Hydrogen Sulfide (H₂S) Poisoning

Overview

• Occupational: petroleum refining, oil/gas industry, sewage/wastewater workers, manure pits, confined spaces
• Natural: geothermal/volcanic emissions
• Deliberate: suicide method — generated by mixing sulfur-containing detergents with acids in enclosed spaces (well-documented globally)

Pathophysiology

1. Cellular (cytotoxic) asphyxia — the primary lethal mechanism, identical to cyanide: H₂S binds to the Fe³⁺ of cytochrome c oxidase (Complex IV) in the mitochondrial electron transport chain, halting oxidative phosphorylation. ATP depletion forces anaerobic glycolysis → pyruvate → lactate accumulation → high anion-gap metabolic acidosis.
2. Pulmonary/mucosal irritation — H₂S is one of few chemical asphyxiants that also directly irritates mucous membranes (eyes, airways), causing conjunctivitis, rhinitis, and at high doses, acute lung injury / pulmonary edema.

H₂S (along with CN and CO) inhibits cytochrome-aa₃ oxidase (Complex IV), blocking the final electron transfer to O₂ and forcing lactate production — Rosen's Emergency Medicine

Olfactory Warning and Its Failure

• Olfactory fatigue: prolonged low-level exposure desensitizes olfactory receptors
• Acute olfactory paralysis: at high concentrations, olfaction is immediately overwhelmed — the smell vanishes precisely when danger is greatest

Clinical Features

• CNS: coma, seizures, agitation
• Cardiovascular: dysrhythmias, bradycardia, cardiovascular collapse
• Metabolic: high anion-gap metabolic acidosis, elevated lactate (>8 mmol/L is highly predictive)
• Pulmonary: delayed pulmonary edema in survivors of massive exposure
• Ocular: corneal injury ("gas eye") — keratoconjunctivitis, can be delayed

Diagnosis

• Clinical: primarily by exposure history and setting (occupational/enclosed space)
• ABG + co-oximetry: normal SpO₂ and ABG (unlike CO, pulse oximetry is accurate in isolated H₂S poisoning)
• Elevated lactate (>8 mmol/L supports cytotoxic asphyxiant poisoning)
• Anion-gap metabolic acidosis
• On-scene testing: field detectors; no widely available clinical blood H₂S assay
• Narrow A-V O₂ difference on co-oximetry is suggestive

Management

1. Scene Safety & Decontamination

• Rescuers must use self-contained breathing apparatus (SCBA) — do NOT enter without PPE
• Remove patient from source immediately
• Decontaminate skin and irrigate eyes

2. Supportive Care (all patients)

• High-flow 100% oxygen (primary treatment)
• Airway management, ventilatory support if needed
• For conscious patients with only mild symptoms, removal + O₂ is often sufficient

3. Antidotal Therapy (severe/unconscious patients)

Note: Amyl nitrite (inhaled) can be used pre-hospital as a MetHb inducer if IV access is unavailable.

4. Disposition

• All symptomatic patients: admit to ICU/critical care
• Monitor for delayed pulmonary edema, corneal injury, and neuropsychiatric sequelae
• Full recovery is achievable with rapid treatment, even in severe cases, provided cardiac arrest has not occurred

Key Differences: H₂S vs. Cyanide vs. CO

Poisonous Gases in Enclosed Spaces & Their Management

Classification of Toxic Gases

1. Carbon Monoxide (CO)

• CO binds hemoglobin with 230–270× greater affinity than O₂ → forms carboxyhemoglobin (COHb) → left-shifts the O₂-dissociation curve → impairs O₂ delivery
• Also binds cytochrome c oxidase → direct cellular poisoning
• Net effect: tissue hypoxia with deceptively normal PaO₂ on standard ABG

• "Cherry-red" skin — rarely seen clinically
• Pulse oximetry is falsely normal — COHb reads as oxyhemoglobin; co-oximetry required
• Lactic acidosis, rhabdomyolysis, pulmonary edema in severe cases
• Delayed neuropsychiatric syndrome: cognitive dysfunction, mood disturbance, memory loss — can appear days to 9 months after recovery

2. Hydrogen Sulfide (H₂S)

1. Cellular asphyxia: binds cytochrome c oxidase (Complex IV) — identical to cyanide, but the bond dissociates rapidly and spontaneously → patients with brief exposure often recover before ED arrival
2. Mucosal/pulmonary irritant: causes conjunctivitis, respiratory irritation, and at high dose, pulmonary edema + corneal injury

