The parameter AM/BL is proportional to the cardiac-induced blood volume increase, which depends on the arterial wall compliance. AM/BL increased after the thoracic sympathectomy treatment (for male patients, from 2.60 ± 1.49% to 4.81 ± 1.21%), as sympathetic denervation decreases arterial tonus in skin. The very low-frequency (VLF) fluctuations of BL or AM showed high correlation (0.90 ± 0.11 and 0.92 ± 0.07, respectively) between the right and left hands before the thoracic sympathectomy, and a significant decrease in the right-left correlation coefficient (to 0.54 ± 0.22 and 0.76 ± 0.20, respectively) after the operation. The standard deviation of the BL or AM VLF fluctuations also reduced after the treatment, indicating sympathetic mediation of the VLF PPG fluctuations. The study also shows that the analysis of the PPG signal and the VLF fluctuations of the PPG parameters enable the assessment of the change in sympathetic nervous system activity after thoracic sympathectomy.
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