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.
Hot Bath. In a bath of this kind, the effects are both stimulant and sedative; and whether one effect or the other shall predominate, must depend on the degree of the temperature above that of the surface of the body. When the elevation is but moderate, the two influences may balance each other; so that neither elevation nor depression of the vital functions shall be perceptible; and this is the condition in which the bath can be longest borne, and which should be aimed at when the mere cleansing effect of water, with as little impression as possible of any kind upon the system, is required. But this equation of effect is obtained only after a short period of immersion. Heat acts more quickly than in which satisfactory results are said to have been obtained from it, are, among others, acute and chronic rheumatism, gout, neuralgia, catarrh, various throat affections, diarrhoea, dysentery, diseases of the liver and stomach, dropsy, scrofulous affections, and incipient phthisis. It is not, however, as a general stimulant exclusively that it operates in such cases. Its strong revulsive influence must greatly contribute to its efficiency; and, as a powerful means of producing diaphoresis, it acts both as an evacuant, and as an eliminative agent with great effect. In reference to these effects of the hot-air bath, it will be more particularly noticed hereafter. (Note to the third edition).
The limits of the hot bath, as determined by the sensations, may be placed, in reference to the lower extreme, at the point when the heat is merely somewhat uncomfortable; in reference to the higher, at that in which it is barely not insupportable. As indicated by thermometrical degrees, they cannot be fixed precisely; because they vary with the variable sensibility and temperature of the surface; but the lowest point may be considered as somewhere between 95° and 100° Fahr., and the highest between 106° and 112°; and it is never advisable to exceed the degree last mentioned. At the mean of 103°, the bath is actively stimulant, producing a strong sensation of heat, reddening and expanding the surface of the body, increasing the frequency, force, and fulness of the pulse, hurrying the respiration, and causing at first an agreeable excitement of the brain, not unlike the effect produced by wine, but ending. if the immersion continue, in painful sensations of fulness, distension, and vertigo. After a time, perspiration breaks out upon those parts of the body not covered with the water; and, if the patient is removed from the bath, and placed in bed, the whole surface usually becomes relaxed, and bathed in a copious sweat. The excitement gradually subsides; and is followed, if the regular succession of events is not modified through some abnormal state of the system, by universal relaxation, with depression of the pulse, muscular weakness, feelings of languor and drowsiness, and ultimately sleep. The immersion may continue from four or five minutes to half an hour, according to the effects; the patient being always , removed when unpleasant cerebral phenomena are produced, and never allowed to remain when the stimulant effect is the main indication, until evidenced of depression supervene.
The operation of the water in the hot bath is at first, through its conducting power, to hasten the stimulating effects of the heat, but afterwards, by the relaxation it produces, to favour the secondary depression; and the latter result often adds greatly to the beneficial influence of the remedy, when it is designed to act rather as a revulsive agent than as a general excitant.
The conditions indicating the use of the hot bath are 1. coldness of the surface,with either general prostration, or powerful and concentrated internal irritation, inflammation, or congestion; and 2. an abnormal state of system, in which a strong impression is required, to break up long-continued and obstinate morbid associations. The following are affections in which these indications are presented.
In the cold stage of febrile diseases, particularly those of a malignant or pernicious character, there is sometimes a degree of prostration and indisposition or inability to react, which is extremely dangerous. The nervous centres seem to have become almost inert under the violence of the morbid cause, and the most important vital functions are prostrate under the want of their necessary influence. The heart acts feebly, the surface is cold and pale, and the great internal organs are loaded with the venous blood, which accumulates in them because the moving forces are unable to carry it forward. Sometimes death takes place without any reaction; sometimes feeble and insufficient efforts at reaction are made, and the patient sinks back into the same alarming prostration as at first. In aid of other stimulant measures, the hot bath may often be resorted to, in these cases, with great advantage. Through its powerful impression on the surface, it rouses the nervous centres from their torpor, and thus indirectly excites the circulatory and respiratory functions; by heating the blood as it passes through the vessels of the skin, it renders that fluid more stimulating to the great interior organs, as the heart, lungs, and brain, into which it is conveyed; and, while thus acting as a powerful general stimulant, it is no less powerfully revulsive, irritating the whole of the cutaneous capillaries to an active expansion, and drawing into them the blood before accumulated in the central viscera and great venous trunks. It is one of the most efficient agents, in such cases, in bringing about reaction. Typhus fever, pernicious miasmatic fever, smallpox, scarlatina, and epidemic erysipelas are among the diseases, in the initial or cold stage of which the hot bath not unfrequently proves useful on the principles here stated. It has, too, the great advantage over internal stimulants, that it can be at once withdrawn when needed no longer, and does not add, by a prolonged influence, to the violence of the reaction when this is brought about.