Table 1.

Variables used and their significance^a

Variable	Unit of measure	Description	Significance
RMSSD	Milliseconds	Square root of the mean of the sum of the squares of differences between adjacent N-N intervals	Approximate correspondence with HF component
pNN50	Percent	Percentage of difference between adjacent N-N intervals >50 ms	Approximate correspondence with HF component
HF norm	Normalized units	HF power in normalized units	Parasympathetic activity
LF norm	Normalized units	LF power in normalized units	Sympathetic activity
LF:HF ratio		Ratio of LF to HF	Sympathovagal balance

Abbreviations: HF, high frequency; LF, low frequency; N-N interval, interval due to normal sinus depolarization.

^aBased on Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology³ and Sztajzel.⁴