Power spectral analysis of heart rate variability (HRV) has been used frequently to assess cardiac autonomic function; however, the relationship of low frequency (LF) power of HRV 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.

Braak's staging concept of Lewy body disease pathogenesis is based on a spatiotemporal sequence of alpha-synuclein deposition, with autonomic nervous system involvement before synucleinopathy in substantia nigra neurons. A patient with primary chronic autonomic failure underwent biennial brain 6-[18F]DOPA and myocardial 6-[18F]dopamine scanning over 4 years. Low myocardial radioactivity indicated cardiac noradrenergic denervation that persisted. Striatal 6-[18F]DOPA-derived radioactivity initially was normal, 2 years later was decreased subtly, and by 4 years was clearly decreased, accompanied by dementia and parkinsonism. In this case, neuroimaging evidence of cardiac noradrenergic denervation and subsequent progressive striatal dopaminergic denervation fit with Braak staging.