The non-randomness index was developed initially as a nonlinear HRV index [17] and its connection with other traditional HRV indices needs to be explored. Though the nonrandomness index was weakly associated with HRV indices (rsquare,0.1; Table S3 ), it did correlate negatively with HF power and positively with the LF/HF ratio, suggesting that the non-randomness index is a marker related to sympathovagal balance. Our findings of lower non-randomness index and LF% seen in homozygous b 2 -AR Arg16 carriers are in line with prior studies showing that the b 2 -AR Arg16 polymorphism is associated with lower sympathetic activity, manifested as lower blood pressure [53, 54] , lower plasma norepinephrine [54, 55] , and enhanced agonist-mediated desensitization in the vasculature [56] .