Expectorants, Emetics, Haematinics, and Poison Antidotes
Inorganic therapeutics play life-saving roles in respiratory clearance, acute emergency toxicology, and blood formation. In this final unit of BP106T, we explore cough expectorants, emetics that induce vomiting to expel ingested poisons, iron-based haematinics for anemia, the 3 clinical classes of antidotes, and the complete biochemical mechanism of dual antidote therapy in lethal cyanide poisoning.
In this Unit:
1. Expectorants: Definition and Classification
Definition:
An expectorant is defined as a drug or chemical substance that increases the volume or decreases the viscosity of respiratory tract secretions (mucus/phlegm), thereby facilitating its expulsion from the trachea and bronchi by coughing.
1. Sedative (Indirect / Reflex) Expectorants
These agents mildly irritate the sensory nerve endings in the gastric mucosa. This triggers a reflex via the vagus nerve that stimulates the bronchial secretory glands in the lungs to produce abundant, thin, watery fluid.
2. Stimulant (Direct) Expectorants
These volatile or aromatic substances are absorbed into systemic circulation and excreted directly through the respiratory epithelium, directly stimulating bronchial secretory cells and relaxing bronchial smooth muscle.
2. Official Inorganic Expectorant Compounds
Potassium Iodide (KI)
• It is also known as: Kalium iodatum or KI.
• It is used for: Thinning tenacious bronchial secretions in chronic bronchitis and protecting thyroid during radiation emergencies.
- Mechanism of Action: Dual mechanism. It irritates gastric mucosa reflexly to thin secretions, AND iodide ions are partially excreted directly into the bronchial fluid, where they liquefy thick mucoprotein bonds.
- Assay: Andrews titration with 0.05 M Potassium Iodate (KIO3) in concentrated hydrochloric acid using chloroform as internal indicator (violet color in chloroform discharges at end point).
- Caution: Prolonged use may depress thyroid hormone synthesis (Wolff-Chaikoff effect) or cause iodism (skin rashes, metallic taste).
Ammonium Chloride (NH4Cl - Sal Ammoniac)
• It is also known as: Sal Ammoniac or Navsadar.
• It is used for: Promoting productive expectoration, urinary acidification, and treating systemic metabolic alkalosis.
- Mechanism of Action: Gastric irritant that triggers a mild vagal reflex increase in watery bronchial mucus. In higher doses, it is metabolized by the liver into urea and free HCl, acting as a potent systemic acidifier.
- Assay (Formalin Titration): Neutralized formaldehyde solution is added. Formaldehyde reacts with ammonium ions to form hexamethylenetetramine (hexamine), liberating stoichiometric free HCl:4 NH4Cl + 6 HCHO → (CH2)6N4 (Hexamine) + 4 HCl + 6 H2OThe liberated HCl is then titrated against standard 0.1 M NaOH using Phenolphthalein.
3. Emetics: Definition and Physiological Mechanisms
Definition:
An emetic is defined as a drug or chemical agent used to induce vomiting in order to rapidly empty gastric contents and expel ingested non-corrosive poisons or toxins before they can be absorbed into the bloodstream.
A. Local Peripheral Reflex Stimulation
The chemical acts as a direct irritant on the mucosal sensory receptors in the stomach wall. Nerve impulses travel via the vagus and sympathetic afferent nerves directly to the vomiting center in the brainstem, causing retrograde peristalsis, abdominal muscle contraction, and vomiting.
Prototype: Copper Sulphate (CuSO4).B. Central CTZ Stimulation
The drug circulates in the blood and directly stimulates dopamine / serotonin receptors in the Chemoreceptor Trigger Zone (CTZ) located in the area postrema of the fourth ventricle outside the blood-brain barrier.
Prototype: Apomorphine, Antimony Potassium Tartrate.STRICT CONTRAINDICATIONS OF EMETICS (Exam Alert):
Emetics must NEVER be administered in the following medical emergencies:
- Corrosive Poisoning (Strong Acids or Caustic Alkalis): Inducing vomiting re-exposes the esophagus and pharynx to the caustic chemical, causing esophageal rupture and mediastinitis.
