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Fractal Cardiovascular Dynamics and Baroreflex Sensitivity After Stellate Ganglion Block

Taneyama, Chikuni, MD*; Goto, Hiroshi, MD

doi: 10.1213/ane.0b013e3181b018d8
Regional Anesthesia: Research Reports
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BACKGROUND: It has been shown that stellate ganglion block can attenuate baroreflex sensitivity. Our primary purpose in this study was to determine whether fractal dynamics (dynamic change of self-similar fluctuation patterns) of not only heart rate but also systolic blood pressure variability are involved in attenuation of baroreflex sensitivity after stellate ganglion block.

METHODS: Sixteen young, healthy volunteers entered the study. Spectral analysis of heart rate and systolic blood pressure variability was performed before and 30, 60, 90, and 120 min after either right or left stellate ganglion block, separated by a 1 to 1½-mo interval, with 6 mL of 1% mepivacaine. Shortly after each spectral analysis, baroreflex sensitivity was assessed with the head-up tilt test.

RESULTS: Baroreflex sensitivity, assessed by the head-up tilt test, was significantly attenuated at 30 min after either right or left stellate ganglion block (1.26 ± 0.18 to 0.46 ± 0.08 bpm/mm Hg, P < 0.05 and 1.17 ± 0.35 to 0.51 ± 0.13 bpm/min, P < 0.01, respectively). Fractal slopes reflecting the degree of self-similarity of fluctuations were significantly increased at 30 min after either right or left stellate ganglion block (right stellate ganglion block—heart rate; −1.08 ± 0.30 to −1.62 ± 0.22, P < 0.01; right stellate ganglion block—systolic blood pressure; −1.30 ± 0.80 to −2.40 ± 0.80, P < 0.05; left stellate ganglion block—systolic blood pressure; −1.20 ± 0.40 to −2.13 ± 0.50, P < 0.05). Fractal slope did not change after left stellate ganglion block with heart rate variability analysis.

CONCLUSIONS: Loss of complexity (status of being complex behavior) of both heart rate and systolic blood pressure variability, indicated by increased fractal slopes, is one mechanism in attenuating baroreflex sensitivity after stellate ganglion block.

From the *Anesthesia and Pain Relief, Chikuni Taneyama Clinic, Shiojiri City, Nagano Prefecture, Japan; and †Department of Anesthesiology, University of Kansas Medical Center, Kansas City, Kansas.

Accepted for publication May 8, 2009.

Supported by institutional and departmental sources.

Presented in part at the Annual Meeting of the American Society of Anesthesiologists, Chicago, IL, October 14, 2006.

Address correspondence and reprint requests to Hiroshi Goto, MD, Department of Anesthesiology, University of Kansas Medical Center, 3901 Rainbow Blvd., Kansas City, KS 66160. Address e-mail to hgoto@kumc.edu.

Patients occasionally experience dizziness, likely because of orthostatic hypotension, when standing after stellate ganglion block. The symptom may well be caused by impaired compensatory baroreflex integrity because stellate ganglion block affects not only cardiac sympathetic nerves but also vagal afferents, including aortic depressor nerves, and these nerves play important roles in the baroreflex mechanism.1

Heart rate variability is widely used to measure alterations in the autonomic nervous system. Among several measures of heart rate variability, a ratio of low frequency (LF) domain to high frequency (HF) domain (LF/HF) has been considered to reflect sympathovagal balance2 and has been widely used to assess cardiovascular stability after various surgeries.3–6 In addition, the fractal dynamics (dynamic change of self-similar fluctuation patterns) of heart rate variability have recently gained popularity in assessing hemodynamic stability and homeostasis.7,8 There is some degree of similarity in heart rate fluctuations between past heart rate fluctuation patterns and those over a given time period. Similarly, previous fluctuation patterns may be used to predict future fluctuations. Thus, heart rate variability has a component of fractal (self-similarity) dynamics. The greater the fractal component of heart rate fluctuations, the greater the loss of complexity of heart rate variability. This attenuated cardiovascular response to various stresses has been used to predict long-term cardiac mortality and morbidity.7–9 The baroreflex sensitivity study, such as a head-up tilt test, is considered as one form of stress and, therefore, we speculated that impaired baroreflex sensitivity after stellate ganglion block might be assessed by the fractal dynamics of heart rate variability. The main purposes of our study, therefore, were twofold: 1) to confirm that stellate ganglion block attenuates baroreflex sensitivity assessed by conventional head-up tilt test; and 2) to elucidate whether or not complexity of fractal heart rate and systolic blood pressure variability is involved in the impairment of baroreflex sensitivity after stellate ganglion block.

