Autonomic mechanisms of bradycardia during nitrox exposure at 3 atmospheres absolute in humans.
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.
Yamazaki
Fumio
F
Department of Physiology, School of Medicine, University of Occupational and Environmental Health, Kitakyushu, Japan. yamazaki@health.uoeh-u.ac.jp
Wada
Futoshi
F
Nagaya
Ken
K
Torii
Riko
R
Endo
Yutaka
Y
Sagawa
Sueko
S
Yamaguchi
Hitoshi
H
Mohri
Motohiko
M
Lin
Yu-Chong
YC
Shiraki
Keizo
K
D001272:Atmosphere; D001285:Atropine; D001341:Autonomic Nervous System; D001919:Bradycardia; D006339:Heart Rate; D006801:Humans; D011433:Propranolol
D001285:Atropine; D011433:Propranolol