THE EFFECT OF HYPERTHERMIA ON THE SYSTEMIC AND PULMONARY CIRCULATION OF THE OX (Bos taurus)

Abstract

The cardiac output, arterial blood pressure, pulmonary arterial pressure, heart rate, blood volume, respiratory rate and rectal temperature of the unanaesthetized ox have been measured at environmental temperatures of 150 C. and 400 C. (low humidity) and also during hyperthermia induced by increasing the humidity of the hot environment (400 C.). Derived values for total peripheral resistance, central blood volume and cardiac work were obtained from the measured data.The cardiac output, central blood volume and respiratory rate were significantly higher, while the mean blood pressure and calculated total peripheral resistance were significantly lower, in the hot dry environment than they were at 15°C. These results seemed to be consistent with the hypothesis that a peripheral vasodilatation and an increased venous return occurred in the hot dry environment.The increase in systemic arterial blood pressure during hyperthermia was attributed to the concomitant increase in cardiac output because the total peripheral resistance diminished. The increase in the cardiac output was the result of an increased heart rate. At a rectal temperature of 41.50 C. the stroke volume of the heart was considerably lower than its control value at an environmental temperature of 15°C. The work of the heart was increased during hyperthermia.The increased respiratory activity in hyperthermic animals was associated with a diminution of the mean pulmonary arterial pressure and an increase in the central blood volume.No statistically significant differences in plasma or blood volumes of the animals were recorded between environments of 150 C. and 400 C. (low humidity) and 400 C.(high humidity). The plasma volume tended to diminish during hyperthermia while the blood volume tended to be maintained close to its control value as a result of an increase in the proportion of red cells in the blood.At a rectal temperature of 41.50 C. none of the animals was in a state of circulatory failure, but there was evidence that their cardiovascular systems were severely stressed at this temperature.IN a previous paper [Ingram and Whittow, 1963] it was reported that the arterial blood pressure of the unan2esthetized ox increases during hyperthermia. The cause of the increase in blood pressure was not established. One of the purposes of the experiments described in the present report was therefore to examine the relative contribution of an increase in cardiac output, blood volume and total peripheral resistance to the pressor response to hyperthermia. An additional object of the present investigation was to determine if the increase in systemic arterial blood pressure during hyperthermia was associated with a change in pulmonary arterial pressure and in the volume of blood in the lungs. The ox is a panting animal; a considerable increase in both respiratory rate and minute volume occur during hyperthermia [Findlay, 1957; Whittow, 1962, 1963]. Little is known about the effects of thermal polypncea and of the slower deeper type of

Effects of Exposure to High Temperature and Feeding Level on Regional Blood Flow and Oxidative Capacity of Tissues in Piglets

“…Some studies reported an increase in cardiac output in man (Rowell, 1974(Rowell, , 1983, dog (Hales & Dampney, 1975), ox (Whittow, 1965), or chicken (Whittow et al 1964) exposed to high ambient temperatures, whereas no change was observed in the baboon (Hales et al 1979) or sheep (Hales, 1973a). It is of importance to note that those experiments were all performed with animals under acute heat stress, which differs from our study performed with warm-acclimated pigs.…”

The effect of hyperthermia and localized heating of the anterior hypothalamus on the sympatho‐adrenal system of the ox (Bos taurus)

“…Our results confirm those of Symbas et al (1964), who found an increase in total plasma catecholamine concentration when the rectal temperature of anaesthetized dogs had risen to 40.50 C, although the catecholamine levels they reported were very much higher than those recorded here. The plasma volume of the ox decreases during hyperthermia, the mean decrease amounting to approximately 10 % (Whittow, 1965). It is apparent from the magnitude of the increases in plasma catecholamine concentration observed in the present experiments that such haemo-concentration could not be the main factor causing an increase in the plasma catecholamine levels.…”

“…The mean respiratory rate increased to a maximal value at a rectal temperature of 40-5°C and slowly declined at higher rectal temperatures, although there was no significant difference in the mean respiratory rate measured at rectal temperatures of 40.50 and 41-5' C. This pattern of respiratory activity during hyperthermia in the ox has been observed before (Findlay, 1957;Bianca & Findlay, 1962;Ingram & Whittow, 1962b;Whittow, 1965). Analysis of variance indicated that the variation in plasma adrenaline levels associated with hyperthermia was statistically significant at the 0 1 % level.…”

“…In nine animals samples of blood were withdrawn from a cannula inserted into the exteriorized carotid artery and passed down into the aorta (Whittow, 1965). In the two splanchnicotomized animals, aortic blood was obtained by trans-thoracic aortic puncture (Stowe & Good, 1960;Whittow & Robertshaw, 1965).…”

“…However, evidence for an increased sympatho-adrenal activity during hyperthermia in other species is conflicting (Hartman & Hartman, 1923;Symbas, Jellinek, Cooper & Hanlon, 1964) and, as a result of their investigation of the adrenergic sudomotor mechanism of the ox, Findlay & Robertshaw (1965) concluded that mild hyperthermia was not associated with any adreno-medullary stimulation although the sweat glands appear to require an intact nerve supply. Furthermore, although the effects of hyperthermia in the ox can be simulated to a large extent by localized heating of the anterior hypothalamus and preoptic region alone (Ingram & Whittow, 1962aIngram, McLean & Whittow, 1963;Whittow, 1965), it is known that localized heating of this area of the brain in the unanaesthetized goat, another ruminant animal, does not result in an increase in the urinary excretion of catecholamines (Andersson, Gale, Hokfelt & Ohga, 1963).…”