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METHODS

With approval from Ethics Committee at the Chikuni Taneyama Clinic (Shiojiri-City, Nagano Prefecture, Japan), informed consent was obtained from 16 healthy local Japanese volunteers (eight males and eight females, age 24.9 ± 4.4 yr). The subjects were not taking any medications, and their electrocardiograms showed regular sinus rhythm without ventricular ectopies. The study was performed at the private pain clinic facility at midmorning. Each subject was placed on a tilt table with a foot rest (Toshiba Medical, Nasu, Japan) in the supine position, and belts were placed around the arms, waist, and knees. Systolic blood pressure was recorded by a noninvasive, continuous, beat-to-beat blood pressure monitoring system with a tonometry method (ANS508; Nihon Colin, Komatsu, Japan), and electrocardiogram was recorded continuously. The MemCalc method, a combination of the maximum entropy method for spectral analysis of heart rate and systolic blood pressure variability, and the nonlinear least squares method for fitting analysis (Tarawa, Suwa Trust, Tokyo, Japan) were applied during a period of 280 s to obtain control values.10 Ratios of LF power (frequencies between 0.04 and 0.15 Hz) to HF power (frequencies 0.15 and 0.4 Hz) were calculated (LF/HF). The fractal slope of 1/ characteristics of both heart rate and systolic blood pressure variability were obtained from the regression lines (log power spectral density relating to log frequency) over the frequency range from 0.01 to 0.15 Hz, then a controlled head-up tilt test was performed. The subject was placed from the supine position into a 60° head-up tilt position in 10 s, and heart rate and systolic blood pressure were recorded. The ratio of increment of heart rate in response to reduction of systolic blood pressure was calculated to assess arterial baroreflex sensitivity. After obtaining control values, the subjects received either right or left stellate ganglion block (in a random fashion) using sealed envelope assignment only for first-time stellate ganglion block with 6 mL of 1% mepivacaine using the anterior paratracheal approach at the level of C6.11,12 All spectral analysis measurements were repeated at 30, 60, 90, and 120 min. At least 1 to 1½ mo later, stellate ganglion block was performed on the other side, and the measurements were repeated as before. All data were expressed as mean ± sd (n = 16). Statistical analysis was performed for within-group comparisons by repeated measures of a one-way analysis of variance using Excel Statistical Program File ystat 2006, xls. Post hoc analysis was performed using the Bonferroni correction.

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RESULTS

The baseline systolic blood pressure and heart rate values were stable before spectral analyses were performed (Figs. 1A and B). Horner syndrome (ptosis, myosis, and enophthalmos) in the ipsilateral eye was observed in all subjects 30 and 60 min after each stellate ganglion block. Even at 120 min, three and two subjects still showed Horner sign after a right and left stellate ganglion block, respectively (Figs. 1A and B). Baroreflex sensitivity was significantly decreased at 30 min after either right or left stellate ganglion block (right stellate ganglion block; 1.26 ± 0.18 to 0.46 ± 0.08 bpm/mm Hg, P = 0.0070, left stellate ganglion block; 1.17 ± 0.35 to 0.51 ± 0.13 bpm/mm Hg, P = 0.0041) (Fig. 2). Baroreflex sensitivity returned to baseline values at 60 min. There was a significant decrease in LF/HF at 30 min after right stellate ganglion block with heart rate variability analysis (2.46 ± 1.10 to 0.87 ± 0.80, P = 0.0004) (Fig. 3A) and after left stellate ganglion block with systolic blood pressure variability analysis (3.20 ± 1.20 to 1.80 ± 0.90, P = 0.0012) (Fig. 3D). The fractal slopes became more negative (steepness increased) at 30 min after right stellate ganglion block with both heart rate and systolic blood pressure variability analysis (−1.08 ± 0.30 to −1.62 ± 0.22, P = 3.267 × 10−6; −1.30 ± 0.80 to −2.40 ± 0.80, P = 0.0133) (Figs. 4A and B, respectively), whereas there were no changes of fractal slope with heart rate variability analysis after left stellate ganglion block (Fig. 4C) and a significant increase in fractal slope after left stellate ganglion block with systolic blood pressure variability analysis (−1.20 ± 0.40 to −2.13 ± 0.50, P = 0.0384) (Fig. 4D).

Figure 1.

Figure 1.

Figure 2.

Figure 2.

Figure 3.

Figure 3.

Figure 4.

Figure 4.

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DISCUSSION

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.

Heart rate and systolic blood pressure were stable before each measurement (Fig. 1) and this is important to produce reliable results of power spectral analysis. The studies were performed at the midmorning in consideration of circadian rhythm of frequency domains (LF, HF) and fractals of cardiovascular variability.13,14 All volunteers had successful stellate ganglion block with 6 mL of 1% mepivacaine, indicated by Horner syndrome in the ipsilateral eye. The Horner syndrome lasted at least 60 min in all volunteers (Fig. 1), although baroreflex sensitivity returned to baseline values at that time (Fig. 2). This suggests that stellate ganglion block blocks sympathetic innervation to the eye longer than it affects other sympathetic and parasympathetic nerves involved in the baroreflex mechanism. The reduction of baroreflex sensitivity at 30 min (Fig. 2) coincided with significant reductions of the LF/HF (Figs. 3A and D) and significant increases in the fractal slope (Figs. 4A B, and D).

It was postulated by Akselrod et al.15 in 1981 that autonomic nerve balance can be evaluated by power spectral analysis of heart rate variability. LF domain (0.04-0.15 Hz) reflects mainly sympathetic nerve activity, and HF domain (0.15-0.4 Hz) reflects parasympathetic nerve activity. The decreased LF/HF observed in our study was presumably because stellate ganglion block primarily decreases cardiac sympathetic outflow. Because solution injected around the stellate ganglion spreads to surrounding areas,16 adjacently located vagal afferents, such as aortic depressor nerves, may be suppressed with stellate ganglion block.1 The cardiac sympathetic nerves and aortic depressor nerves play important roles in the arterial baroreflex loop; therefore, the autonomic derangement results in impairment of baroreflex sensitivity assessed by the head-up tilt test.

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.

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ACKNOWLEDGMENTS

The authors wish to thank Mrs. Tomiko Kodama for technical assistance and data analysis of the study.

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Appendix: GLOSSARY

  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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.
  6. 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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