Syn. Prussic Acid.

As hydrocyanic acid is employed in a number of different forms, and obtained from different sources, it will be most convenient to treat first of its general effects on the system, and therapeutic applications, and subsequently of the several forms and sources alluded to, with all that is peculiar to each.

1. Effects On The System

In its concentrated state, hydrocyanic acid is one of the strongest poisons known; but, properly diluted and guarded, it is a perfectly safe medicine. With slight local irritant properties, it is a powerful sedative to the whole nervous system, operating with special force on the brain.

In small medicinal doses, it often controls abnormal nervous excitement, without any other observable impression. More largely taken, it occasions a sense of irritation in the throat when swallowed, sometimes nausea, a feeling of confusion in the brain, vertigo, perhaps headache, faintness, dimness of vision, and drowsiness; and all these cerebral symptoms may result from simply smelling a bottle of the strong acid. The pulse is sometimes retarded, sometimes accelerated, but generally if not always weakened. The respiration is also disturbed, being either hurried or diminished. its direct influence on the nervous tissue is depressing, as evinced by the numbness it produces, and the relief which it sometimes affords to pain. its sedative powers, however, are much more obvious when it is taken in poisonous doses.

Poisonous Effects

Death has frequently resulted from prussic acid, taken either accidentally, or with a view to self-destruction. The small quantity required, the rapidity of its operation, and the absence of painful effect, have recommended it to the suicide and the poisoner; but the suspicion which sudden death excites, and the means of detection afforded by the odour and chemical reactions of the acid, have had a tendency to deter the latter; so that poisoning by this agent is probably less frequent now than formerly.

Insensibility and death usually take place so soon after a poisonous dose of the acid, that opportunities are not often afforded of observing the early effect of it in the human subject. it may be inferred, however, from the recollections of those who have recovered, from occasional observation of fatal cases from the commencement, and from various circumstances noticed before or after death implying purposed movements on the part of the patient, that a short period generally if not always elapses, after the application of the poison, before consciousness and the power of action are lost. The symptoms noticed during this interval have been a feeling of heat or acrimony in the mouth and fauces, sometimes nausea with salivation, confusion of head, dizziness, noises in the ears, vertigo, stiffening of certain muscles, constriction of the throat, general weakness, faintness, etc. These, however, are soon lost in complete insensibility or profound coma, which sometimes comes on in from ten to twenty seconds, and is rarely postponed beyond one or two minutes; though a case is on record, in which it did not occur until a quarter of an hour after the taking of the poison.

It is in this condition of insensibility that the patient is usually first seen. The eyes are fixed and glistening, and the pupil generally dilated, and perfectly insensible to light; the respiration is deep, laboured, and slow, sometimes sobbing, and occasionally attended with frothing of the mouth; an odour of prussic acid not unfrequently exhales with the breath; the pulse is extremely feeble, irregular, often imperceptible, and the skin pale or livid, and bathed in a cool sweat; convulsions are very frequent, with tetanic rigidity of the muscles, and especially trismus, though sometimes the body is quiet and motionless; involuntary evacuations occur in some instances; and, finally, death takes place, it may be so early as two minutes from the ingestion of the acid, and generally if not always within an hour. Of seven cases of epileptic patients in one of the Parisian hospitals, poisoned at one time by the exhibition, through mistake, of a strong instead of a weak preparation of prussic acid, all of whom perished, the first died at the end of fifteen, and the last of forty-five minutes. The period is said to be shorter from a large than a small poisonous dose; though there is no fixed relation in this respect.

Should death not take place within an hour, the patient generally survives, waking up as from a deep sleep, and gradually returning to complete consciousness and health. The recovery is usually rapid. it may occur in half an hour, or be postponed for three or four hours. Sometimes vomiting takes place with returning sensibility, and probably proves salutary by the evacuation of the poison.

The lowest quantity which has been known to cause death is, I believe, nine-tenths of a grain of the anhydrous acid, equivalent to about forty-nine drops of the diluted acid of the U. S. Pharmacopoeia. So small a quantity as thirty-six drops of the latter has produced the most violent and threatening symptoms of poisoning, from which, however, the patient recovered. Occasionally a considerably larger amount has been taken without causing death. Dr. Taylor, in his Treatise on Poisons (Lond., a.d. 1848, p. 672), states that the largest dose of the anhydrous acid, from which recovery is known to have taken place spontaneously, is one grain and a half, equal to somewhat more than a drachm of the diluted acid.

Attention has been called by Dr. Chanet to a kind of chronic poisoning from prussic acid, produced by frequent exposure to its vapours, in quantities insufficient to cause immediate death, as in certain manufacturing processes. The symptoms are dull pain in the head, darting pains over the eyes, abnormal sounds in the ears, dizziness, vertigo, pain in the precordial region, difficult respiration, constriction of the throat, feelings as of suffocation, and alternate wakefulness and somnolency. (Gaz. des Hopit, Juil. 24, 1847.) it has been questioned whether hydrocyanic acid has, like digitalis, the cumulative property; whether, in other words, having been given for some time in regularly successive doses, without observable effect, it suddenly acts at last with the accumulated force of the whole, or a considerable portion of the quantity taken. I have never seen any tendency of this kind in hydrocyanic acid, and do not think that danger need be apprehended, with due care not to increase the dose to an amount which might itself prove hazardous. There may be a temporary insusceptibility to the action of the poison, which may at length suddenly cease; and a dose which, a short time previously, would not have been felt, might now prove dangerous. But it is the last dose here that acts, and not the accumulated doses before taken. The influence of the poison is too brief for this danger; and, if it do not operate within a few hours, it will probably be decomposed or eliminated without effect.