryepiglottic folds, and false vocal cords.
2. Insidious Edema Accumulation: Capillary hyperpermeability in loose pharyngeal connective tissue causes progressive, dramatic swelling. Crucially, upper airway edema is time-dependent and resuscitation-dependent.
3. The 2 to 24-Hour Danger Window: A patient who initially speaks with a clear voice upon scene arrival may develop complete, catastrophic upper airway occlusion 6 to 18 hours later as aggressive crystalloid resuscitation accelerates soft tissue swelling.

Timeline of Upper Airway Compromise:
  Hour 0-2: Mild mucosal erythema, soot deposition, voice clear.
  Hour 4-8: Progressive epiglottic and arytenoid swelling; hoarseness, stridor, muffled ("hot potato") voice.
  Hour 8-18: Maximum soft tissue edema; obliteration of laryngeal aperture; catastrophic complete airway obstruction.

[!WARNING] Clinical Rule of Airway Management: Never wait for stridor or respiratory distress to intubate a patient with suspected supraglottic burn injury. Stridor signifies that ≥80% of the airway lumen is already occluded. Early, prophylactic endotracheal intubation with a large-caliber tube (≥7.5–8.0 mm ID to facilitate subsequent therapeutic bronchoscopy and pulmonary toilet) is mandatory before anatomical landmarks are obliterated.

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3. Subglottic Chemical Injury: Incomplete Combustion & Tracheobronchitis

While direct heat rarely penetrates below the vocal cords in dry air exposures, the chemical byproducts of incomplete combustion carried on microscopic soot particles (<5 µm) penetrate deeply into the tracheobronchial tree and terminal alveoli.

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4. Lower Airway Cellular Pathophysiology: Cast Formation, Atelectasis & Shunt

The cellular destruction in subglottic chemical injury initiates a sequence of events that severely compromises pulmonary gas exchange:

┌─────────────────────────────────────────────────────────────┐
│         Toxic Chemical Inhalation on Mucosal Surface        │
└──────────────────────────────┬──────────────────────────────┘
                               │
                               ▼
┌─────────────────────────────────────────────────────────────┐
│ • Complete loss of ciliated epithelial cells                │
│ • Arrest of the mucociliary escalator                       │
│ • Massive bronchial artery vasodilation & transudation      │
└──────────────────────────────┬──────────────────────────────┘
                               │
                               ▼
┌─────────────────────────────────────────────────────────────┐
│ Sloughed Necrotic Cells + Neutrophils + Fibrinogen Exudate  │
│               = OBSTRUCTIVE ENDOBRONCHIAL CASTS             │
└──────────────────────────────┬──────────────────────────────┘
                               │
        ┌──────────────────────┴──────────────────────┐
        ▼                                             ▼
┌──────────────────────────────┐            ┌──────────────────────────────┐
│ Complete Airway Plugging     │            │ Partial "Ball-Valve" Action  │
├──────────────────────────────┤            ├──────────────────────────────┤
│ • Segmental/lobar atelectasis│            │ • Air trapping / Hyper-      │
│ • True intrapulmonary shunt  │            │   inflation                  │
│ • Refractory hypoxemia       │            │ • Alveolar barotrauma /      │
│ • Bacterial pneumonia (VAP)  │            │   pneumothorax               │
└──────────────────────────────┘            └──────────────────────────────┘

1. Denudation of the Mucociliary Escalator: Toxic acids and alkalis cause rapid necrosis and sloughing of pseudostratified ciliated columnar epithelial cells. The natural clearance mechanism for secretions and debris is instantly paralyzed.
2. Bronchial Hyperperfusion: Injury stimulates a massive 10- to 20-fold increase in bronchial artery blood flow, mediated by nitric oxide, substance P, and calcitonin gene-related peptide (CGRP). This hyperperfusion drives intense microvascular transudation of plasma proteins (especially fibrinogen) into the airway lumen.
3. Endobronchial Cast Formation: Within 24 to 48 hours, sloughed epithelial sheets, activated neutrophils, copious viscous mucus, and polymerizing fibrin combine to form dense, rubbery, obstructive casts and pseudomembranes that mold to the bronchial tree.
4. Surfactant Inactivation: Direct chemical toxicity and alveolar flooding with plasma proteins inactivate pulmonary surfactant. Alveoli collapse, drastically increasing work of breathing and reducing lung compliance.
5. Intrapulmonary Shunting & V/Q Mismatch: Collapsed and plugged lung segments remain perfused by pulmonary capillaries, creating a large right-to-left intrapulmonary shunt ($Q_s/Q_t$) that is refractory to supplemental oxygen alone.
6. Bacterial Colonization & Pneumonia: Denuded basement membranes, stagnant necrotic debris, and impaired alveolar macrophage function provide an ideal culture medium. Over 50% of patients with severe inhalation injury develop secondary bacterial tracheobronchitis or ventilator-associated pneumonia (VAP), typically caused by Staphylococcus aureus or Pseudomonas aeruginosa.

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5. Systemic Chemical Asphyxiants: Carbon Monoxide vs. Hydrogen Cyanide

In closed-space structural fires, toxic systemic asphyxiants represent the leading cause of immediate, on-scene prehospital death.

