Which hypoxia occurs in carbon monoxide poisoning? a) Stagnant b) Anaemic c) Histo-toxic d) Hypoxic
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The textbooks reveal an important and nuanced answer. Here is the complete explanation:
Answer: b) Anaemic Hypoxia (for MCQ/exam purposes)
However, this question deserves a careful, layered explanation because CO poisoning actually causes more than one type of hypoxia.
Types of Hypoxia - Quick Classification
| Type | Mechanism | Examples |
|---|---|---|
| Hypoxic | Low O₂ in arterial blood (low PaO₂) | High altitude, lung disease |
| Anaemic | Reduced O₂-carrying capacity of Hb | Anaemia, CO poisoning, methaemoglobinaemia |
| Stagnant (Ischaemic) | Reduced blood flow to tissues | Heart failure, shock |
| Histotoxic | Cells cannot use O₂ (mitochondrial poison) | Cyanide poisoning |
Why CO Poisoning Causes Anaemic Hypoxia (Primary/Classic Answer)
CO binds haemoglobin with ~250 times the affinity of O₂, forming carboxyhaemoglobin (COHb). This:
- Reduces the O₂-carrying capacity of blood (less Hb available to carry O₂) - directly analogous to anaemia
- Shifts the O₂-dissociation curve to the left, so even the remaining oxyhaemoglobin releases less O₂ at the tissue level
- The PaO₂ (measured by pulse oximeter) appears normal, masking the problem
This fits the definition of anaemic hypoxia - normal O₂ tension in blood, but reduced O₂ content/delivery.
(Plum and Posner's Neurology, p.377)
The Nuance: CO Also Causes Histotoxic Hypoxia
Plum and Posner's explicitly states:
"Most of the toxicity from carbon monoxide poisoning is not due to hemoglobin binding but is histotoxic, a result of its binding to cytochromes."
"Histotoxic hypoxia results from agents that poison the electron transport chain. Such agents include cyanide and carbon monoxide."
CO also binds cytochrome c oxidase (Complex IV) in mitochondria, preventing cellular use of O₂ - which is the histotoxic component. This explains why CO toxicity can be far more severe than simple anaemia of equivalent degree.
(Plum and Posner's Diagnosis and Treatment of Stupor and Coma, p.377-378)
For Your MCQ - How to Choose
| Context | Answer |
|---|---|
| Standard exam MCQ / pharmacology / physiology | b) Anaemic hypoxia (classic teaching) |
| If "histotoxic" is among the options and question asks about mechanism of cell death | Histotoxic is also correct |
In most medical school and competitive exam settings (USMLE, UKMLA, Indian PG exams), the expected answer is b) Anaemic hypoxia, because CO's primary mechanism taught in physiology is reduction of Hb oxygen-carrying capacity.
The histotoxic label is more commonly reserved for cyanide in standard curricula, with CO poisoning being the "anaemic" example.
Final Answer: b) Anaemic hypoxia
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