Autonomic control of the cardiovascular system during acute hypobaric hypoxia, assessed by time-frequency decomposition of the heart rate

The time from onset of severe hypoxia to loss of consciousness (TUC) varies among healthy individuals. We hypothesized that this variance is due to difference in autonomic modulation of the cardiovascular system. ECG and finger oxygen saturation patterns were recorded from twenty-one consenting aircrew members, during controlled altitude chamber training at 25.000 feet. Autonomic activity was assessed by heart rate variability (HRV) using a selective frequency-dependent-window algorithm (SDA) in the LF (0.04-0.15 Hz) and HF (0.15-0.50 Hz) bands. Subjects who displayed the highest HRV integrals during baseline and experimental states, were able to remain conscious longer without supplemental oxygen (mean 364 sec) `at altitude' compared to subjects with low HRV integrals (mean 231 sec). Thus, hypoxia resistance may be related to autonomic activity and its ability to achieve selective blood shunting.