title: Autonomic mechanisms of bradycardia during nitrox exposure at 3 atmospheres absolute in humans.
abstract: BACKGROUND
  This experiment was designed to examine the involvement of the autonomic nervous systems and intrinsic component in the occurrence of hyperbaric bradycardia.
  
  
  METHODS
  Eight male divers were exposed to a N2-O2 (Nitrox) environment at 3 atmospheres absolute (ATA) for 7 d. The heart rate (HR), plasma norepinephrine (NE), and a spectral power of the variability of cardiac interval were measured during a 4-d predive control period, a 7-d saturation period at 3 ATA, and a 4-d postdive period. In each dive period, atropine and propranolol were administered intravenously for cholinergic blockade and beta-adrenergic blockade, respectively.
  
  
  RESULTS
  Basal HR decreased by -10% (p < 0.05) during the saturation period compared with that of the predive control. The HR after an administration of atropine was attenuated by 5.5 +/- 2.4% (p < 0.05) during hyperbaric exposure. The HR after a simultaneous administration of atropine and propranolol, the intrinsic HR, was similar throughout the dive periods. Plasma NE decreased at 3 ATA (p < 0.05). The basal level of high-frequency power of cardiac interval variability, an index of cardiac parasympathetic modulation, remained unchanged throughout the dive period, whereas this power was eliminated by atropine administration.
  
  
  CONCLUSIONS
  These results suggest that reduced sympathetic activity plays a primary role in the reduction of HR in the present hyperbaric environment.
authors.affiliation: Department of Physiology, School of Medicine, University of Occupational and Environmental Health, Kitakyushu, Japan. yamazaki@health.uoeh-u.ac.jp
authors.forename: Fumio
authors.initials: F
authors.lastname: Yamazaki
authors.forename: Futoshi
authors.initials: F
authors.lastname: Wada
authors.forename: Ken
authors.initials: K
authors.lastname: Nagaya
authors.forename: Riko
authors.initials: R
authors.lastname: Torii
authors.forename: Yutaka
authors.initials: Y
authors.lastname: Endo
authors.forename: Sueko
authors.initials: S
authors.lastname: Sagawa
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Yamaguchi
authors.forename: Motohiko
authors.initials: M
authors.lastname: Mohri
authors.forename: Yu-Chong
authors.initials: YC
authors.lastname: Lin
authors.forename: Keizo
authors.initials: K
authors.lastname: Shiraki
mesh_terms: D001272:Atmosphere; D001285:Atropine; D001341:Autonomic Nervous System; D001919:Bradycardia; D006339:Heart Rate; D006801:Humans; D011433:Propranolol
chemical_list: D001285:Atropine; D011433:Propranolol