Table 1.

Abbreviations of heart rate variability indices

Time-domain
RR	Mean value of all normal-to-normal interbeat (N–N) intervals
SDNN	Standard deviation of all N–N intervals
SDNNindex	Average of the standard deviations of N–N intervals for each 5-min period
SDANN	Standard deviation of the average N–N intervals for each 5-min period over 24 h
NN50	Number of N–N intervals differing by >50 ms from the preceding interval
pNN50	Percentage of adjacent cycles that are >50 ms apart
RMSSD	Root mean square successive differences in milliseconds
Frequency-domain
TP	Total power (0.0–0.5 Hz)
VLF	Very low frequency (0.0033–0.04 Hz)
LF	Low frequency (0.04–0.15 Hz)
HF	High frequency (0.15–0.40 Hz)
LF/HF	Ratio of low to high frequency