• High anion-gap metabolic acidosis + elevated lactate
• Delayed pulmonary edema and corneal burns in survivors

3. Hydrogen Cyanide (HCN)

• Headache, dyspnea, anxiety → seizures, coma, cardiovascular collapse
• "Arterialization" of venous blood (high venous O₂ saturation)
• High anion-gap metabolic acidosis; lactate >10 mmol/L correlates with toxic cyanide levels in fire victims
• Not cyanotic initially (hemoglobin O₂-carrying capacity unaffected)
• Bitter almond odor — present in only ~50% of people (genetic anosmia)

Critical: If CO co-exposure is suspected (fire victim), use hydroxocobalamin — do NOT use nitrites (adding MetHb to CO-poisoned patient worsens O₂ delivery catastrophically).

4. Chlorine (Cl₂)

• Immediate: Pungent odor (good warning), eye/throat irritation, nausea/vomiting (low exposure)
• High exposure: Cough, hoarseness, bronchospasm, pulmonary edema within 6 hours; ARDS possible

• Remove from source
• Humidified 100% O₂
• Nebulized bronchodilators (β-agonists ± anticholinergics)
• Nebulized sodium bicarbonate — may neutralize acids, symptom relief
• Ocular irrigation
• Supportive respiratory care; no proven pharmacologic antidote

5. Phosgene (COCl₂)

1. Immediate: Mild eye/throat irritation or none; characteristic "freshly mown hay" odor
2. Latent: Apparent improvement
3. Delayed (6–24 h): Dyspnea, pulmonary edema — onset within 4 hours = poor prognosis

• Mandatory rest — exertion dramatically worsens pulmonary edema
• O₂ only if SpO₂ <92% (avoid hyperoxia)
• Nebulized β-agonists, nebulized N-acetylcysteine, NSAIDs (animal/case report evidence)
• Early corticosteroids for lung injury
• Protective mechanical ventilation (low tidal volume, high PEEP) if intubation needed
• Observe all exposed patients ≥24 h even if initially asymptomatic

6. Nitrogen Dioxide (NO₂)

1. Initial: Mild dyspnea, flu-like symptoms
2. Transient improvement
3. Worsening dyspnea → pulmonary edema (12–72 h later)

• Supportive (humidified O₂, ventilatory support)
• Early corticosteroids — case reports show benefit for ALI
• Methylene blue if significant methemoglobinemia

7. Ammonia (NH₃)

• Humidified O₂, nebulized bronchodilators
• Anticholinergics for secretion control
• Ocular irrigation (alkaline burns); evaluate for corneal injury

General Management Principles for All Enclosed-Space Gas Exposures

Scene Response

1. Do NOT enter without appropriate respiratory protection (SCBA minimum)
2. Identify the gas type if possible (on-scene monitors, history, odor)
3. Evacuate all exposed persons

Emergency Management Algorithm

ALL EXPOSURES:
  ├── Remove from source
  ├── High-flow O₂ (100%)
  ├── Decontaminate skin/eyes as needed
  ├── Airway assessment (singed hair, hoarseness, soot = early intubation)
  └── IV access + monitoring (ECG, SpO₂, ABG, COHb, lactate)

CHEMICAL ASPHYXIANT (CO/HCN/H₂S suspected)?
  ├── CO → 100% O₂ ± HBO
  ├── HCN → Hydroxocobalamin 5 g IV (first-line)
  │         If no CO co-exposure: cyanide antidote kit acceptable
  └── H₂S → 100% O₂; sodium nitrite if severe + within minutes

PULMONARY IRRITANT (Cl₂/phosgene/NO₂/NH₃)?
  ├── O₂, humidification, bronchodilators
  ├── Bicarbonate nebulization (Cl₂, NH₃)
  ├── Corticosteroids (NO₂, phosgene)
  └── Observe ≥24 h (phosgene/NO₂ — delayed edema)

Lab/Diagnostic Workup

• Co-oximetry: COHb, MetHb levels (pulse oximetry is unreliable for CO/MetHb)
• ABG + lactate: Lactate >8–10 mmol/L → chemical asphyxiant until proven otherwise
• ECG: Arrhythmias, ischemia (CO, cyanide)
• Chest X-ray: Pulmonary edema, infiltrates
• Whole-blood cyanide: Confirmatory, not for acute management decisions

Disposition