Sympathetic nervous system representation in time and frequency domain indices of heart rate variability

This study evaluated the contributions of sympathetic and parasympathetic m odulation to heart rate variability during situations in which vagal and sy mpathetic tone predominated. In a placebo-controlled, randomized, double bl ind blockade study, six young healthy male individuals received propranolol (0.2 mg.kg(-1)), atropine (0.04 mg.kg(-1)), propranolol plus atropine, or placebo infusions over 4 days. Time-domain indices were calculated during 4 0 min of rest and 20 min of exercise at 70% of maximal exercise intensity. Spectrum analysis, using fast Fourier transformation, was also performed at rest and during the exercise. The time-domain indices standard deviation o f R-R intervals, mean of the standard deviations of all RR intervals for al l 5-min segments, percentage of number of pairs of adjacent R-R intervals d iffering by more than 50 ms, and square root of the mean of the sum of squa res of differences between adjacent R-R intervals were reduced after atropi ne and propranolol plus atropine. Propranolol alone caused no appreciable c hange in any of the time-domain indices. At rest, all spectrum components w ere similar after placebo and propranolol infusions, but following parasymp athetic and double autonomic blockade there was a reduction in all componen ts of the spectrum analysis, except for the low:high ratio. During exercise , partial and double blockade did not change significantly any of the spect rum components. Thus, time and frequency-domain indices of heart rate varia bility were able to detect vagal activity, but could not detect sympathetic activity. During exercise, spectrum analysis is not capable of evaluating autonomic modulation of heart rate.