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CO & Cyanide Poisoning for MRCP Part 1

TL;DR

Tox: Carbon Monoxide & Cyanide Poisoning is a high-yield topic for MRCP Part 1, focusing on oxygen delivery versus utilisation failure. Carbon monoxide reduces oxygen carriage via carboxyhaemoglobin, while cyanide blocks cellular respiration causing severe lactic acidosis. Recognising diagnostic clues and choosing the correct antidote are key exam skills.


Why this matters

In MRCP Part 1, toxicology questions frequently test your understanding of physiology applied to acute scenarios. Carbon monoxide (CO) and cyanide poisoning are repeatedly examined because they highlight two fundamentally different mechanisms of hypoxia.

These topics also integrate with emergency medicine, respiratory physiology, and metabolic interpretation—making them highly examinable. For a broader overview of the syllabus, refer to the MRCP Part 1 overview.


Core sections

1. Pathophysiology (Most tested concept)

Carbon Monoxide (CO):

  • Binds haemoglobin with ~200× greater affinity than oxygen

  • Forms carboxyhaemoglobin (COHb)

  • Causes a left shift of the oxyhaemoglobin dissociation curve

  • Leads to impaired oxygen delivery (functional anaemia)

Cyanide:

  • Inhibits cytochrome c oxidase (Complex IV)

  • Prevents oxidative phosphorylation

  • Causes histotoxic hypoxia (oxygen present but unusable)

👉 Key distinction:

  • CO poisoning → problem with oxygen delivery

  • Cyanide poisoning → problem with oxygen utilisation

2. Clinical Features

Feature

Carbon Monoxide

Cyanide

Onset

Gradual

Rapid

CNS

Headache, confusion, coma

Agitation, seizures, coma

Skin

“Cherry red” (rare)

Flushed

Lactate

Mild–moderate ↑

Marked ↑ (>10 mmol/L)

Oxygen saturation

Falsely normal

Normal

3. Diagnostic Clues

Carbon Monoxide:

  • Pulse oximetry appears normal

  • Diagnosis via co-oximetry (COHb levels)

  • Exposure history: enclosed spaces, faulty heaters

Cyanide:

  • Severe lactic acidosis

  • Elevated venous oxygen saturation

  • Seen in house fires (synthetic materials)

4. Management Principles

Carbon Monoxide:

  • Immediate 100% oxygen via non-rebreather mask

  • Consider hyperbaric oxygen therapy (HBOT) if:

    • COHb >25%

    • Neurological symptoms

    • Cardiovascular instability

    • Pregnancy (lower threshold)

Authoritative reference:

Cyanide:

  • Immediate high-flow oxygen

  • Hydroxocobalamin (first-line in UK)

  • Sodium thiosulfate (adjunct)

Further reading:

5. Most Tested Subtopics

  1. Mechanistic differences (delivery vs utilisation)

  2. Limitations of pulse oximetry in CO poisoning

  3. Indications for hyperbaric oxygen

  4. Lactate as a diagnostic clue in cyanide poisoning

  5. Combined toxicity in fire exposure

6. High-Yield Exam Points (Rapid Revision List)

  1. CO binds haemoglobin → reduced oxygen delivery

  2. Cyanide inhibits electron transport chain

  3. Pulse oximetry is unreliable in CO poisoning

  4. CO shifts dissociation curve to the left

  5. Cyanide causes profound lactic acidosis

  6. Fire victims → suspect dual poisoning

  7. COHb measured using co-oximetry

  8. Hyperbaric oxygen reduces delayed neurological sequelae

  9. Hydroxocobalamin discolours urine red

  10. Venous oxygen is elevated in cyanide poisoning


Practical examples / mini-cases

MCQ Example:

A 40-year-old woman is rescued from a house fire. She is confused, tachycardic, and hypotensive. Lactate is 12 mmol/L. Oxygen saturation is 99%.

What is the most appropriate immediate treatment?

A. Hyperbaric oxygenB. HydroxocobalaminC. Sodium bicarbonateD. Observe

Answer: B. Hydroxocobalamin

Explanation: Severe lactic acidosis (>10 mmol/L) strongly suggests cyanide poisoning, especially in fire exposure. Immediate antidote therapy with hydroxocobalamin is life-saving.


Medical students are studying

Common pitfalls (5 bullets)

  • Relying on normal SpO₂ to exclude CO poisoning

  • Missing cyanide toxicity in fire victims

  • Overinterpreting “cherry red” skin (rare finding)

  • Ignoring lactate levels in suspected poisoning

  • Using hyperbaric oxygen indiscriminately


Practical study-tip checklist

  • Learn mechanism contrasts (delivery vs utilisation)

  • Practise ABG interpretation in poisoning scenarios

  • Memorise antidotes and indications

  • Reinforce learning with Free MRCP MCQs

  • Simulate exam pressure with a Start a mock test

💡 Cross-link suggestion: Combine this topic with salicylate and paracetamol toxicity for integrated revision.


FAQs

1. Why is pulse oximetry misleading in carbon monoxide poisoning?

Pulse oximeters cannot differentiate oxyhaemoglobin from carboxyhaemoglobin, resulting in falsely normal readings.

2. What is the hallmark lab finding in cyanide poisoning?

Severe lactic acidosis due to impaired oxidative phosphorylation is the key feature.

3. When should hyperbaric oxygen be used in CO poisoning?

In severe poisoning (COHb >25%), neurological symptoms, or pregnancy with lower thresholds.

4. What is the first-line antidote for cyanide poisoning?

Hydroxocobalamin, which binds cyanide to form vitamin B12 for excretion.

5. Can both poisonings occur together?

Yes, particularly in enclosed-space fires involving synthetic materials.


Ready to start?

Consolidate your toxicology knowledge with structured, exam-focused revision. Start with the MRCP Part 1 overview and strengthen retention using our Free MRCP MCQs.


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