The main findings of the current study are 1) both right and left stellate ganglion block decreased baroreflex sensitivity, assessed by the head-up tilt at 30 min, but no longer after 60 min; and 2) stellate ganglion block caused an imbalance of autonomic nerve activity as indicated by the reduction of LF/HF. The sympathovagal imbalance is likely one mechanism that impairs baroreflex integrity. A loss of complexity of heart rate and systolic blood pressure variabilities, indicated by increased steepness of the fractal slope, is likely another mechanism to attenuate compensatory baroreflex integrity.
The healthy heart rate fluctuations appear quite random but there is some degree of correlation of these fluctuations over time. Heart rate fluctuation at every timepoint is partially dependent on heart rate fluctuations at all previous points: fractal (self-similarity) dynamics. The fractal heart rate dynamics, in addition to sympathovagal balance, have been found to be useful to assess long-term cardiovascular stability and homeostasis.7–9 To assess the degree of fractal dynamics, the regression line over frequency between 0.01 and 0.15 Hz has been designated as a fractal slope,4 and healthy individuals have heart rate fractal slopes with β = approximately 1.0 (1/f) characteristics of heart rate variability with both random and highly correlated characteristics.8
It has been reported that loss of complexity of heart rate variability assessed by increased fractal slope is associated with orthostatic hypotension and impending syncope.17 The loss of complexity of heart rate variability has been implicated in adverse cardiovascular outcomes and impaired hemodynamic homeostasis after coronary artery surgery,4 in congestive heart failure,18 in the aging process,9 and in smoking.19 Our study showed that fractal slopes of not only heart rate but also systolic blood pressure variability get steeper after stellate ganglion block. Our study also shows that normal β values of systolic blood pressure variability are slightly higher than 1.0, similar to those of heart rate variability (Fig. 4). The loss of complexity of heart rate and systolic blood pressure variability, indicated by the increased fractal slope at 30 min, is likely another mechanism for decreased baroreflex sensitivity. However, further study is necessary to elucidate other possible mechanisms by which stellate ganglion block results in a steeper fractal slope of heart rate and systolic blood pressure variability.
The effects of right and left stellate ganglion block on heart rate and systolic blood pressure are different because of hemilateralization of autonomic cardiovascular control; there is sympathetic predominance in the right hemisphere and parasympathetic predominance in the left hemisphere.20–22 Our data showed that right stellate ganglion block affected both heart rate and systolic blood pressure variability, but left stellate ganglion block affected only systolic blood pressure and not heart rate variability.
In summary, our study indicates that either right or left stellate ganglion block attenuates baroreflex sensitivity at 30 min in healthy volunteers, likely not only because of autonomic imbalance but also because of loss of complexity of heart rate and systolic blood pressure variability. Our study also indicates that clinicians should caution their patients when standing within 1 h after stellate ganglion block because there may be excessive regularity of cardiovascular variability.
The authors wish to thank Mrs. Tomiko Kodama for technical assistance and data analysis of the study.
- Heart Rate Variability: The beat-to-beat fluctuations in heart rate (RR interval alterations) due to summations of various cycles (e.g., circadian, respiratory, neural, intravascular volume, seasonable cycles) and uncorrelated random fluctuations.
- Spectral Analysis of Heart Rate Variability: A common method used for analyzing heart rate variability (frequency domain analysis); plotting the power spectral density on the y axis in relation to frequency on the x axis. Derived mathematically from the time domain analysis of heart rate variability; plotting fluctuations of RR intervals on the y axis in relation to time on the x axis.
- Low Frequency Domain, High Frequency Domain: In the power spectral density-frequency plot, several specific frequency bands have been identified: low and high frequency bands. The areas within those bands are low frequency and high frequency domains, reflecting the degree of mainly sympathetic and parasympathetic nerve activity, respectively.
- Fractals of Heart Rate Variability: By nature, heart rate variability possesses fractals (self-similarity). Namely, the pattern of RR interval fluctuations over a long period of time (e.g., 0-300 min) are similar to those of the short period of time (e.g., 0-30 min) which, in turn, are similar to the shorter period of time (e.g., 0-3 min). Therefore, the future R-R interval fluctuations are influenced by, and partially dependent upon, interbeat fluctuations from any previous period of time.
- Fractal Slope of Heart Rate Variability: The slope of the regression line (log power spectral density-log frequency plot) over frequency between 0.01 and 0.15 Hz. The steeper the slope, the higher the degree of fractals. Another important slope of the regression line is, for point of reference, the sympathovagal slope over frequency between 0.01 and 0.4 Hz. The steeper the sympathovagal slope, the greater the sympathetic predominance.
- Loss of Complexity of Heart Rate Variability: Decreased degree of difficulty in predicting future patterns of heart rate variability (increased fractals of heart rate variability or decreased uncorrelated randomness in heart rate variability).
Similar descriptions can be applied to systolic blood pressure variability.
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