STEPTRACKERPHARMACOLOGY + TOXICOLOGY | TOXICOLOGY | HIGH-YIELD REVIEW

Specific Poisonings

Gases that block oxygen use, metals that bind sulfhydryl groups, and household or environmental toxins each leave a recognizable fingerprint and have a matching treatment.

FOCUS ON
Carbon monoxide: binds Hb (↑ affinity, left shift) + complex IV; headache, confusion, "cherry-red" (rare); normal pulse oximetry and PaO₂ → co-oximetry; 100% O₂, hyperbaric for severe; delayed neuro sequelae (globus pallidus). Cyanide (fires, nitroprusside, amygdalin): complex IV block → lactic acidosis, venous blood bright red, almond odor → hydroxocobalamin (→ cyanocobalamin) or nitrites (→ methemoglobin binds CN) + thiosulfate (→ thiocyanate). Methemoglobinemia (dapsone, benzocaine, nitrites, nitrates in well water, primaquine): Fe³⁺ → chocolate-brown blood, cyanosis unresponsive to O₂, SpO₂ ~85% → methylene blue (not in G6PD), vitamin C. Lead: anemia (basophilic stippling, ring sideroblasts — ALA dehydratase, ferrochelatase), neuropathy (wrist/foot drop), encephalopathy, lead lines → succimer (children), EDTA + dimercaprol. Arsenic: garlic breath, rice-water diarrhea, QT, Mees lines → dimercaprol. Mercury: erethism, acrodynia, kidney → dimercaprol/succimer. Copper → penicillamine. Caustics, hydrocarbons: no emesis/charcoal.
POISON
MECHANISM
CLUES
TREATMENT
Carbon monoxide
Binds Hb 200× O₂, left shift, inhibits cytochrome oxidase
Headache, dizziness, several family members ill, winter heater; normal SpO₂
100% O₂, hyperbaric O₂ (coma, pregnancy, high COHb)
Cyanide
Blocks complex IV (cytochrome a₃)
Smoke inhalation, nitroprusside, plant pits; lactic acidosis, ↑ venous O₂
Hydroxocobalamin; nitrites + sodium thiosulfate
Methemoglobinemia
Fe²⁺ → Fe³⁺ (can't carry O₂, left shift)
Chocolate-brown blood, SpO₂ ~85% despite O₂, normal PaO₂
Methylene blue (reduces via NADPH) — avoid in G6PD; ascorbic acid
Hydrogen sulfide
Complex IV block
Rotten-egg smell, sewer/manure workers, sudden collapse
Remove, O₂, supportive (nitrites debated)
Lead
Inhibits ALA dehydratase, ferrochelatase
Microcytic anemia, basophilic stippling, abdominal pain, foot/wrist drop, encephalopathy, gum lines
Succimer (mild), EDTA + dimercaprol (severe); remove source
Arsenic
Binds sulfhydryls, blocks pyruvate dehydrogenase
Garlic breath, vomiting, rice-water stools, QT, neuropathy, Mees lines, skin cancer (chronic)
Dimercaprol, succimer
Mercury
Sulfhydryl binding
Erethism (shyness, irritability), tremor, gingivitis, nephrotoxicity, acrodynia
Dimercaprol, succimer
Copper · gold · Wilson disease
—
Hepatitis, neuropsychiatric (Wilson)
Penicillamine, trientine
Thallium
Mimics K⁺
Painful neuropathy, alopecia
Prussian blue
Caustics
Alkali → liquefactive necrosis (deep); acid → coagulative
Oral burns, drooling, stridor
No emesis, no neutralization; endoscopy
Hydrocarbons
Low viscosity → aspiration
Cough, chemical pneumonitis
No emesis or charcoal
Black widow · scorpion · snakes
Latrotoxin ↑ ACh/NE release · Na⁺ channels · hemotoxins
Abdominal rigidity · autonomic storm · coagulopathy
Supportive, antivenom
Mushrooms (Amanita)
Amatoxin inhibits RNA polymerase II
Delayed GI symptoms → liver failure
Silibinin, NAC, transplant

Mechanism — cyanide antidotes

Hydroxocobalamin + CN⁻
→
Cyanocobalamin (B12)Excreted in urine
Nitrites → methemoglobin
→
MetHb binds CN⁻
→
Thiosulfate → thiocyanate(rhodanese) → urine

Avoid nitrites in smoke inhalation with CO — they worsen oxygen delivery.

Oxygen-delivery poisons

POISON
SpO₂
PaO₂
CLUE
CO
Normal (false)
Normal
COHb ↑
MetHb
~85%
Normal
Brown blood
Cyanide
Normal
Normal
Lactate ↑, venous O₂ ↑

How it's tested

COMMON CONFUSIONIn CO and methemoglobin poisoning, PaO₂ (dissolved O₂) is normal — oxygen content is low; pulse oximetry misleads.
THE TRAPMethylene blue in a G6PD-deficient patient doesn't work (no NADPH) and can cause hemolysis — use exchange transfusion or hyperbaric O₂.
YOUR APPROACHExposure clue (fire, workplace, water, cosmetics, folk remedy) → mechanism → specific test (co-oximetry, lactate, level) → antidote.

If you see… think…

Family with headaches in winter, normal SpO₂ → carbon monoxide
House-fire victim with severe lactic acidosis → cyanide → hydroxocobalamin
Chocolate-colored blood after topical anesthetic → methemoglobinemia → methylene blue
Child with anemia, basophilic stippling, abdominal pain → lead → succimer
Garlic breath, rice-water diarrhea → arsenic
Painful neuropathy and hair loss → thallium
Sewer worker collapses, rotten-egg smell → hydrogen sulfide
Bilateral globus pallidus lesions after poisoning → carbon monoxide
Liver failure after foraged mushrooms → Amanita amatoxin
Shy, irritable, tremulous worker making thermometers → mercury