Abbreviation	Meaning
HRV	Heart Rate Variability
IBI	Interbeat Interval
HF-HRV	High-Frequency Heart Rate Variability
LF-HRV	Low-Frequency Heart Rate Variability
VLF-HRV	Very Low-Frequency Heart Rate Variability
LF/HF ratio	Ratio of Low-Frequency to High-Frequency HRV
RMSSD	Root Mean Square of Successive Differences
SDNN	Standard Deviation of Normal-to-Normal Intervals
PNN50	Percentage of successive interbeat intervals differing by more than 50 ms
SMD	Standardized Mean Difference
MDD	Major Depressive Disorder
g	Hedges’ effect size
r	Correlation coefficient
CI	Confidence Interval
N	Sample size
k	Number of included studies