A moderate negative correlation was found between the
STT and both SD2 and LF (ms2) in the group of smokers.
Thus, a greater sympathetic response, as reflected by LF
(ms2), denotes better mucociliary transportability. This
finding confirms the influence of the autonomic nervous
system, specifically sympathetic activity, on the increase
in mucociliary clearance. The SD2 index represents both
sympathetic and parasympathetic activities, but does not
allow one to distinguish whether a change is due to increased sympathetic tone or the withdrawal of vagal tone.59
Because this index refers to the response to exercise, it is
suggested to be influenced mainly by sympathetic activity.