The Immediate‐effects of Acute Hypoxia on the Heart Rate, Arterial Pressure, Cardiac Output and Ventilation of the Unanæsthetized Rabbit

Abstract

The circulatory and respiratory changes occurring during the first 5 min. of hypoxia were studied in unanawsthetized rabbits. Mild degrees of hypoxia produced an increase in ventilation in the rabbit, without eliciting a detectable circulatory response. The early circulatory effects consisted of bradycardia, a rise in mean arterial pressure and a fall in cardiac output, indicating predominant systemic vasoconstriction. The magnitude of the bradyeardia and rise in arterial pressure were related to the fall in arterial 02 saturation. Atropine or vagotomy reduced or abolished the bradycardia, but greatly accentuated the rise in blood pressure. Denervation of the carotid baroreceptors and chemoreceptors almost abolished the bradyeardia and diminished the rise in blood pressure.IN the first few minutes after induction of acute hypoxia most mammalian species develop tachycardia, a moderate rise in mean arterial pressure and an increase in ventilation. It is generally agreed that the role of the arterial chemoreceptors is of paramount importance in the early respiratory drive during oxygen lack [Bjurstedt, 1946;Otis, 1947 and1949]. Up till recently the tachycardia which develops in the early phase of hypoxia has also been considered to be of chemoreceptor origin. Recent observations in dogs on the effects of stimulation of the arterial chemoreceptors by perfusion with venous blood cast considerable doubt on this interpretation, since these procedures usually produce bradyeardia [Bernthal, Greene and Revzin, 1951; Neil, 1956;Scott, 1958 and.The unanaesthetized rabbit develops during the first few minutes of hypoxia a pronounced bradycardia rather than the usual tachyeardia and some rise in arterial pressure. The purpose of this paper is to report the results of an analysis of the mechanism of this early circulatory response to acute hypoxia. METHODSUnanaesthetized rabbits were used in these experiments, except when otherwise stated. The operative techniques were carried out under local anaesthesia and included tracheotomy, central ear artery cannulation and right atrial catheterization

The effect of progressive hypoxia on the respiratory and cardiovascular systems of the chicken

“…The initial cardiovascular changes observed in the unanaesthetized rabbit in response to systemic hypoxia (Korner & Edwards, 1960) resemble closely those of the artificially ventilated dog, and the fall in heart rate is related closely to blood oxygen tension, which is in itself determined by ventilation volume. Korner (1965) points out that the rabbit, because of its shallow breathing, is less able than other species to increase alveolar ventilation when exposed to low oxygen concentrations.…”

“…Both bilateral vagotomy and denervation of the carotid body chemoreceptors abolish the bradycardia in rabbits (Korner & Edwards, 1960). Recently Chalmers, Isbister, Korner & Mok (1965) have reported that the sympathetic nervous system is important during hypoxia in maintaining cardiac output close to normal values in these animals.…”

The role of the sympathetic nervous system in the circulatory response of the rabbit to arterial hypoxia.

“…The influence of the sympathetic nervous system on the chronotropic effects is more difficult to assess. This is because the bradycardia in the rabbit is largely vagally mediated (Korner & Edwards, 1960a), and is the result of simultaneous activation in these experiments of the arterial chemoreceptors by hypoxia (Bernthal, Green & Revzin, 1951;Daly & Scott, 1958;Downing, Remensnyder & Mitchell 1962; Korner, 1965a) and of the baroreceptors by the blood-pressure changes. These vagal effects would oppose the chronotropic effects of initial beta-adrenergic activity.…”

Effects of hyperventilation on the circulatory response of the rabbit to arterial hypoxia

“…These sympatho-adrenal effects probably constitute the primary reflex response to arterial chemoreceptor stimulation in the anaesthetized rabbit since they are virtually abolished by section of the carotid sinus and aortic nerves. They also constitute part of the primary arterial chemoreceptor response of the unanaesthetized rabbit, where, in addition to the sympatho-adrenal effects, efferent vagal activity also increases (Korner & Edwards, 1960;Korner, 1965a;Korner, Langsford et al 1968).…”