Table 1: The definitions of heart rate variability parameters.
Variable 
Unit 
Description 
Time domain analysis 

SDNN 
ms 
Standard deviation of all NN intervals (normal to normal) 
RMSSD 
ms 
The square root of the mean of the sum of the squares of differences between adjacent NN intervals 
pNN50 
% 
Percentage of successive NN intervals that differ by more than 50 ms 
Frequency domain analysis 

TOTAL power 
ms^{2} 
Variance of all NN intervals (≤ 0.4 Hz) 
VLF 
ms^{2} 
Power in very low frequency range (≤ 0.04 Hz) 
LF 
ms^{2} 
Power in low frequency range (0.040.15 Hz) 
HF 
ms^{2} 
Power in high frequency range (0.150.4 Hz) 
LF/HF 

Ratio LF/HF 