Autonomic response to hypobaric hypoxia assessed by time-dependent frequency decomposition of heart rate

Background: Acute hypoxia tolerance varies substantially among healthy indi viduals. We hypothesized that this variability results from a difference in autonomic (ANS) response to hypoxic stress. Methods: Peripheral oxygen sat uration, respiration and ECG were recorded from 21 healthy subjects (age, 2 9 +/- 7 yr) in an altitude chamber during normoxia, severe hypoxia (282 mm Hg), and mild hypoxia (360 mm Hg). Cardiovascular control was assessed by t ime-frequency decomposition of the heart rate signal applying the Selective Discrete Transform Algorithm (SDA). This procedure uses a variable time wi ndow, thus providing reliable physiological data even during transient stat es. Autonomic activity was quantified by power spectral density integrals o ver a 3-dimensional time-dependent spectral distribution of heart rate fluc tuations. Results: Subjects who had slower peripheral oxygen desaturation d uring severe hypoxia onset (mean 92.9 vs. 58.4 s) were those who displayed higher ANS activity in all ambient states, namely normoxia and hypoxia. The se same subjects withstood hypoxia for significantly longer time periods (m ean 313 vs. 244 s), Conclusion. Improved hypoxia tolerance is linked to enh anced autonomic activity, involving a better management of peripheral blood flow.