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DrX Whiz Niraj
Poisons & Antidotes: Cyanide & General Poisoning
Master the pharmacology of Antidotes! Complete notes on Activated Charcoal, Sodium Nitrite, and Sodium Thiosulphate (The Cyanide Antidote Kit).
A Poison is any substance that, when introduced into or absorbed by a living organism, destroys life or injures health. An Antidote is an agent that counteracts the effects of a poison.
Types of Antidotes:
- Physical/Mechanical Antidotes: Interfere with the absorption of the poison (e.g., Activated Charcoal absorbs the poison in the stomach).
- Chemical Antidotes: Change the chemical nature of the poison to make it non-toxic (e.g., Sodium Thiosulphate for cyanide).
- Physiological/Pharmacological Antidotes: Produce opposite physiological effects to counteract the poison (e.g., Atropine for organophosphate poisoning).
Mechanism of Action
Activated charcoal acts as a Mechanical/Physical Antidote. It has a highly porous structure which gives it a massive surface area. When administered orally, it adsorbs (binds to the surface) toxic molecules and drugs in the gastrointestinal tract, preventing them from being absorbed into the blood.
Uses & Properties
It is the most common emergency treatment for most drug overdoses (like Aspirin, Barbiturates) and oral poisonings.
- It is a fine, black, odorless, and tasteless powder.
- Limitation: It does NOT bind well to heavy metals (Iron, Lithium), alcohols, or strong acids/bases.
Cyanide is a deadly poison because it binds to the enzyme Cytochrome oxidase in the mitochondria, instantly stopping cellular respiration and ATP production.
The standard treatment is the sequential administration of two chemical antidotes: Sodium Nitrite followed by Sodium Thiosulphate.
A. Sodium Nitrite (NaNO₂)
It acts as the first line of defense in the cyanide kit.
- Action: It oxidizes the Iron in Hemoglobin (Fe²⁺) to form Methemoglobin (Fe³⁺).
- Methemoglobin has a very high affinity for cyanide. It pulls the deadly cyanide out of the tissues (freeing the cytochrome oxidase enzyme) and binds with it to form Cyanmethemoglobin.
B. Sodium Thiosulphate (Na₂S₂O₃)
It is administered immediately after Sodium Nitrite to permanently clear the poison.
- Action: It acts as a sulfur donor. An enzyme in the liver called Rhodanese uses the sulfur from Sodium Thiosulphate to convert the toxic cyanide into Thiocyanate (SCN⁻).
- Thiocyanate is relatively non-toxic and is safely excreted in the urine.
Clinical Alert: Time Sensitivity!
Cyanide poisoning works extremely fast, causing death within minutes by suffocating the cells (even if there is plenty of oxygen in the blood). The antidote kit must be administered sequentially without delay. Amyl Nitrite (inhalant) is often given first while the IV line for Sodium Nitrite and Sodium Thiosulphate is being prepared.