The Heart Rate Variability (HRV) and the Photoplethysmographic Pulse Amplitude Variability (PPGAV) were analyzed during clinostatism and orthostatism in intervals of six minutes each on twenty h e a l t h y subjects. The power spectrum density and the spectral coherence of each signal were obtained. While the indices of HRV were as in other reports in healthy subjects, the PPGA showed a decrease in orthostatism, which can be attributed to sympathetic activity. There was significant spectral coherence between both signals, both in low (LF) as in high frequencies, although with a different distribution for each of studied subject. In clinostatism, 12 subjects showed significant coherence in LF band between 0.10-0.15 Hz, while in orthostatism the results shifted and 11 subjects showed significant coherence between 0.071 to 0.118 Hz. This may be evidence of that there is sympathetic activity operating over the PPGAV.
