Sinus Node Function
In order to ascertain baseline sinus function, we identified 86 patients who had a 24-hour Holter monitor performed when they were not using any chronotropic medications (beta blockers, calcium channel blockers, Class I-IV antiarrhythmics). Average, minimum and peak heart rate recorded as well as metrics of heart rate variability including SDNN (standard deviation of NN intervals in ms) and RMSDD (Root mean square of successive RR interval differences in ms) were documented10. We used SDNN as the index of heart rate variability over a 24 h period and RMSSD as the primary time-domain measure for estimation of the vagally mediated changes reflected in HRV11.