Circulatory responses to hypocapnia in the anesthetized dog

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Abstract
The circulatory responses to hypocapnia were studied in 40 anesthetized dogs. Hypocapnia induced without a change in ventilation caused slight increase in limb vascular resistance in six dogs and decrease in one. Hypocapnia induced by hyperventilation caused increase in limb vascular resistance in six dogs and decrease in four. Following administration of phenoxybenzamine into the femoral artery, hypocapnia induced by either method invariably caused increase in limb vascular resistance (8 dogs). These results show that hypocapnia has a direct vasoconstrictor effect on limb blood vessels. In the intact limb this response may be opposed by vasodilator effects mediated through nerves. Hypocapnia induced without change in ventilation had no significant effect on cardiac output, systemic vascular resistance or arterial blood pressure (8 dogs). Hypocapnia induced by increased ventilation was associated with significant decreases in cardiac output and systemic arterial blood pressure and significant increase in systemic vascular resistance (9 dogs). These responses were probably related to the effects of increased intermittent positive pressure used to augment ventilation.

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