Causes. The immediate cause of sun-stroke is exposure to undue heat, but this need not be the heat of the sun’s rays direct. A large proportion of cases in the human subject are attacked during the night, and again at sea where an attack in a passenger is practically unknown, it is terribly common among stokers working in a close atmosphere of 100° to 150° F.
The attendant conditions have much influence in determining an attack, thus it is generally held that heat with excess of moisture is the most injurious, yet in Cincinnati, statistics showed a greater number of cases in man when the air was dry. The suppression of perspiration and the arrest of cooling by evaporation in the latter case would tend to a rapid increase of the body temperature, and the condition would be aggravated by the electric tension usually present with the dry air. With the hot, moist air perspiration might continue, but evaporation would be hindered, and there would be arrest of the cooling process and an extreme relaxation of the system.
Again, if is usually found that seizures take place during or after hard muscular exertion in a hot period, and much importance is attached to the attendant exhaustion, the excess of muscular waste, and the alteration of the myosin, which latter coagulates at a lower temperature in the overworked animal. But on the other hand, experiment shows that the animal confined to absolute inactivity in the hot sunshine or in a high temperature (at 90°), dies in a few hours, whereas another animal left at liberty in the same temperature does not suffer materially. The explanation appears to be that the dog, kept absolutely still, has the continuous action of the heat on the same parts and on the same blood, for the capillaries dilate, and the blood is delayed, overheated, and surcharged with carbon dioxide, and the result is either syncope from heart failure, or asphyxia from excessive carbonization of the blood. Back of these and concurring with them is the paralysis of the vaso-motor and heat generating nerve centres, from the high temperature or the condition of the blood.
The excessive carbonization of the blood deserves another word. The prolonged contact of the blood and air in the lungs is essential to the free interchange of oxygen and carbon dioxide. Vierordt showed that with sixty respirations per minute the expired air became charged with but 2.4 per cent. of this gas, whereas with fourteen respirations it contained 4.34 per cent. Therefore, with violent muscular work (which charges the blood with carbon dioxide) and rapid breathing (which fails to secure its elimination), the overdriven animal soon perishes from asphyxia. Under a high temperature of the external air, this condition is aggravated since the rarefied air contains just so much the less oxygen, the absorption of which is the measure of the exhalation of carbon dioxide.
Dr. H. C. Wood, who has experimented largely on the subject in animals, finds the cause of heart failure in the coagulation of the myosin, which takes place under ordinary circumstances at 115° F., but at a much lower temperature when a muscle has been in great activity immediately before death. As the temperature of thermic fever frequently reaches 113°, or even higher, he easily accounts for the sudden syncope occurring during active work in a high temperature. As an example of such sudden rigor, he adduces the sudden stiffening of the bodies of some soldiers killed in battle during hot weather.
Wood further shows that all the symptoms of thermic fever can be produced in the rabbit by concentrating the temperature on its head, which seems to imply a direct action on the brain and in particular on the heat producing and vaso-motor centres. This becomes the more reasonable that the temperature attained does not impair the vitality of the blood but, leaves the leucocytes possessed of their amœboid motion. He found, moreover, that if the heat were withdrawn before it has produced permanent injury to the nervous system, blood or other tissues, the convulsions and unconsciousness are immediately relieved and the animal recovers.
Other conditions may be adduced as predisposing or concurrent causes of thermic fever. Whatever impairs the animal vigor has this effect. Fatigue, as already noticed, is a potent factor, in man a drinking habit; in all animals a long persistence of the heat during the night as well as the day; impure air in badly ventilated buildings; and mechanical restriction on the freedom of breathing. In military barracks with the daily temperature at 118° F. and the night temperature 105, the mortality became extreme, and in close city car stables the proportion of sun-strokes is enhanced. In all such cases, the air becomes necessarily more and more impure continually. The atmosphere has the same heat as the animal body, so that no upward current from the latter can be established, to create a diffusion. The carbon dioxide and other emanations from the lungs, the exhalations from the skin, dung and urine, accumulate in the air immediately surrounding the animal and respiration becomes increasingly imperfect and difficult. This condition is further aggravated by the accumulation of the animal heat in the body. The blood circulating in the skin can no longer be cooled, to return with refrigerating effect on the interior of the body, the cooling that would come from the evaporation of sweat is obviated by the suppression of that secretion, as well as by the saturation of the zone of air immediately surrounding the body, and thus the tendency is to a steady increase of the body temperature until the limit of viability has been passed.
The mechanical restriction of respiration should not be overlooked. In European soldiers landed in India and marched in the tight woolen clothing and close stocks a high mortality has been induced and in horses with tight girths or collars and short bearing reins, and oxen working in collars a similar result is observed. Any condition of fever is a potent predisposing factor.
Horses or cattle that are put to violent or continued exertion when too fat or out of condition are especially subject to sun-stroke. Fat cattle driven to market under a hot sun, or shipped by rail, crowded in a car and delayed on a siding under a hot sun, with no circulation of air, often have insolation in its most violent form. The same may be seen in the hot stockyard, with a still atmosphere and the fat animals subjected to the full blaze of a July sun. The chafed feet caused by travel, and the muscular weariness caused by standing in the moving car are material additions to the danger.
Similarly horses suffer on the race track when subjected to protracted and severe work in hot weather, or again dragging loads in a heated street under a vertical sun, or on a side hill with the sun’s rays striking perpendicularly to its surface.