Low-frequency power of heart rate variability is not a measure of cardiac sympathetic tone but may be a measure of modulation of cardiac autonomic outflows by baroreflexes

Abstract

Power spectral analysis of heart rate variability has often been used to assess cardiac autonomic function; however, the relationship of low-frequency (LF) power of heart rate variability to cardiac sympathetic tone has been unclear. With or without adjustment for high-frequency (HF) power, total power or respiration, LF power seems to provide an index not of cardiac sympathetic tone but of baroreflex function. Manipulations and drugs that change LF power or LF:HF may do so not by affecting cardiac autonomic outflows directly but by affecting modulation of those outflows by baroreflexes.

Figure 1

(Left) Group mean (± SEM) values for the log of low frequency (LF) power of heart rate variability (in ms²) as a function of mean values for the log of baroreflex-cardiovagal gain, calculated from the slope of the linear relationship between cardiac interbeat interval and systolic blood pressure during the descent of pressure in Phase II of the Valsalva maneuver (in ms/mm Hg). (Middle) Individual values for the log of LF power as a function of baroreflex-cardiovagal gain. (Right) Group mean (± SEM) values for the log of LF power of heart rate variability as a function of mean values for septal myocardial ¹⁸F-fluorodopamine-derived radioactivity in the 5-minute interval beginning about 5 minutes after initiation of 3-minute infusion of the tracer. Abbreviations: Den. Nl. BRS=denervated, normal baroreflex sensitivity; PD+OH=Parkinson disease with orthostatic hypotension; PD No OH=Parkinson disease without orthostatic hypotension; Baro. Failure=baroreflex failure from head/neck cancer and neck irradiation; PAF=pure autonomic failure; MSA=multiple system atrophy; SNS-x=bilateral; thoracic sympathectomies; PDRisk=individuals with multiple statistical risk factors for Parkinson disease (at least 3 of the following: family history of Parkinson disease; symptoms of REM behavior disorder; decreased olfaction; orthostatic intolerance from orthostatic hypotension. Dashed line shows linear line of best fit. Note positive correlation between the log of LF power and baroreflex-cardiovagal gain and no correlation between the log of LF power and myocardial radioactivity. Data adapted and updated from Rahman et al. (Rahman et al., 2011)