Cyanide poisoning vs Carbon monoxide poisoning
How to tell Cyanide poisoning and Carbon monoxide poisoning apart when a board question makes them look alike.
Cyanide is complex IV blockade with severe lactic acidosis/shock; CO is carboxyhemoglobin with normal PaO2, low oxygen content, left shift, and faulty pulse-ox reassurance.
Side by side
Cyanide poisoning causes abrupt cellular hypoxia, severe lactic acidosis, cardiovascular collapse, seizures, coma, and may follow smoke inhalation, industrial exposure, or nitroprusside.
Carbon monoxide poisoning causes headache, dizziness, confusion, syncope, myocardial ischemia, normal PaO2, falsely reassuring pulse oximetry, and elevated carboxyhemoglobin.
Cyanide inhibits cytochrome c oxidase in complex IV, blocking oxidative phosphorylation despite oxygen delivery.
Carbon monoxide binds hemoglobin with high affinity and shifts the oxygen dissociation curve left, impairing oxygen delivery.
Profound lactic acidosis, high venous oxygen saturation, smoke inhalation with shock, and response to hydroxocobalamin support cyanide poisoning.
Elevated carboxyhemoglobin, exposure to fire/heater/exhaust, headache, normal PaO2, high/falsely reassuring pulse oximetry despite low oxygen content, and left-shifted oxygen unloading support CO poisoning.
The trap
Both occur in smoke inhalation, so severe lactic acidosis and cardiovascular collapse point to cyanide while high carboxyhemoglobin points to CO.
The repair move
For inhalational poisoning, separate hemoglobin binding from mitochondrial oxygen-use failure.
Try a free question
CoreA patient rescued from a house fire has coma, hypotension, severe lactic acidosis, and high venous oxygen saturation. Which poisoning is most likely?
Show answer and explanation
Answer: A. Cyanide poisoning. Smoke exposure with cardiovascular collapse and severe lactic acidosis from blocked oxidative phosphorylation supports cyanide poisoning.
Why the other choices are tempting
- B. Carbon monoxide poisoning is confirmed by elevated carboxyhemoglobin and may have normal PaO2.
- C. Opioid overdose causes miosis and respiratory depression.
- D. Methanol causes visual injury and anion-gap metabolic acidosis.
- E. Salicylate poisoning causes tinnitus and mixed acid-base disturbance.
Flashcards
Tap a card to flip it.
What enzyme does cyanide inhibit?
Cyanide inhibits cytochrome c oxidase in mitochondrial complex IV.
What antidote treats cyanide poisoning?
Hydroxocobalamin treats cyanide poisoning by binding cyanide.
Quick answers
What is the fastest way to tell Cyanide poisoning from Carbon monoxide poisoning?
Cyanide is complex IV blockade with severe lactic acidosis/shock; CO is carboxyhemoglobin with normal PaO2, low oxygen content, left shift, and faulty pulse-ox reassurance.
What is the key clue for Cyanide poisoning?
Cyanide poisoning causes abrupt cellular hypoxia, severe lactic acidosis, cardiovascular collapse, seizures, coma, and may follow smoke inhalation, industrial exposure, or nitroprusside.
What is the key clue for Carbon monoxide poisoning?
Carbon monoxide poisoning causes headache, dizziness, confusion, syncope, myocardial ischemia, normal PaO2, falsely reassuring pulse oximetry, and elevated carboxyhemoglobin.
What is the common trap with Cyanide poisoning vs Carbon monoxide poisoning?
Both occur in smoke inhalation, so severe lactic acidosis and cardiovascular collapse point to cyanide while high carboxyhemoglobin points to CO.