- Petroleum Hydrocarbon Poisoning (Kerosene, Petrol, Diesel): Vomiting risks aspiration of volatile vapors into the lungs, producing fatal chemical pneumonitis and asphyxiation.
- Unconscious, Comatose, or Convulsing Patients: Absence of protective cough and gag reflexes causes immediate asphyxiation by inhaled vomitus.
4. Official Inorganic Emetic Compounds
Copper Sulphate (CuSO4 • 5H2O - Blue Vitriol)
• It is also known as: Blue Vitriol, Bluestone, or Cupric Sulphate Pentahydrate.
• It is used for: Emergency emesis induction and chemical neutralization in acute phosphorus poisoning.
- Role in Toxicology: Used as an emergency emetic (0.25 to 0.5 g dissolved in 100 mL warm water). Produces rapid vomiting in 5 to 10 minutes.
- Specific Antidote for White Phosphorus Poisoning: Copper sulphate coats ingested unabsorbed yellow/white phosphorus particles with an inert black layer of insoluble copper phosphide (
Cu3P2), preventing toxic absorption. - Caution: If vomiting does not occur, stomach lavage must be performed immediately to prevent copper poisoning.
Antimony Potassium Tartrate (Tartar Emetic)
• It is also known as: Tartar Emetic or Potassium Antimonyl Tartrate.
• It is used for: Inducing central CTZ emesis and second-line treatment of parasitic schistosomiasis.
- Role: Acts both reflexly on the gastric mucosa and centrally on the medulla CTZ. Highly toxic; historical interest only in modern clinical medicine.
- Other Uses: Historically used as an antiparasitic intravenous agent against Schistosoma japonicum (Schistosomiasis / bilharziasis) and Leishmaniasis.
5. Haematinics: Definition and Physiological Role of Iron
Definition:
A haematinic is defined as a drug or pharmaceutical substance containing iron, vitamin B12, or folic acid, which is essential for the physiological synthesis of hemoglobin and the maturation of erythrocytes (red blood cells), used to treat anemia.
Distribution of the Body's ~4.0 Grams of Iron:
O2) from lungs to tissue capillaries.Fe³⁺) must be reduced to ferrous iron by gastric hydrochloric acid and ascorbic acid (Vitamin C). Inside mucosal cells, iron binds apoferritin to form ferritin, and in plasma it is carried by the transport protein transferrin.6. Official Pharmaceutical Iron Compounds
Ferrous Sulphate (FeSO4 • 7H2O - Green Vitriol)
20% Elemental Fe• It is also known as: Green Vitriol or Iron Vitriol.
• It is used for: Prevention and treatment of iron deficiency hypochromic microcytic anemia.
- Appearance: Pale bluish-green monoclinic prisms. Efflorescent in dry air and oxidizes to brownish ferric salt when exposed to moisture.
- Assay (Cerimetry): Dissolved in dilute H2SO4 and titrated against 0.1 M Ceric Ammonium Sulphate using Ferroin indicator (end point: red turns to pale blue):Fe²⁺ + Ce⁴⁺ → Fe³⁺ + Ce³⁺
- Clinical Pearls: First-line oral drug for iron-deficiency microcytic hypochromic anemia. Ingested after meals to minimize gastric nausea. Stools become black (harmless iron sulfide).
Ferrous Gluconate [Fe(C6H11O7)2 • 2H2O]
12% Elemental Fe- Appearance: Yellowish-grey or pale greenish-yellow powder with a faint caramel-like odor. Highly soluble in water.
- Clinical Advantage over FeSO4: Far less irritating to the gastric mucosa. Produces significantly lower incidence of epigastric cramps, nausea, and severe constipation. Preferred for sensitive stomachs and pediatric patients.
- Assay: Direct cerimetric titration with standard ceric sulphate in the presence of dilute H2SO4 using Ferroin indicator.
7. Poisons and the Three Classes of Antidotes
A poison is defined as any substance that, when administered, inhaled, or absorbed into the human body, causes cellular damage, severe biochemical illness, or death. An antidote is defined as a therapeutic agent used to neutralize, counteract, or reverse the toxic manifestations of a poison.
