njit-etd2003-081 - New Jersey Institute of Technology

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5.2.6 Vagal Tone and Sympathovagal Balance via Time-Frequency Distribution of
COPD Subjects
Although the inherent rhythmicity of the heart is due to a natural pacemaker situated in
the sinoatrial node, continuous beat-to-beat control of the heart is dependent on the
relative balance of the sympathetic and the parasympathetic impulses delivered from the
brain to the sinus node. From drug studies, three discrete frequency ranges in the
spectrum of HRV were found to be of importance: a very low frequency range (VLF,
0.02 to 0.04 Hz), a low frequency range (LF, 0.04 to 0.15 Hz) and a high frequency range
(HF, 0.15 to 0.5 Hz). The VLF band was equated with the rennin-angiotensin system, the
LF band with blood pressure and baroreflex control, and the HF band with respiration.
The LF band is mediated by both the sympathetic and the parasympathetic pathways
while the HF band is mediated by the parasympathetic pathways only.
Assessment of the parasympathetic activity from spectral analysis is obtained via
a measurement of the area under the HF peak. Sympathetic activity is less easy to
quantify using this methodology [49]. A realizable goal is that of the sympathovagal
balance measurements (LF/HF ratio), which recognizes both the reciprocal and the nonreciprocal
parasympathetic and sympathetic influences on heart rate. [49] Factors other
than the cardiac nerves can also alter the heart rate (HR). The HR is also sensitive to
changes in body temperature, plasma electrolyte concentrations and hormones. However,
these factors are normally less important than the sympathetic and the parasympathetic
nerve pathways to the heart [49].