title: Influence of CO₂ on upper airway muscles and chest wall/diaphragm corticomotor responses assessed by transcranial magnetic stimulation in awake healthy subjects.
abstract: BACKGROUND
  Functional interaction between upper airway (UA) dilator muscles and the diaphragm is crucial in the maintenance of UA patency. This interaction could be altered by increasing respiratory drive. The aim of our study was to compare the effects of hypercapnic stimulation on diaphragm and genioglossus corticomotor responses to transcranial magnetic stimulation (TMS).
  
  
  METHODS
  10 self-reported healthy men (32 ± 9 yr; body mass index = 24 ± 3 kg/m(-2)) breathed, in random order, room air or 5% and then 7% Fi(CO(2)), both balanced with pure O(2). Assessments included ventilatory variables, isoflow UA resistance (at 300 ml/s), measurement of lower chest wall/diaphragm (LCW/diaphragm), and genioglossus motor threshold (MT) and motor-evoked potential (MEP) characteristics. TMS twitches were applied during early inspiration and end expiration at stimulation intensity 30% above LCW/diaphragm and genioglossus MT.
  
  
  RESULTS
  Compared with room air, CO(2) inhalation significantly augmented minute ventilation, maximal inspiratory flow, tidal volume, and tidal volume/respiratory time ratio. UA resistance was unchanged with CO(2) inhalation. During 7% CO(2) breathing, LCW/diaphragm MT decreased by 9.6 ± 10.1% whereas genioglossus MT increased by 7.2 ± 9%. CO(2)-induced ventilatory stimulation led to elevation of LCW/diaphragm MEP amplitudes during inspiration but not during expiration. LCW/diaphragm MEP latencies remained unaltered both during inspiration and expiration. Genioglossus MEP latencies and amplitudes were unchanged with CO(2).
  
  
  CONCLUSIONS
  In awake, healthy subjects, CO(2)-induced hyperventilation is associated with heightened LCW/diaphragm corticomotor activation without modulating genioglossus MEP responses. This imbalance may promote UA instability during increased respiratory drive.
authors.affiliation: Unité de Recherche en Pneumologie, Centre de Recherche, Institut Universitaire de Cardiologie et de Pneumologie de Québec, Université Laval, Québec, Canada.
authors.forename: Jean-Christian
authors.initials: JC
authors.lastname: Borel
authors.forename: Cesar Augusto
authors.initials: CA
authors.lastname: Melo-Silva
authors.forename: Simon
authors.initials: S
authors.lastname: Gakwaya
authors.forename: Frédéric
authors.initials: F
authors.lastname: Sériès
mesh_terms: D000328:Adult; D002245:Carbon Dioxide / Q000008:administration & dosage*; D003964:Diaphragm / Q000187:drug effects / Q000502:physiology*; D019054:Evoked Potentials, Motor / Q000502:physiology; D006801:Humans; D006935:Hypercapnia / Q000503:physiopathology; D006985:Hyperventilation / Q000139:chemically induced / Q000503:physiopathology; D008297:Male; D009044:Motor Cortex / Q000502:physiology*; D018482:Muscle, Skeletal / Q000187:drug effects / Q000502:physiology*; D015656:Respiratory Mechanics / Q000187:drug effects / Q000502:physiology*; D035441:Thoracic Wall / Q000187:drug effects / Q000502:physiology*; D013990:Tidal Volume / Q000502:physiology; D050781:Transcranial Magnetic Stimulation / Q000379:methods; D014851:Wakefulness / Q000187:drug effects / Q000502:physiology*
chemical_list: D002245:Carbon Dioxide