Contrasting effects of hypoxia and hypercapnia on ventilation and sympathetic activity in humans

We compared the effects of isocapnic hypoxia (IHO) and hyperoxic hypercapnia (HC) on sympathetic nerve activity (SNA) recorded from a peroneal nerve in 13 normal subjects. HC caused greater increases in blood pressure (BP), minute ventilation (V̇E), and SNA [53 ± 14% (SE) during HC vs. 21 ± 7% during IHO; P < 0.05]. Even at equivalent levels of V̇E, HC still elicited greater SNA than IHO. However, apnea during HC caused a lesser (P < 0.05) increase in SNA (91 ± 26% compared with apnea on room air) than apnea during IHO (173 ± 50%). Hypercapnic hypoxia resulted in a greater absolute increase in V̇E (23.6 ± 2.8 l/min) than the additive increases due to HC alone plus IHO alone (18.0 ± 1.8 l/min, P < 0.05). SNA also increased synergistically by 108 ± 23% with the combined stimulus compared with the additive effect of HC alone plus IHO alone (68 ± 19%; P < 0.05). We conclude that 1) HC causes greater increases in V̇E and SNA than does hypoxia; 2) for the same increase in V̇E, hypercapnia still causes a greater increase in SNA than hypoxia; however, during apnea, hypoxia causes a much greater increase in SNA than hypercapnia; 3) the inhibitory influence of ventilation on SNA is greater during hypoxia (i.e., predominantly peripheral chemoreceptor stimulation) than hypercapnia (i.e., predominantly central chemoreceptor stimulation); and 4) combined hypoxia and hypercapnia have a synergistic effect on SNA as well as on V̇E.