Table 1.

Definitions of time-domain and frequency-domain heart rate variability (HRV) measurements.

HRV Measures	Definition
Time-Domain
SDNN	Standard deviation of the average normal-to-normal (NN) intervals
pNN50	Percentage of successive NN intervals that differ by more than 50 ms
rMSSD	Root mean square of successive NN interval differences
Frequency-Domain
HF	Absolute power of the high-frequency band (0.15–0.4 Hz)
LF	Absolute power of the low-frequency band (0.04–0.15 Hz)
VLF	Absolute power of the very-low frequency band (0.003–0.04 Hz)
ULF	Absolute power of the ultra-low frequency band (≤0.003 Hz)
log HF	Log-transformed HF
log LF	Log-transformed LF
log LF/HF	Log-transformed ratio of LF to HF