A hot bath applied to the whole body, or a large portion of it, produces an acceleration of the pulse and an increase of animal heat proportionate to the temperature of the bath. A bath at 106° to 108° F. will increase the pulse from the normal standard to one hundred or one hundred and twenty beats in a minute, in a short time. A bath four or five degrees hotter has been known to increase the pulse to more than one hundred and fifty beats in a minute.
When a hot bath is prolonged, the face becomes flushed, and the whole skin very red; the head aches; sight is sometimes dimmed; ringing in the ears, faintness, a stinging pain in the skin, and intense desire to urinate are symptoms which are often present. Copious perspiration and intense congestion of the skin are constant effects. The cutaneous congestion, from relaxation of the blood-vessels, is apt to continue to exist after the bath, if it is greatly prolonged, to the serious injury of the subject.
The effects of the vapor bath are essentially the same as those described, though a somewhat higher degree of heat is tolerated without injury. In the hot-air bath, a still higher heat is borne with impunity.
Rationale of Effects of the Hot Bath.—It scarcely need be repeated that all of the effects noticed, as well as those of all other baths, are chiefly the results of modifications of vital action occasioned by the agent employed. The application of heat to the body occasions relaxation of the muscular coats of the small arteries, and increased action of those vessels. No doubt this is for the purpose of bringing moisture to the surface to protect the tissues against the unnatural heat. As is the case with cold baths, the causes which modify the heat are three; viz.,—
1. The increased quantity of blood circulating through the part brings to it an increased amount of heat;
2. Increased vital and chemical action increases the production of heat;
3. The body absorbs heat from the surrounding medium as any other colder object would do.
In the general application of hot water or vapor, effects similar to its local effects are produced upon the whole surface of the body, involving, also, to a considerable extent, the deeper structures. The pulse is accelerated because the small arteries are distended and more active, creating a demand for a greater quantity of blood, requiring an increase in the heart’s action. It is also quite probable that the action of the heart is somewhat quickened as the result of the influence of heat upon the pneumogastric nerve which controls it.
The cerebral symptoms, faintness, etc., which occur when heat is applied in excess, are the result of the diversion of so large a proportion of the blood into the superficial vessels. A prolonged hot foot bath or leg bath will often produce faintness.
There are few agents which will so rapidly produce such powerfully excitant and stimulant effects as the hot bath. The painful and undesirable results occasioned by its incautious use are evidences of its power.