Sympathovagal Balance: A Reappraisal
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To the Editor:
The article by D.L. Eckberg1 will surely promote a florid discussion. Its structure is based on a number of arguments about which our disagreement is substantial.
The term “sympathovagal balance,” belonging to traditional physiology, was introduced by us in the study of heart rate variability (HRV) when we wrote in 1983 that upright posture “is expected to shift the sympathovagal balance toward a sympathetic predominance.”2 Two years later, Pomeranz et al3 wrote that “autonomic control of the heart in response to postural movements strikes a balance between the activities of the parasympathetic and sympathetic nervous system.” After the emphasis provided by Pagani et al,4 the term became widely used.
A hypothesis is a highly democratic entity, and its acceptance cannot be imposed. In our mind, like a horizontal beam pivoted at its center, “sympathovagal balance” refers to a reciprocal functional relationship,5 implying that when 1 of the 2 components of the autonomic outflow is excited, the other is inhibited, according to a central push-pull pattern of organization.
We shall now analyze the major issues of disagreement in the sequence in which they appear in Eckberg’s article.1
Beginning from Figure 1,1 reproduced from our own work,5 Eckberg writes: “My integration of the two [low-frequency] spectral powers in this figure suggests that sympathetic spectral power is ≈15% greater than vagal spectral power”1 and, subsequently, “sympathetic contributions to 0.1 Hz spectral power are only marginally greater than vagal contributions.”
It is hard to understand how this comparison was made. In that experiment, the impulse activities of 2 distinct nerve filaments, unlikely to contain an equal number of active units, were simultaneously recorded with different amplifications, providing 2 variability signals with different variance. In the frequency domain, considering in the Figure the …