The difference in arterial baroreflex sensitivity between the supine and standing positions in healthy subjects

Figures

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  Fig. 1. Sequences of R-R interval (RRI) changes unrelated to respiratory movement in the supine position. (A) Top panel: Respiratory movement-induced thorax volume changes monitored with a thoracic belt incorporating a pair of stretchable strain sensors in a subject breathing quietly with a steady breathing volume of 15 breaths/min (0.25 Hz) with the aid of a metronome. The y-axis value indicates the output voltage of the strain sensor. Bottom panel: 320-s RRI time series with 0.25 Hz oscillation synchronized to respiratory movement. This time series also contains three sequences of large RRI changes, initial decreases followed by abrupt reversals to increases not associated with respiratory frequency and volume changes. (B) Detailed time courses of the three sequences of RRI changes. The rapid RRI increase from nadir to peak occurred over 2 s in all three cases. The case indicated by a red arrow was used for cross-spectral analysis whose results are shown in Fig. 4. (C) Power spectral densities (PSDs) of 100-s RRI time sequences containing the two sequences of RRI changes (a) and no such event (b) indicated by the arrows in the bottom panel in (A). In the PSD (a), in addition to the…

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  Fig. 2. Temporal relationships of changes in 0.05–0.35 Hz frequency powers with the sequence of R-R interval (RRI) and interbeat interval (IBI) changes in the supine position. (A) Temporal relationships of RRI spectral power changes over 0.05–0.35 frequency ranges with the sequence of RRI changes in the RRI time series shown in Fig. 1A (top panel). The RRI spectral powers at 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, and 0.35 Hz were calculated consecutively in the 20-s window of the RRI time series with a 1-s shift. The time series of RRI spectral power amplitudes thus obtained were plotted at 1-s intervals in the second and third panels. The second panel shows the results for 0.05, 0.10, 0.15, and 0.20 Hz powers, and the third panel results for 0.20, 0.25, 0.30, and 0.35 Hz powers. The 0.20 Hz power changes are presented in both panels to facilitate power amplitude comparison over the seven frequency ranges. (B) The same analyses as (A) for the IBI time series. No substantial difference was observed between the results of spectral analyses of RRI and IBI time series.

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  Fig. 3. Temporal relationships of changes in 0.05–0.25 Hz frequency powers with the sequence of interbeat interval (IBI) changes during standing. Example of the IBI time series and the sequence of IBI changes in spectral powers at 0.05- 0.25 Hz during standing. Data was obtained from the subject, whose RRI time series in the supine position is shown in Fig. 2, after postural change from the supine position. The top panel shows the IBI time series containing three sequences of IBI changes, and the bottom panels show spectral power increases associated with the IBI changes. The vertical lines through the top and bottom panels indicate the temporal relationship between the sequence of transient IBI changes and spectral power increases. Postural change from supine to standing markedly reduced the sequence-related power increases in the frequency range higher than 0.15 Hz, although slight 0.15–0.20 Hz components remained as indicated by arrows. Note that standing also markedly reduced the RSA-generated 0.25 Hz component. The second sequence of IBI changes indicated by a red arrow in the top panel was used for cross-spectral analysis, whose results are shown in Fig. 4.

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  Fig. 4. Example of spectral and cross-spectral analysis of time series of heart rate and blood pressure variabilities taken from a subject in the supine and standing positions. (A) Results of analyses in the supine position. Thirty-second IBI and mean arterial pressure (MAP) data segments containing the transient arterial baroreflex (ABR) response enclosed by a blue rectangle (a. Baroreflex) and immediately before it (b. Control) were prepared from the data indicated by a red arrow in Fig. 2A. The results of analyses of (a) and (b) are plotted in graphs (c), (d), (e), and (f) in red and black dots, respectively. Graphs A(c) and A(d) show the averaged power spectra of MAP and IBI variabilities, respectively. Graph A(e) shows high coherences (>0.7) between IBI and MAP variabilities in all frequency ranges between 0.05 and 0.30 Hz both during and before the ABR response in this case. Graph A(f) shows that ABR caused a modest increase in transfer function gain (TFG) at 0.05, and 0.10 Hz, and the increase became marked at 0.15 and 0.20 Hz. In contrast, before the ABR response, an increase in TFG was detected at frequencies higher than 0.20 Hz which reached the level during the ABR response at…

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  Fig. 5. Summary of transfer function gains (TFGs) between heart rate and blood pressure variabilities during and before the transient arterial baroreflex response in the supine and standing positions. TFGs during and before the transient baroreflex response are shown in red and black, respectively. The box indicates the 25th and 75th percentiles, the solid horizontal line indicates the median, the x indicates the mean, and the whisker indicates the range of data. The Wilcoxon signed-rank test statistically compared TFGs during and before baroreflex. n = 13 both in the supine (A) and standing (B) positions. Note that the y-axis in graph (B) is magnified fourfold.