ASSESSMENT OF CARDIAC AUTONOMIC NERVOUS ACTIVITIES DURING HELIOX EXPOSURE AT 24 ATM ABS

Background: This experiment was designed to examine the involvement of the autonomic nervous system in the production of hyperbaric bradycar dia. Methods: Four male divers were exposed to a He-O-2 (heliox) envir onment at 24 atmosphere absolute (atm abs) for 7 d. The heart rate (HR ) and respiratory rate were recorded at rest in the morning (0700 h) a nd at night (2230 h) on 1 d during a 5-d predive control, 2 d during a 7-d saturation dive at 24 atm abs, 2 d during decompression, and on 1 d during a 4-d postdive period. Cardiac sympathetic and parasympathet ic activities were estimated by using a spectral analysis of the varia bility of R-R intervals. Results: The morning HR did not fluctuate thr oughout the experimental days. The night time HR decreased (p < 0.05) by 11.8% on the first day at 24 atm abs compared with that of the pred ive control. The bradycardia diminished gradually and returned to the predive level with continued exposure at 24 atm abs. The high-frequenc y power of the cardiac variability, an index of cardiac parasympatheti c activity, increased (p < 0.05) only in the first night at 24 atm abs , whereas the low-frequency power and a ratio of low- to high-frequenc y power, an index of cardiac sympathetic activity, were unchanged. Con clusions: The present results suggest that an increased parasympatheti c activity rather than a decrease in the sympathetic activity is respo nsible for the bradycardia on exposure to heliox dry saturation dive a t 24 arm abs. The mechanism of the gradual disappearance of the bradyc ardia is unknown, but perhaps it may be related to the development of cardiovascular deconditioning.