njit-etd2003-081 - New Jersey Institute of Technology

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The normal respiration rate can be as low as only a few breaths per minute at
rest and as high as up to 40 breaths per minute during intense exercise [15]. This
stresses the fact that, when doing research on heart rate variability to determine
parasympathetic activity, the frequency of respiration must be known. More
specifically, the power spectrum of the respiration waveform should be computed. Once
the power spectrum of the respiration is obtained, the coherence between the respiration
spectrum and the heart rate variability spectrum could be computed. Frequencies with a
high coherence (i.e. >0.9) could then be considered of parasympathetic origin. Although
this is the theoretical approach, in the field, the area under the frequencies of 0.15 to 0.4
Hz is considered parasympathetic in origin.
Unlike parasympathetic activity, the sympathetic activity is not easily separated
from the power spectrum of heart rate variability [14]. It has been hypothesized that the
low frequency peak (0.04 to 0.15 Hz) is a mixture of both parasympathetic activity and
sympathetic activity. A better concept that is used to isolate the sympathetic activity is
that of "sympatho-vagal balance" which recognizes both reciprocal and non-reciprocal
parasympathetic and sympathetic influences on heart rate by computing the low
frequency to high frequency ratio [49]. An increase in the low frequency to high
frequency ratio indicates either an increase of sympathetic activity, a decrease in
parasympathetic activity, or a reciprocal change in both.