Interpreting power-spectral-measures extracted from sympathetic nerve activity: possibilities and pitfalls

This paper discusses the possibilities and potential pitfalls associated with interpretations based on the low-frequency power (LFP), the high-frequency power (HFP) and their ratio: PR=LFP/HFP, computed from muscle sympathetic nerve activity (MSNA). Segments of MSNA from several subjects were analyzed by Capon´s minimum variance distortionless response (MVDR) method. The power spectra revealed peaks within [0-0.5] Hz, apart from the peak at the cardiac-related frequency (CF). Our investigations reveal that: (1) the presence of nearly periodic (groups of) bursts can produce strong harmonics that affect the HF component and hence the accuracy of HFP and PR, (2) changes in the strong CF-component can affect the components within the lower frequency range, and (3) small changes in the location of the needle electrode can affect the spectral-measures.