Hypoxia and its influences on the cardiovascular system of self breathing cats

ABSTRACT

Effects of acute hypoxia with/without the presence of hypocapnia on the cardiovascular system [CVS] were studied in anesthetized spontaneously breathing cats. After preparation, hypoxic air [8% O[2] and 92% N[2]] was given to the animal to reduce arterial O[2] tension to 45 +/- 3 mmHg. Stimulation of carotid chemoreceptors by hypoxic blood induced hyperventilation and the outcome was hypocapnia and alkalosis. It is shown that these two parameters reduce carotid chemoreceptor sensitivity to hypoxia. Hence, we tried to determine if this combination would hamper the influences of hypoxia on the CVS. Therefore, in a second run in the same cat induction of hypocapnia and alkalosis was prevented by adding 5% C0[2] to hypoxic air. The results of this study indicated that, hypoxia alone did not change vascular tone and heart rate; but it did increase respiratory rate, aortic flow, and mean arterial blood pressure [Pa]. The combination of hypoxia with hypocapnia diminished the stimulatory influences of carotid chemoreceptors on respiration. Despite the presence of bradycardia, aortic flow, Pa, and vessel tone did not change. From this observation we conclude that the reflex effect of hypoxia on vascular tone is not affected by hypocapnia; whereas, its negative chronotropic effect on the heart is potentiated