HRV Variables. The following variables were evaluated, mean R-R interval (Mean RR), the standard deviation between r-r intervals (SDNN), the number of pairs of adjacent NN intervals differing by more than 50 ms (NN 50 Count) and the proportion of NN intervals differing by more than 50 ms = NN50 divided by total number of NN intervals (pNN 50%), the very low frequency between (VLF); the normalized spectral components between 0 and 0.04 Hz , the low frequency (LF); similarly, the normalized spectral components between 0.04 and 0.15 Hz, the high frequency (HF); the normalized spectral power components were summed between 0.15 and 0.4 Hz and the ratio LF/HF 16. The HF band is related to the parasympathetic nervous system efferent activity 17 and is synchronized to respiratory rhythms 16. The LF band is related to both the sympathetic nervous system and parasympathetic nervous system efferent activity 18 and the cardiac sympathetic and parasympathetic nerve activity 16. The VLF band is not well defined but appears to be related to thermoregulation 16 renin-angiotensin-aldosterone systems 19-20, parasympathetic mechanisms 21, both the parasympathetic 21 and sympathetic branches of ANS affect VLF 22 and the humoral system 23, 24. The LF/HF ratio is used to indicate balance between the sympathetic nervous system and parasympathetic nervous system efferent tone 18. Thus, a change in this score might indicate either a parasympathetic nervous system or a sympathetic nervous system tonal change.