Physiological Antidotes
Counteracts the toxic effects of the poison by producing opposite physiological actions on the body's receptors (antagonism) without binding directly to the poison molecule itself.
• Naloxone for respiratory depression in morphine overdose.
Chemical Antidotes
Chemically reacts with, chelates, or alters the poison to form a non-toxic, insoluble, or readily excretable chemical complex.
• Dimercaprol (BAL) and EDTA for arsenic, lead, and mercury toxicity.
Mechanical (Physical) Antidotes
Prevents the systemic absorption of the poison into the blood by physically adsorbing it onto an inert surface or coating the mucosal wall of the stomach.
• Egg albumin / Milk: Coats mucosal wall and precipitates heavy metals.
8. Biochemical Toxicity Mechanism of Cyanide Poisoning
Cyanide (HCN, NaCN, KCN) is one of the most rapidly fatal chemical poisons known. Death occurs within minutes through a devastating cellular asphyxiation mechanism called Histotoxic Anoxia:
How Cyanide Kills at the Mitochondrial Level:
- Under normal conditions, mitochondria produce ATP by passing electrons through the respiratory chain to Cytochrome c Oxidase (Complex IV), which contains ferric iron (
Fe³⁺). Cytochrome oxidase transfers electrons to molecular oxygen, generating water and cellular energy. - The ingested or inhaled cyanide ion (
CN⁻) enters the bloodstream and binds with immense chemical affinity to the ferric (Fe³⁺) moiety of Cytochrome Oxidase. - This completely freezes and arrests the enzyme: electrons can no longer be transferred to oxygen.
- Even though arterial blood is 100% saturated with oxygen (the patient has bright cherry-red skin and blood), the tissues are completely unable to utilize the oxygen!
- Cellular ATP collapses instantly, triggering acute metabolic lactic acidosis, respiratory arrest, and cardiac failure within 3 to 5 minutes.
9. Dual Antidote Therapy Mechanism: Sodium Nitrite + Sodium Thiosulphate
The official emergency pharmacopoeial treatment for cyanide poisoning is a coordinated, sequential two-step intravenous therapy employing Sodium Nitrite (NaNO2) followed by Sodium Thiosulphate (Na2S2O3):
Step 1: Intravenous Infusion of Sodium Nitrite (300 mg NaNO2)
Sodium nitrite is a powerful oxidizing agent. It oxidizes approx. 20–30% of normal circulating ferrous hemoglobin (Fe²⁺) into Methemoglobin (Fe³⁺).
The Molecular Decoy: Methemoglobin has an even greater binding affinity for cyanide than cytochrome oxidase! Methemoglobin aggressively pulls the cyanide ions away from the poisoned cytochrome oxidase enzyme, forming harmless Cyanmethemoglobin. This frees cytochrome oxidase, instantly restarting mitochondrial cellular respiration!
Step 2: Intravenous Infusion of Sodium Thiosulphate (12.5 g Na2S2O3)
However, cyanmethemoglobin is reversible and will slowly release cyanide back into the body. Therefore, Sodium Thiosulphate is infused immediately as an indispensable sulphur donor.
The Hepatic Rhodanese Miracle: The human liver contains an endogenous mitochondrial enzyme called Rhodanese (Thiosulphate-Cyanide Sulphurtransferase). Rhodanese uses the sulphur atom from sodium thiosulphate to convert the deadly cyanide into Sodium Thiocyanate (NaSCN). Sodium thiocyanate is completely non-toxic and is safely cleared by the kidneys in urine. Methemoglobin is simultaneously recycled back to normal hemoglobin!
Master Flowchart of Cyanide Antidote Therapy
↓ (+ Sodium Nitrite)
Generates Methemoglobin [Fe³⁺] → Scavenges CN⁻ → Forms [Cyanmethemoglobin] (Enzyme liberated!)
↓ (+ Sodium Thiosulphate + Liver Rhodanese Enzyme)
Converts CN⁻ into [Sodium Thiocyanate - NaSCN] → Excreted harmlessly in Urine!