Respiratory function during thermal tachypnoea in sheep

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Abstract
Four Merino wethers were exposed to dry bulb temperatures ranging from approximately 20 to 60[degree]C, and the concurrent changes in respiratory frequency, tidal volume, respiratory minute volume, alveolar ventilation, dead space ventilation, CO2 output, rectal temperature, and arterial and mixed venous blood, CO2 content, CO2 partial pressure and pH were established. The respiratory response to heat exposure showed 2 phases. Respiratory minute volume was initially increased by a rise in the respiratory frequency, while tidal volume decreased. After more prolonged exposure there was a 2nd phase in which respiratory minute volume was further increased by an increase in the tidal volume; respiratory frequency was now slower than in the 1st phase but was still well above control values. The increase in respiratory minute volume during the 1st phase of the response was restricted almost entirely to the respiratory dead space; changes in blood CO2 and pH were slight. In the 2nd phase, respiratory minute volume showed a much greater increase, and a change of alveolar ventilation to about 5 times the control level resulted in severe respiratory alkalosis. Contrary to findings in cattle, the slower, deeper form of respiration could be elicited even with rectal temperature in the normal range. This change in respiration appears to be the result of either peripheral thermoreceptor function or mechanical demands of the respiratory system. The neglect of control of acid-base balance during the 2nd phase indicates the existence of a dominant thermal stimulus or modification of respiratory control mechanisms.

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