RELATIONSHIP BETWEEN SPECTRAL COMPONENTS OF CARDIOVASCULAR VARIABILITIES AND DIRECT MEASURES OF MUSCLE SYMPATHETIC-NERVE ACTIVITY IN HUMANS

Background Spectral analysis of RR interval and systolic arterial pres sure variabilities may provide indirect markers of the balance between sympathetic and vagal cardiovascular control. Methods and Results We examined the relationship between power spectral measurements of varia bilities in RR interval, systolic arterial pressure, and muscle sympat hetic nerve activity (MSNA) obtained by microneurography over a range of blood pressures. In eight healthy human volunteers, MSNA, RR interv al, intra-arterial pressure, and respiration were measured during bloo d pressure reductions induced by nitroprusside and during blood pressu re increases induced by phenylephrine. Both low-frequency (LF; 0.10+/- 0.01 Hz) and high-frequency (HF; 0.23+/-10.01 Hz) components were dete cted in MSNA variability. Increasing levels of MSNA were associated wi th a shift of the spectral power toward its LF component. Decreasing l evels of MSNA were associated with a shift of MSNA spectral power towa rd the HF component. Over the range of pressure changes, the LF compon ent of MSNA variability was positively and tightly correlated with LF components of RR interval (in normalized units; P<10(-6)) and of systo lic arterial pressure variability (both in millimeters of mercury squa red and normalized units; P<5x10(-5) and P<5x10(-6), respectively). Th e HF component of MSNA variability was positively and tightly correlat ed with the HF component (in normalized units) of RR-interval variabil ity (P<3x10(-4)) and of systolic arterial pressure variability (P<.01) . Conclusions During sympathetic activation in normal humans, there is a predominance in the LF oscillation of blood pressure, RR interval, and sympathetic nerve activity. During sympathetic inhibition, the HF component of cardiovascular variability predominates. This relationshi p is best seen when power spectral components are normalized for total power. Synchronous changes in the LF and HF rhythms of both RR interv al and MSNA during different levels of sympathetic drive are suggestiv e of common central mechanisms governing both parasympathetic and symp athetic cardiovascular modulation.