This study found that b 2 -AR Arg16Gly genotype was associated with clusters based on acceleration/deceleration patterns of heart rate. Although b 2 -AR is expressed in the heart at lower concentrations than is the b 1 -AR subtype, it also expresses on the presynaptic sympathetic nerve terminals and expresses abundantly in vascular and bronchial smooth muscle. Therefore, changes in b 2 -AR function may not only alter sympathetic activity [50] , but also respiration and vascular responses [51, 52] . It can be reasonable to speculate that these factors have influences in HRV. Of note, the association between b 2 -AR Arg16Gly polymorphism and heart rate fluctuation patterns may simply reflect a functional genetic component nearby due to linkage disequilibrium and warrants future genetic research targeting on this region.