An effect of hypoxemia on vascular responsiveness and blood pressure regulation has not been demonstrated in man. The response of forearm resistance vessels to several vasoconstrictor stimuli was compared during normoxia and acute hypoxia. Forearm vasoconstrictor responses to lower body negative pressure and to the application of ice to the forehead, which are neurogenic stimuli, were decreased during acute hypoxia. Lower body negative pressure caused a decrease in mean arterial pressure during hypoxia, but not during normoxia. Because norepinephrine is the neurotransmitter released during reflex vasoconstriction, we considered the possibility that decreased responsiveness to norepinephrine might be one mechanism for diminished responses to lower body negative pressure and ice on the forehead during hypoxia. Hypoxia decreased the response of forearm resistance vessels to infusions of norepinephrine and angiotensin into the brachial artery. In addition, the effectiveness of intravenous infusions of norepinephrine in elevating mean arterial pressure was decreased during hypoxia. Since exposure to acute hypoxia stimulates hyperventilation and hypocapnia, experiments were done to determine the contribution of hypocapnia during hypoxia to the decreased vasoconstriction. The results indicate that hypocapnia may diminish the vascular response to some stimuli, but the reduction in oxygen appears to be the primary mechanism for decreased vasoconstrictor responses during acute hypoxia.
Donald D. Heistad, Robert C. Wheeler