Cardiac Baroreflex Variability and Resetting during Sustained Mild Effort

Figures

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  Figure 1. Continuous measurements of RR-interval, systolic blood pressure (SBP), and muscle sympathetic nerve activity (MSNA) made simultaneously during the three phases of the experiment.

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  Figure 2. Episodes of baroreflex engagements as normally presented in spontaneous sequence analysis, based on data from one subject at baseline. An episode consists of three or more consecutive cardiac cycles (red circles) during which SBP and RRI both increase or decrease from the first cycle to the second and from the second to the third etc. The episode thus provides three or more data points from which to extract a regression line (thin blue lines). The slope of each of these lines is considered to be a measure of the “gain” or “sensitivity” of the baroreflex during that engagement episode, and the average of all such slopes (thick blue line) is considered to be a measure of the average sensitivity of the baroreflex during that particular experimental session.

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  Figure 3. Episodes of baroreflex engagements as normally presented in spontaneous sequence analysis (Figure 2) are here placed along the timeline of the experiment. Episodes in which SBP and RRI were increasing in tandem are shown in green, and those in which SBP and RRI we decreasing in tandem are shown in red. Thin blue lines indicate periods in which the baroreflex was not engaged.

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  Figure 4. SBP-RRI data as determined by spontaneous sequence analysis from one subject. The dots refer to the spontaneous operating points of the SBP-RRI data in each of the Baseline (BLN), first 2 min of handgrip exercise (HBm1–2), minutes 3–5 of handgrip (HGm3–5) and during a 4 min period of post-exercise circulatory occlusion (PECO). Logistic curves represent the mean (middle curve of each group) plus or minus two standard deviations (outside curves to the right or left of the center average curve. The position and slopes of the baroreflex “space” shift to the right and downward as exercise progresses, recovering somewhat during PECO.

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  Figure 5. The range of RR interval values [RRI-range, normalized to a value of 1.0 at baseline (time “0”)] observed at each minute during the handgrip exercise (HG) and post-exercise circulatory occlusion (PECO). Specifically, while RRI is seen to decline monotonically during the 5 min HG in Figure 1, RRI-range is seen here to drop dramatically in the first 2 min of HG then stabilizing and rising again. The thin red lines representing individual subjects are shown to illustrate the differences that exist among subjects, while the heavy blue line represents the average of all subjects. Significantly different from baseline (p < 0.01). Significantly different from baseline (p < 0.05).

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  Figure 6. The range of systolic blood pressure values [SBP-range, normalized to a value of 1.0 at baseline (time “0”)] observed at each minute during the handgrip exercise (HG) and post-exercise circulatory occlusion (PECO). Again there is a drop in SBP-range in the first 2 min, then a rise to above pre-HG level in the next 3 min. By contrast, SBP itself rises monotonically during the 5 min of HG, as seen in Figure 1. The thin red lines representing individual subjects are shown to illustrate the differences that exist among subjects, while the heavy blue line represents the average of all subjects. Significantly different from baseline (p < 0.05).

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  Figure 7. Muscle sympathetic nerve activity [MSNA, normalized to a value of 1.0 at baseline (time “0”)] during the timeline of the experiment. There is a distinct change in the rate of increase following the first 2 min or so of HG. The thin red lines representing individual subjects are shown to illustrate the differences that exist among subjects, while the heavy blue line represents the average of all subjects. The dashed blue line represents a continuation of the slope during the first 2 min, to highlight the change in slope during the next 3 min. Significantly different from baseline (p < 0.01). Significantly different from baseline (p < 0.05).

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  Figure 8. Baroreflex sensitivity during the timeline of the handgrip (HG) and post-exercise circulatory occlusion (PECO) phases of the experiment [nornmalized to a value of 1.0 at baseline (time “0”)]. There is a steep drop in sensitivity during the first 2 min of HG, followed by a less steep but continuing drop in the next 3 min. The thin red lines representing individual subjects are shown to illustrate the differences that exist among subjects, while the heavy blue line represents the average of all subjects. The dashed blue line represents a continuation of the slope during the first 2 min, to highlight the change in slope during the next 3 min. Significantly different from baseline (p < 0.01).

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  Figure 9. (top) Correlation between baroreflex sensitivity and RRI-range. Low RRI-range is associated with low baroreflex sensitivity. (bottom) Lack of correlation between baroreflex sensitivity and SBP-range. The 10 data points in each panel represent the 10 data points of average of the 5 subjects in each of Figures 5,6,8.