Depression of the reflex tachycardia from the left atrial receptors by acidaemia.

1. The effects are described of an acidaemia, produced either by the inhalation of carbon dioxide or by an intravenous infusion of hydrochloric acid, on the reflex increase in heart rate which results from stimulation of the left atrial receptors in the anaesthetized dog.2. The results show that during acidaemia the reflex increase in heart rate from stimulation of the left atrial receptors is reduced. The extent of the reduction is related to the degree of the acidaemia.3. It is suggested that it is necessary to monitor and correct changes in the pH of arterial blood in experiments which involve reflex changes in heart rate, especially if the efferent pathway involves the sympathetic nerves.4. It is also suggested that the occurrence of such an acidaemia in anaesthetized surgically traumatized animals may be a factor contributing to the variability in results from investigations involving reflex changes in heart rate in response to stimulation of the left side of the heart.