                                  ┌────────────────────────────────────────┐
                                  │      SYSTEMIC CHEMICAL ASPHYXIANTS     │
                                  └───────────────────┬────────────────────┘
                                                      │
                         ┌────────────────────────────┴────────────────────────────┐
                         ▼                                                         ▼
┌────────────────────────────────────────────────────────┐        ┌────────────────────────────────────────────────────────┐
│ Carbon Monoxide (CO)                                   │        │ Hydrogen Cyanide (HCN)                                 │
├────────────────────────────────────────────────────────┤        ├────────────────────────────────────────────────────────┤
│ • Hemoglobin affinity: 200 - 250x > Oxygen             │        │ • Combusts: Polyurethane, wool, silk, synthetic resins │
│ • Forms: Carboxyhemoglobin (COHb)                      │        │ • Target: Inhibits Cytochrome Oxidase a3 (Complex IV)  │
│ • Shifts Oxy-Hb Curve: Marked LEFT shift (Haldane)     │        │ • Effect: Halts mitochondrial oxidative phosphorylation│
│ • Tissue O2 delivery: Severely impaired unloading      │        │ • Cellular state: Histotoxic hypoxia (aerobic failure) │
│ • Intracellular: Inhibits Cytochrome c Oxidase         │        │ • Lab hallmark: Extreme lactic acidosis (>8-10 mmol/L) │
│ • SpO2: Falsely NORMAL (absorbs identically at 660nm)  │        │ • SvO2 / ScvO2: Supranormal (tissues cannot extract O2)│
│ • Half-life: 240-320m (room air) -> 60-90m (100% O2)   │        │ • Specific Antidote: Hydroxocobalamin (Cyanokit)       │
└────────────────────────────────────────────────────────┘        └────────────────────────────────────────────────────────┘

Carbon Monoxide (CO) Pathophysiology:

• Mechanism: Carbon monoxide diffuses rapidly across the alveolar-capillary membrane and binds to the ferrous ($Fe^{2+}$) iron sites of hemoglobin with an affinity 200 to 250 times greater than oxygen, forming carboxyhemoglobin (COHb).
• Haldane Effect & Left-Shift: The binding of CO to one of the four hemoglobin heme sites alters the quaternary tetramer conformation, increasing the oxygen affinity of the remaining three sites. This causes a dramatic leftward shift of the oxyhemoglobin dissociation curve, severely hindering the release (unloading) of oxygen to hypoxic peripheral tissues.
• Mitochondrial & Myoglobin Binding: CO binds to intracellular myoglobin (impairing cardiac contractility) and inhibits cytochrome c oxidase in the electron transport chain, promoting free radical generation and delayed neurologic sequelae (DNS).
• The SpO2 Trap: Standard dual-wavelength pulse oximeters measure light absorption at 660 nm and 940 nm. Because oxyhemoglobin and carboxyhemoglobin have nearly identical absorption characteristics at 660 nm, standard pulse oximetry falsely reads COHb as normal oxyhemoglobin. A patient with 40% COHb may display an SpO2 of 99%.
• CO Clearance Kinetics:
  • Room Air ($FiO_2$ 0.21): Elimination half-life ($t_{1/2}$) is 240 to 320 minutes.
  • 100% Normobaric Oxygen ($FiO_2$ 1.0 via non-rebreather/ETT): $t_{1/2}$ drops to 60 to 90 minutes.
  • Hyperbaric Oxygen (HBO at 2.8–3.0 ATA): $t_{1/2}$ drops to 20 to 30 minutes.

Hydrogen Cyanide (HCN) Pathophysiology:

• Mechanism: Cyanide gas is generated by the thermal decomposition of nitrogen-containing polymers (plastics, melamine, polyurethane foam, carpets, synthetic textiles). Inhaled HCN enters the circulation and binds with high affinity to the ferric ($Fe^{3+}$) iron atom of cytochrome oxidase a3 (Complex IV) within the mitochondrial electron transport chain.
• Histotoxic Hypoxia: The electron transport chain is completely blocked. Cellular aerobic respiration ceases instantly, even though arterial blood is fully saturated with oxygen ($PaO_2$ and $SaO_2$ remain completely normal).
• Severe Lactic Acidosis: Unable to perform oxidative phosphorylation, cells shift entirely to anaerobic glycolysis, producing massive quantities of pyruvate that convert to lactic acid. Serum lactate levels characteristically surge to >8 to 10 mmol/L.
• Supranormal Venous Oxygen: Because the peripheral tissues cannot extract or consume oxygen, venous blood returning to the heart remains fully oxygenated. Central venous oxygen saturation ($ScvO_2$) and mixed venous oxygen saturation ($SvO_2$) are paradoxically elevated (>85–90%).
• Antidotal Reversal: Hydroxocobalamin (Cyanokit, 5.0 g IV infusion) contains a cobalt ion ($Co^{3+}$) that binds cyanide with higher affinity than cytochrome a3, forming non-toxic cyanocobalamin (Vitamin B12), which is safely excreted in the urine.