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The reflex autonomic effects due to inhalation of low concentrations of oxygen (Pao2 30 mm Hg) were assessed in unanesthetized rabbits from the differences in responses between groups of normal and autonomically “deefferented” rabbits and those with selective effector block. Changes due to autonomic effects were estimated for heart rate, total and regional (portal, renal, muscle, and cutaneous) peripheral resistance, and the rate of epinephrine secretion. The autonomic effects could be described by an early component having a time course similar to the initial vagal effects, where the maximum change occurred soon after the start of hypoxia and then declined to 37% of this value in 5 minutes, and a late component similar to the neural effects in muscle, starting 5 minutes after the start of hypoxia and rising slowly to 63% of the maximum in 15 minutes. Four autonomic effector patterns involving different combinations of early and late components were distinguished: (1) vasoconstriction in the portal and renal beds followed the time course of the sum of early and late components; (2) inhibition of cutaneous constrictor tone was the mirror image of the above response; (3) the biphasic sympathoadrenal effect on heart rate was the sum of a negative early and positive late component; (4) the neural constrictor effect in muscle and the rate of epinephrine secretion had only a late component. The early effects resulted in reduction in cardiac output and major redistribution of peripheral blood flow, probably due to chemoreceptor stimulation. During the late phase, cardiac output rose and blood flow was redistributed further, probably mainly through baroreceptor mechanisms.

Dynamic Characteristics of the Cardiovascular Autonomic Effects during Severe Arterial Hypoxia in the Unanesthetized Rabbit