Linear and nonlinear assessment of heart rate variability in nonacclimatized middleaged subjects at different hypoxic...

Studies of autonomic cardiac nervous system activity during acute exposure to hypobaric hypoxia have suggested a depression of autonomic functions and a shift in the sympatho-vagal balance towards relatively more sympathetic and less parasympathetic activity at higher hypoxic levels . This study was performed on nine non-acclimatized middle-aged healthy men (age 43.7 +/- 7.3 years), to evaluate the linear (autoregressive spectra) and non-linear (Poincaré plot, b coefficient, fractal dimension) heart rate variability (HRV) parameters at rest in supine position at three different altitudes: 400 m, 3200 m and 4200 m. A statistically significant increase of heart rate (HR) was detected at both higher altitudes in comparison with the reference level as well as comparing the values observed at 3200 m and 4200 m, respectively. Acute exposure to both hypoxic levels (3200 m and 4200 m) induced a shift of sympatho-vagal balance towards more sympathetic and less parasympathetic activity in comparison to the basal condition at 400 m asl, as indicated by autoregressive LF and HF spectral components and in particular by LF/HF ratio. The non-linear ss coefficient values demonstrated a statistically significant decrease of the complexity of the system at both hypoxic conditions as effect of hypobaric hypoxia on ANS activity of myocardium. However this fact was not confirmed by the fractal dimension parameter.