title: Contrasting effects of hypoxia and hypercapnia on ventilation and sympathetic activity in humans.
abstract: 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 (VE), and SNA [53 +/- 14% (SE) during HC vs. 21 +/- 7% during IHO; P less than 0.05]. Even at equivalent levels of VE, HC still elicited greater SNA than IHO. However, apnea during HC caused a lesser (P less than 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 VE (23.6 +/- 2.8 l/min) than the additive increases due to HC alone plus IHO alone (18.0 +/- 1.8 l/min, P less than 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 less than 0.05). We conclude that 1) HC causes greater increases in VE and SNA than does hypoxia; 2) for the same increase in VE, 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 VE.
authors.affiliation: Department of Medicine, University of Iowa, Iowa City.
authors.forename: V K
authors.initials: VK
authors.lastname: Somers
authors.forename: A L
authors.initials: AL
authors.lastname: Mark
authors.forename: D C
authors.initials: DC
authors.lastname: Zavala
authors.forename: F M
authors.initials: FM
authors.lastname: Abboud
mesh_terms: D000328:Adult; D001049:Apnea; D002245:Carbon Dioxide / Q000494:pharmacology*; D002628:Chemoreceptor Cells / Q000502:physiology; D005260:Female; D006439:Hemodynamics; D006801:Humans; D000860:Hypoxia; D008297:Male; D008564:Membrane Potentials; D010100:Oxygen / Q000494:pharmacology*; D012119:Respiration / Q000502:physiology*; D012129:Respiratory Function Tests; D013564:Sympathetic Nervous System / Q000502:physiology*
chemical_list: D002245:Carbon Dioxide; D010100:Oxygen