This section is from the book "Materia Medica Pharmacy, Pharmacology And Therapeutics", by W. Hale White. Also available from Amazon: Materia Medica Pharmacy, Pharmacology And Therapeutics..
C5H11No2=116.78. A liquid containing about 80 per cent. of Amyl (principally Iso-Amyl) Nitrite, together with variable quantities of undetermi-|ned compounds.
A clear yellow, or pale-yellow liquid, of a peculiar, ethereal, fruity odor, and a pungent, aromatic taste. Sp. gr., 0.870 to 0.880. Solubility. - Insoluble in water; soluble in Ether, Chloroform, or Alcohol.
Free acid and amyl nitrate.
Dose, 1 to 3 m.; .06 to .20 c.c cautiously inhaled from a handkerchief in which a glass capsule containing the Amyl Nitrite has been crushed; 1/2 to 1 m.; .03 to .06 c.c. internally, dissolved in alcohol.
Locally applied it diminishes the activity of the sensory nerves, but they quickly recover.
Amyl nitrite is rarely given by the mouth, so the following account will refer to the effects of inhalation. The effects of a single inhalation pass off in two or three minutes.
From the medical point of view by far the most important effects of amyl nitrite are those produced upon the heart and vessels. Within a minute of inhalation the face flushes, the heart beats very rapidly, and violently, there is a throbbing in the head, and the vessels, e.g., the carotids, may be seen to pulsate actively. Headache, giddiness, dilatation of the pupils, and increased respiratory movements quickly supervene. All the vessels of the body rapidly dilate, hence the flushing. They may be actually seen to widen in the ear of a rabbit or in the retina. This is due to a direct action on the muscular coats of the arterioles, for it happens if the cord is destroyed.
The blood-pressure and arterial tension, of course, fall very low. The increase in the rate of the pulse is unaccompanied by any alteration in the force of the beat; it is apparently due to a depressing influence on the inhibitory vagus centre; the vaso-motor paralysis will, however, produce a rapid pulse. In toxic doses the heart may be arrested in diastole from direct action on the cardiac muscle.
The rapidity and depth of respiration are at first increased, probably from central stimulation; the respiratory centres are later depressed, the breathing becoming slower and shallower, and usually death finally occurs from paralytic asphyxia of central origin.
Many of the symptoms referable to the nervous system are secondary effects of the dilatation of the vessels of the brain and spinal cord. Such are the throbbing, sense of fulness, giddiness and headache noticed directly after inhalation. The headache may remain some time. If much has been inhaled there is unsteadiness of gait and general restlessness. The pupil dilates, and disturbances of vision are present. The motor centres of the cord are profoundly depressed; therefore after large doses reflex actions are abolished. The function of sensory nerves, motor nerves, and muscles is depressed by the local application of the drug to them, but not after inhalation until shortly before death.
Amyl nitrite causes this to fall considerably, both in fever and health. The fall is due to the peripheral vascular dilatation, and if large doses are given, to the changes in the blood.
The drug probably escapes in the urine as nitrites and nitrates; it is slightly diuretic, and may cause glycosuria, due, it is said, to dilatation of the vessels of the liver or of the medulla.
Nitrites, given in medicinal doses, circulate as sodium nitrite. Outside the body they greatly diminish oxidation, and the same takes place in the blood. After the inhalation of a considerable amount (more than is usually given to a man) the arterial and venous blood both become a uniform chocolate color.
This is due to the formation of methaemoglobin and another body, nitric oxide haemoglobin. The haemoglobin can no longer absorb oxygen, and hence its oxidizing power is abolished. It is by this action on the blood that, in man, nitrites kill, not by their vaso-dilator action; therefore the treatment for poisoning by them is inhalation of oxygen, that more may be dissolved in the plasma. In some of the lower animals they kill by acting as a direct poison to animal tissues.
Brunton in 1867 observed that in a patient suffering from angina pectoris the peripheral vessels were strongly contracted during an attack of pain. This induced him to make the patient inhale amyl nitrite, and it was found that the vessels dilated and the pain passed off. Inhalation of amyl nitrite is now used for all sorts of cardiac pain, especially when it comes on in paryoxysms. Generally the drug affords relief in a minute or so after inhalation, but by no means always. We do not sufficiently understand the pathology of angina pectoris to know how it acts. It may be by dilating the peripheral vessels; but against that view is the fact that they are not always contracted during attacks of angina pectoris, and amyl nitrite may relieve patients in whom the vessels are not contracted. The attacks of pain common in thoracic aneurism may be relieved by it. It is used to avert the dangerous pallor sometimes seen during the administration of chloroform, and may be inhaled for other forms of syncope. The peculiar hot flushes experienced by some women during the menopause are benefited by inhalation of it; this is probably untrue.
If it is inhaled when the aura is felt, an epileptic fit may sometimes be prevented, and it has also been found useful during the status epilepticus. Because in migraine the vessels of the head are contracted, it has been used, and sometimes successfully, for this complaint. Its depressing action on the cord has suggested its employment in tetanus and strychnine poisoning.