Acidum Hydrocyanicum

Hydrocyanic Acid

Prussic Acid

Hydrocyanic or Prussic Acid. Acide hydrocyanique, Fr.; Blausậure, Ger.

Acidum Hydrocyanicum Dilutum

Diluted hydrocyanic acid. A colorless liquid, having a peculiar odor, and wholly volatilized by heat. It imparts a faint, evanescent red color to litmus, and is not discolored by hydrosulphuric acid. With solution of nitrate of silver, added in slight excess, one hundred grains of it produce a white precipitate, which, when washed with water until the washings are tasteless, and dried at a temperature not exceeding 212°, weighs ten grains, and is wholly soluble in boiling nitric acid.

The official diluted acid contains two per cent of anhydrous acid and ninety-eight per cent of alcohol and water. Dose, τη j—τη v.

Antagonists and Incompatibles

The metallic salts are generally, incompatible; also the red oxide of mercury and the sulphides. Freshly-precipitated oxide of iron (hydrated sesquioxide) has been proposed as a chemical antidote, but its action is too slow. In cases of poisoning, the remedies of the greatest utility are cold affusion to the spine, the inhalation of ammonia, the stomach administration, as also the intra-venous injection of this substance, and the subcutaneous injection of ether. Atropine has been proposed as a physiological antagonist by Preyer; but the rate at which atropine is diffused, as compared with the diffusion of prussic acid, obviously will render such antagonism powerless, how much soever it may be approved on theoretical grounds. The results of experiments, as the author and others have shown, are, however, opposed to the existence of this antagonism. In addition to these measures, artificial respiration should be practiced.

Physiological Effects

Applied to the unbroken skin, it is doubtful whether hydrocyanic acid is absorbed, but in contact with a wound or an abrasion, and with the mucous membrane, it diffuses into the blood with great rapidity.

The vapor has a rather fragrant odor, similar to that of bitter almonds. Inhaled, it has speedily caused death. When the effects of the vapor are short of lethal, giddiness, faintness, embarrassed breathing, a weak, small pulse, and great muscular weakness, are produced; and there may be even coma and profound insensibility, and yet recovery ensue (Taylor).

In small medicinal doses, beyond a fugitive and very slight calmative effect, no symptoms are produced by it. When the dose somewhat exceeds the medicinal standard, there may occur transient giddiness, nausea, faintness, a feeble pulse, and general muscular weakness. The effects follow very speedily. When a very large toxic dose is taken, a few seconds only intervene from the act of swallowing until its effects are manifest, and death may ensue in two minutes or be postponed to five. Under these circumstances, the following phenomena have been observed: sudden insensibility; eyes protruding and glistening; pupils dilated and unaffected by light; extremities cold, relaxed; the skin covered with a clammy sweat; breathing convulsive, slow; the pulse extremely feeble or imperceptible; evacuations involuntary (Taylor). When the effects are slower, in consequence of the ingestion of a merely lethal dose, there are occasionally tetanic convulsions, opisthotonos, trismus, etc.

Although the effects of prussic acid are exceedingly rapid, a fatal result is not instantaneous. Various acts of volition may be gone through, provided but a few seconds are required for their performance. Several instructive instances of this kind are narrated by Taylor. The effects of hydrocyanic acid are not more rapid than can be accounted for by its distribution through the blood

Most contradictory opinions have been expressed as to the action of prussic acid on the blood: that it at first arterializes and afterward arrests decarbonization of the blood; that it destroys the ozonizing power, and does not impair the capacity of the red blood-globules to carry and to yield up oxygen; that cyanohaemoglobin is formed by the combination of the acid with haemoglobin, and that this combination can not take place, owing to the rapidity of the action of the poison. From this chaotic state of scientific opinion the following may be evolved: the blood is dark, owing to deficient decarbonization, but this is probably due to a spasm of the pulmonary arterioles and paresis of the muscles of respiration, whence it follows that rapid asphyxia ensues. The primary action of prussic acid on the terminal filaments of the pneumogastric, as shown by Preyer, is confirmatory of this view.

Although the action of the heart ceases after respiration, prussic acid undoubtedly exerts a direct paralyzing action on the cardiac ganglia.

The cerebral effects of this poison are, probably, indirect, the result of rapid carbonic-acid poisoning, and the sudden withdrawal of oxygen from the cerebral tissues. Direct application of prussic acid to the medulla oblongata causes (in the alligator) a sudden and complete expiration, and collapse of the lung (Jones). The tetanic convulsions which have been observed in many cases of poisoning, in animals and in man, indicate a direct action of this agent on the spasm-center; but the disappearance of the excitability of the motor nerves, and of the contractility of muscles which it causes, shows that it quickly exhausts the irritability of the spinal cord. These effects on the cord, on the nerve-trunks, and on the muscles, are also, probably, in part due to the circulation through them of blood deprived of oxygen and charged with carbonic acid. The fact that instances of recovery from a condition of profound insensibility are numerous, is confirmatory of the view just expressed. Moreover, artificial respiration exerts an undeniable influence over the lethal effects of the acid in animals (Preyer), whence it may be concluded that to supply oxygen to the blood is sufficient to arrest all of the symptoms produced by the want of oxygen and by the excess of carbonic acid.

Post-mortem rigidity sets in early after death from prussic acid, and is very pronounced. The fingers are tightly closed, the toes strongly flexed, the jaws rigid, the eyes prominent and staring. The blood is dark-colored, fluid, and the venous trunks and the cerebral sinuses are gorged.

The quantity of medicinal, diluted hydrocyanic acid necessary to produce death will vary with the age, size, and bodily vigor. Habit, also, influences to a remarkable degree the susceptibility to its toxic influence. A quantity equivalent to forty minims of the diluted hydrocyanic acid (United States Pharmacopoeia) has proved fatal. As the effects of a medicinal dose are expended in a half hour to one hour, the repetition of the doses hourly will not be unsafe. Hydrocyanic acid is not a cumulative poison.