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 pressure 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 variabilities in RR interval, systolic arterial pressure, and muscle sympathetic nerve activity (MSNA) obtained by microneurography over a range of blood pressures. In eight healthy human volunteers, MSNA, RR interval, intra-arterial pressure, and respiration were measured during blood pressure reductions induced by nitroprusside and during blood pressure increases induced by phenylephrine. Both low-frequency (LF; 0.10±0.01 Hz) and high- frequency (HF; 0.23±0.01 Hz) components were detected in MSNA variability. Increasing levels of MSNA were associated with a shift of the spectral power toward its LF component. Decreasing levels of MSNA were associated with a shift of MSNA spectral power toward the HF component. Over the range of pressure changes, the LF component of MSNA variability was positively and tightly correlated with LF components of RR interval (in normalized units; P<10^-6) and of systolic arterial pressure variability (both in millimeters of mercury squared and normalized units; P<5 x 10^-5 and P<5 x 10^-6, respectively). The HF component of MSNA variability was positively and tightly correlated with the HF component (in normalized units) of RR- interval variability (P<3 x 10^-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 relationship is best seen when power spectral components are normalized for total power. Synchronous changes in the LF and HF rhythms of both RR interval and MSNA during different levels of sympathetic drive are suggestive of common central mechanisms governing both parasympathetic and sympathetic cardiovascular modulation.