This section is from the book "Materia Medica And Therapeutics Inorganic Substances", by Charles D. F. Phillips. Also available from Amazon: Materia medica and therapeutics.
Continued Small Doses - 1 To 5 Min. Hydrochloric Acid (Acidum Hydrochloricum) Treatment given at moderate intervals of two to four hours, lessen the force and rapidity of the heart-action: 10 to 20 min. taken by the mouth, or inhaled, may cause giddiness and faintness, with slowing, or sometimes quickening, of the pulse, and suffusion of the face. With animals, full or large doses cause a sudden arrest in diastole; this continues for a variable time, and is followed by quickened action, and afterward by diminution, and then by either gradual return to the normal number of beats, or total cessation according to the dose, and to the age, strength, etc., of the animals (Laschkewitsch, Preyer).
A point of much interest is the statement that section of the vagi in the neck prevents this primary diastolic arrest, and Preyer, who has studied the subject with the greatest care, and has made very numerous experiments, affirms that after such section, no slowing of the heart-action occurs under doses that would with uncut vagi stop the heart (Op. cit., erst. Theil, 1868, p. 35); also that death, under toxic doses, is much slower when these nerves are divided than when they are not. We know from Pfluger's researches that weak stimulation of the vagus causes slowing of the heart, and a very strong stimulus of it causes stoppage in diastole, and Preyer argues that the action of prussic acid on the heart is exerted through the vagi in accordance with these results, and the secondary and temporary quickening which occurs with certain doses is due to a secondary paralysis of the control-influence of the same nerves.
On the other hand, we have directly contradictory observations upon cats by Boehm and Knie, who found no primary diastolic arrest, and no influence exerted either way by section of the pneumogastrics; but their animals, though more accurately dosed, were in still less natural condition than those of Preyer, for they were chloralized, tracheotomized, and injected through an exposed jugular vein: we cannot think their observations conclusive (v. p. 265).
With very large toxic doses death is instantaneous, and the heart is arrested in diastole without any recurrence of ventricular contraction, though some movement of the auricles may be perceived on opening the chest (Lecorche and Meuriot, Archives Gen., t. xi., 6e serie): with such doses the result is not influenced by section of the vagi, and death is presumed to follow direct cardiac paralysis (Preyer). Applied directly to the heart, the acid arrests its movement and destroys its muscular irritability.
Arterial Pressure in the vessels is said to be increased under the action of prussic acid (Wahl), but according to Boehm and Knie, such increase is temporary only; the pressure soons falls below normal, and after large doses remains so for some time.
The startling rapidity of action of prussic acid suggests an immediate toxic effect on the blood, and there is indeed a remarkable color-change induced, which has been thought to give a clue to the intimate working of the poison. Thus, if the jugular vein of a rabbit be exposed, and seen to contain dark blood, and a toxic dose of acid be then given by the mouth, so soon as convulsive movements indicate its taking effect, will the stream of venous blood take on a clear red color, and the vessel greatly enlarge in size. If the blood be let flow from an incision, a similar change is observed, and if the right heart be examined in situ, the dark blood contained in it is equally seen to become red; it is so also in the nose and ears (Gaeth-gens: Med.-Chem. Untersuch., drit. Heft., Berlin, 1868, Hoppe-Seyler). This had been noticed, though with less detail, by earlier observers, by Vietz and others, by Claude Bernard (who got a similar result with carbonic oxide), and by Preyer, who found the same appearance caused not only by diluted sulphuretted hydrogen, but also by the mere removal of any obstructions placed in the air-passages (Op. cit., p. 88). It is not therefore, due to a specific action of prussic acid, but is secondary to altered respiration, and although very interesting, has not the importance attached to it by Gaethgens. The apparently contradictory observations of Bischoff and others, to the effect that all the blood found in the body after cyanic poisoning is unusually dark and venous, are explained by a difference mainly in the rapidity of the poisonous action: if life be prolonged for a few minutes, the red color is gradually replaced by dark, while if death be very sudden, red blood only is found in the heart- sometimes even on the following day (C. Bernard). In cold-blooded animals, the red color persists much longer than in the warm-blooded (Preyer).