Table 2 Time-domain linear and nonlinear and frequency-domain HRV measures used in this study.

mean NN

ms

Mean of normal-to-normal (NN) intervals

SDNN

ms

Standard deviation of NN intervals; an estimate of overall HRV

SDSD

ms

Standard deviation of differences between adjacent NN intervals; an estimate of short-term HRV; associated with parasympathetic activity

RMSSD

ms

Square root of the mean of the sum of the squares of differences between adjacent NN intervals; an estimate of short-term HRV; associated with parasympathetic activity

pNN50

%

Number of pairs of adjacent NN intervals differing by more than 50 ms divided by the total number of all NN intervals

Nonlinear measures

SD1

ms

Standard deviation of the Poincaré plot perpendicular to the line-of-identity; the shortest diameter of the fitted ellipse; an estimate of short-term HRV; associated with parasympathetic activity

SD2

ms

Standard deviation of the Poincaré plot along the line-of-identity; the longest diameter of the fitted ellipse; an estimate of long-term HRV; associated with sympathetic activity

SD1/SD2

-

Ratio of SD1 to SD2; describes the ratio of short-term to long-term HRV; describes the relationship between parasympathetic and sympathetic activity

Frequency-domain measures

LF

ms²

Power in low frequency range (0.04-0.15 Hz); estimate of long-term HRV; reflects both sympathetic and parasympathetic activity

nLF

n.u.

Normalized power of LF; LF/(LF+HF)x100; estimate of long-term HRV; reflects both sympathetic and parasympathetic activity

HF

ms²

Power in high frequency range (0.15-0.4 Hz); estimate of short-term HRV; reflects parasympathetic (vagal) activity

nHF

n.u.

Normalized power of HF; HF/(LF+HF)x100; estimate of short-term HRV; reflects parasympathetic activity

LF/HF

-

Ratio between LF and HF range powers; describes the relationship between sympathetic and parasympathetic activity, i.e. sympathovagal balance