Cepeda FX, et al. - Researchers evaluated the hypotheses that nonlinear heart rate variability (HRV) indices accurately reflect cardiac vagal control, and their changes in response to low dose atropine were associated with those in RR interval. Progressive low dose atropine was used to induce changes in cardiac vagal tone. Low dose atropine increased average RR interval (vagotonic effect), as predicted. Either within or among subjects, no strong connection was seen between vagotonic changes in RR interval and the majority of nonlinear HRV indices. There was an inconsistent relationship between responses of nonlinear HRV indices and RR interval to changes in cardiac vagal tone. Nonlinear HRV indices could not serve as consistent indices of cardiac vagal control in healthy humans.