The Effect of Thermally-induced Hyperpnea on the Acid-base Status of the Blood of Calves

Abstract

SUMMARY Eight bull calves were exposed to severe heat (40°C. dry bulb, 38°C. wet bulb temperature) until their body temperature reached 42.0°C., which, on average, occurred after 2½ hours. This treatment produced the following mean changes in the animals: ventilation increased from 29 to 120 l./min., which in a first phase was accomplished by rapid and shallow breathing and in a second phase by slower and deeper breathing. The total carbon dioxide content and carbon dioxide tension of the blood plasma decreased from 30 to 23 mM./l. and from 44 to 17 mm. Hg respectively, while the pH of the blood increased from 7.43 to 7.78 and that of the urine from 7.2 to 8.2 respectively. These changes in respiration and in the acid-base status of the blood were accompanied by increases in heart rate, blood glucose and blood lactic acid, and there was a general tendency for plasma calcium to decline. These findings were interpreted as representing a respiratory alkalosis due to overventilation, which, however, was being counteracted by the excretion of alkali from the kidneys and accumulation of lactic acid in the blood. It was further inferred that hypocalcaemic tetany did not constitute a serious hazard in calves exposed to severe heat. In a supplementary experiment 3 other calves were exposed for 4 hours at a time to ‘mild heat’, i.e. to thermal conditions which just allowed respiration and body temperature to attain steady states. Under these conditions, which are comparable to those found in many tropical areas, body temperature rose to 40.1°C. and heart rate to 116 beats/min. Ventilation increased by means of rapid and shallow breathing to 79 l./min. There was, however, no substantial change in the acid-base status of the animals as judged by the behaviour of the carbon dioxide content and tension and the pH of the blood. The levels of blood glucose and blood lactic acid did not rise. It is concluded that under conditions of ‘mild heat’ there is no serious threat of a respiratory alkalosis. However, any additional stress involving a further increase in ventilation, such as high altitude, physical exercise or excitement might tip the balance to the alkaline side.