This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
It will be remembered that opium, in its primary action, stimulates the circulation moderately, and the nervous system energetically. In reference to these properties, it is indicated in most acute affections in which the vital powers and functions generally are enfeebled, as in low fevers, etc. An important consideration, in this application of opium, is that its stimulant operation continues longer, in quantities just insufficient to produce its narcotic effects, than in the full anodyne and soporific dose. Hence, as a mere stimulant, it should be given in rather small doses, which should be repeated at such intervals as to sustain a steady operation, without allowing the effects of successive doses to accumulate, and thus to bring about the indirect sedative influence of the medicine. One-eighth, one-quarter, or one-half a grain may be given for this purpose every two, four, or six hours, the smaller quantity being given at the shorter, and the larger at the longer interval. This rule applies to cases possessing the ordinary healthy susceptibility to the action of opium. Should the susceptibility be materially diminished by the disease, the dose must be increased accordingly.
In quantities insufficient to produce any material or very sensible impression on the brain, opium acts precisely like the nervous stimulants, and may be employed for similar purposes. Through the general equalizing influence upon the nervous functions which characterizes this class of medicines, it acts very happily in a great number of slight nervous derangements and vascular irritations. It is, however, only in slight affections that it acts on this principle; for, when given in doses calculated to produce an energetic impression, it ceases to be a mere general nervous stimulant, and concentrates its direct action mainly upon the brain. General or local uneasiness, restlessness, moderate wakefulness, slight pains and spasms, nausea, etc., are illustrative examples.
The indication founded on this property of opium is highly important. Not a few disorders consist essentially in a depressed or debilitated condition of the cerebral nervous centres; and it is often important to be able to apply to these centres a stimulant, which shall not also materially excite the circulation. Thus, in the collapse sometimes attendant upon the cold stage of fevers, there is not unfrequently vast depression of the nervous centres; yet, as high febrile reaction may soon supervene, the indication is very obvious for a medicine which, while it may rouse the depressed centres, and for a time may stimulate the heart, shall cease to produce the latter effect before reaction is established. Exactly such a medicine is opium, the excitant action of which upon the heart, at all times moderate, is comparatively brief, and is not likely in any degree to aggravate the coming fever. It is obvious, however, that, in such cases, the medicine must be given in its full dose, on account not only of the insusceptibility apt to exist, but also of the relative brevity of its stimulant operation on the heart. Delirium tremens, certain conditions of insanity, and various other nervous disorders call for this cerebral stimulation of opium.
The antiperiodic action, which is often strongly evinced by opium, may be considered as dependent directly upon its stimulant operation on the cerebral centres. In all regular periodical diseases, it is highly probable that the paroxysm makes its first approaches through the nervous centres, and that, if anticipated in its attack by a sufficiently strong preoccupation of those centres, it may be warded off, and set aside altogether; and it is a law of these diseases that, if the regularity of the succession be broken, the complaint itself will cease, at least for a time. Now opium, through its stimulant operation upon the cerebrum and medulla oblongata, is capable of effecting the required preoccupation; and, if given so as to be in full action at the time for the expected return of the paroxysm, will often set it aside. The fullest dose of opium is generally required to answer this indication. Not only intermittent fever, but all other regularly intermittent diseases, may very often be interrupted in this way.
Allusion has been repeatedly made to the diminished frequency of pulse which follows, in a short time, the excitement produced by a full dose of opium. A diminution in its force, also, comes on after a time; and a positive depression of the circulation is thus effected. The larger the quantity of opium taken, the more quickly is this effect produced; and the inference is that, for the fulfilment of the present indication, the medicine should be given in its fullest regular dose. It is in inflammations, and vascular irritations that opium is called for upon this ground. Its first stimulant action, however, is in the way; and it should not be employed until the force of the circulation has been subdued by suitable preliminary measures, or has somewhat subsided in the course of the disease. The opium should also, as a general rule, be associated with some other medicine calculated to modify its stimulant action; and ipecacuanha or tartar emetic is usually selected for the purpose. The diaphoretic tendency of such a combination, independently of the depressing properties of the adjuvant, contributes much to the desired object.