Cardiac vagal modulation impairment is a typical feature of Chagas disease (ChD) and is correlated to antagonistic anti-M2 autoantibodies activity. In this letter, we analyze the hypothesis that, in ChD, anti-muscarinic antibody activity could simultaneously increase parasympathetic tonus (decreasing heart rate) and decrease vagal modulation over cardiac sinus node (reducing heart rate variability - HRV). Although attractive, this hypothesis is not supported by empirical data, since reduction of vagal-specific HRV indexes has been demonstrated in the absence of bradycardia. Moreover, anti-muscarinic antibody levels correlate with HRV indexes, but not with heart rate. In the absence of reduced heart rate and, consequently, of tonic vagal enhancement, it is impossible to explain the reduction of vagal-mediated HRV by continuous and strong muscarinic activity of autoantibodies in ChD.