Power spectral and time based analysis of heart rate variability following 15 days head-down bed rest

Power spectral and time based analyses were applied to the cardiac inter- beat interval (RRI) of 8 healthy men before and after 15 d of bed rest in the 6° head-down tilt position (HDT) to determine changes in indices of cardiac parasympathetic and sympathetic activity after this exposure. At 24 h prior to HDT and on MDT day 15, a minimum of 256 RRI's were obtained from an electrocardiogram (ECG) while the subjects were in the supine position. RRI was subjected to power spectral and two methods of time-based analyses. Power spectral and two methods of time-based analyses. Power spectral analysis demonstrated that the index of cardiac vagal activity was reduced (95.2 ± 28.5 to 48.2 ± 17.4 ms²) without affecting the index of cardiac sympathetic activity (1.18 ± 0.7 to 0.69 ± 0.4). The two methods of time-based analyses, time series and standard deviation analyses, further demonstrated a reduction of cardiac vagal activity post-NDT (5.5 ± 4 to 4.8 ± 0.6 ms²; and 42.8 ± to 33.9 ± 3.3 ms, respectively). These data suggest that exposure to 15 d of HDT reduces cardiac vagal activity, while changes in cardiac sympathetic activity were indistinguishable.