Hypoxia augments apnea-induced peripheral vasoconstriction in humans

Obstructive apnea and voluntary breath holding are associated with transien t increases in muscle sympathetic nerve activity (MSNA) and arterial pressu re. The contribution of changes in blood flow relative to the contribution of changes in vascular resistance to the apnea-induced transient rise in ar terial pressure is unclear. We measured heart rate, mean arterial blood pre ssure (MAP), MSNA (peroneal microneurography), and femoral artery blood vel ocity (V-FA, Doppler) in humans during voluntary end-expiratory apnea while they were exposed to room air, hypoxia (10.5% inspiratory fraction of O-2) , and hyperoxia (100% inspiratory fraction of O-2). Changes from baseline o f leg blood flow ((Q) over dot) and vascular resistance (R) were estimated from the following relationships: (Q) over dot infinity V-FA, corrected for the heart rate, and R infinity MAP/(Q) over dot . During apnea, MSNA rose; this rise in MSNA was followed by a rise in MAP, which peaked a few second s after resumption of breathing. Responses of MSNA and MAP to apnea were gr eatest during hypoxia and smallest during hyperoxia (P < 0.05 for both comp ared with room air breathing). Similarly, apnea was associated with a decre ase in Q and an increase in R. The decrease in Q was greatest during hypoxi a and smallest during hyperoxia (-25 +/- 3 vs. -6 +/- 4%, P < 0.05), and th e increase in R was the greatest during hypoxia and the least during hypero xia (60 +/- 8 vs. 21 +/- 6%, P < 0.05). Thus voluntary apnea is associated with vasoconstriction, which is in part mediated by the sympathetic nervous system. Because apnea induced vasoconstriction is most intense during hypo xia and attenuated during hyperoxia, it appears to depend at least in part on stimulation of arterial chemoreceptors.