This section is from the book "Materia Medica And Therapeutics: An Introduction to the National Treatment of Disease", by John Mitchell Bruce. Also available from Amazon: The pharmacology and therapeutics of the materia medica.
Impurities. - Nitric and hydrocyanic acids.
Applied directly to peripheral nerves and muscles, nitrite of amyl depresses or paralyses them. It is never so employed in. man. Internally, the drug is seldom given by the mouth, except in cholera.
Nitrite of amyl is almost invariably administered by inhalation, a few drops being kept ready for use in a glass capsule (enveloped in cotton wool), which may be broken between the fingers and thumb when required. The vapour instantly enters the circulation through the lungs, converts a certain amount of haemoglobin into methaemoglobin, and thus interferes with the oxygenating function of the red corpuscles; the amount of oxygen absorbed (in animals) being quickly lowered, as well as the excretion of carbonic acid. The blood of animals killed by nitrite of amyl is of a chocolate colour; but the effect of an ordinary inhalation in man is very transitory.
Nitrite of amyl almost instantaneously reaches the tissues, and produces striking phenomena. Two to five drops, inhaled as directed, immediately produce a sense of fulness and throbbing in the head; flushing of the face, neck, and trunk; tingling over the surface generally; dilatation of the pupils, and disturbance of vision; giddiness and unsteady gait; increased frequency and force, that is, palpitation, of the heart; visible pulsation of the carotids; restlessness and anxiety of mind. These symptoms quickly disappear, leaving possibly slight headache. Larger doses aggravate all the phenomena, but never produce unconsciousness; mental confusion, intense bodily depression, coldness of the extremities, and sweats being the result, followed by severe headache which may last for hours. Very rarely convulsions occur in man as in some of the lower animals.
The specific action of nitrite of amyl proves on analysis to be almost confined to the circulatory system, the other parts being chiefly involved secondarily. Two distinct effects are produced on the circulation; the heart is greatly accelerated, with but little, if any, increase of its force; the peripheral vessels are dilated by relaxation of their muscular coat. Some authorities hold that the cardiac acceleration is duo to depression of the cardiac centre, others to depression of the vagus in the heart; some refer the vascular relaxation to the action of the nitrite on the vasor centre in the medulla, others to its action on the vaso-motor nerves and muscular walls. Be this as it may, the fact remains that the blood pressure falls to a remarkable degree, that is, the resistance to the discharge of the left ventricle is correspondingly diminished; whilst this discharge is accomplished much more frequently within a given time. In other words, the left ventricle, under the influence of nitrite of amyl, has at once less work to accomplish, and more force wherewith to accomplish it; that is, is greatly relieved. These considerations led Dr. Lauder Brunton to the employment of the drug in those cases of the complex class of disease known as angina pectoris, in which agonising pain in the breast and neighbourhood is due to distension of the left ventricle, from its inability to empty itself against the pressure in the aorta, and in which fatal paralysis of the heart, or even rupture of its walls, is the result of the unequal effort. Clinical experience has fully confirmed the value of amyl nitrite, in cases where spasm of the arteries is damming the blood back upon the ventricle, the channels being instantly opened and the ventricle rapidly emptied by the double effect of the drug. The pain of the aneurism of the aorta, and of other forms of cardiac disease and disorder, can often be relieved by amyl, but caution must be exercised in the first trial. Threatening death from cardiac paralysis in chloroform anaesthesia, and sea-sickness in which the blood pressure is greatly disturbed, are sometimes successfully treated with amyl. Some cases of epilepsy, accompanied by spasm of the cerebral vessels and facial pallor, and of megrim or sick-headache, due to similar spasm in the trigeminal area, are also benefited by this drug.
The reflex irritability of the cord is reduced (in animals) by nitrite of amyl, which has therefore been proposed as a remedy in poisoning by strychnia. Neither the peripheral nerves nor the muscles are affected when it is given through the blood. Respiration is disturbed, apparently by the alteration of the haemoglobin and circulation, not through the nervous system. The nitrite sometimes affords immediate relief in asthma, but the dyspnoea may as quickly return. The body temperature falls, from obvious causes.
Nitrite of amyl probably escapes from the body by the urine, which is decidedly increased in amount, and may contain sugar. Both of these effects are probably results of local disturbance of the pressure in the kidneys and liver respectively.