Heart rate variability to monitor autonomic nervous system activity during orthostatic stress

Exposure to microgravity with space flight, or to earth-based analogs such as head-down tilt (HDT) bedrest, results in cardiovascular deconditioning. With reapplication of gravity, deconditioning is most often observed as an elevated heart rate, a narrowed pulse pressure, and possibly even a failure to maintain blood pressure with symptoms of presyncope or syncope. Noninvasive measurements of heart rate variability (HRV) have been used to study cardiovascular control mechanisms during orthostatic stress (head-up tilt and lower body negative pressure [LBNP]). Recently, the authors developed a new approach to study heart rate and blood pressure control mechanisms. Coarse graining spectral analysis (CGSA) allows simultaneous extraction of not only parasympathetic (PNS) and sympathetic (SNS) indicators from the HRV signal, but also determines the fractal dimension (DF), calculated from the slope (beta) of the log spectral power-log frequency relationship. The fractal dimension is an index of the complexity of the cardiovascular control system. Our investigations have indicated patterns of HRV consistent with reduced PNS and increased SNS activity and reduced DF with the HRV signal during orthostatic stress. These findings also indicate that the cardiovascular system may be less stable when operating at a reduced level of complexity.