Cross-Wavelet Time-Frequency Analysis Reveals Sympathetic Contribution to Baroreflex Sensitivity as Cause of Variable Phase Delay Between Blood Pressure and Heart Rate

Figures

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  FIGURE 1. Schematic diagram to explain how the simulated IBI-data were generated. The systolic pressure is considered to rapidly affect the length of the ongoing IBI, due to the vagal effect. In addition, the values of diastoles/systoles activate the sympathetics: lower pressure values cause more activity, inducing a shorter IBI. After a pressure change, the effect on IBI starts slowly, but lasts for some seconds, and so the considered IBI is affected by several past pressure-values, as depicted by the arrows. The dots on the red line indicate the relative contributions of each of these previous pressure values to the ongoing beat duration, in addition to the vagal effect.

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  FIGURE 2. Example registration of IBI (A) and SAP (B) data for a resting supine subject, who is passively moved to a 70° head-up-tilt position and back (between the vertical lines). In this case, a large increase in heart-rate during HUT is observed, while the blood pressure level is hardly affected.

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  FIGURE 3. Wavelet analysis of IBI and SAP variability, and resulting BRS and phase between SAP and IBI for the data from Figure 2. The colors in the upper panels show the spectral power for each frequency and for each point of time, (A) for IBI and (B) for SAP. The 0.1 Hz frequency is indicated by a dotted line. (C) shows the wavelet coherence r2 (color) and phase (arrows) between SAP and IBI. Arrows pointing in the 3 o’clock indicate zero phase difference. Phase is shown only where r2 > 0.5. In (D), the BRS is calculated from the data in the first two panels (only where r2 > 0.5). In (E,F), the mean spectral power in the LF band (red line) and in HF band (black line) is shown for IBI and SAP variability. (G,H) show the mean phase between SAP and IBI and the mean BRS for both frequency bands. A negative phase implies SAP-variations leading IBI-variations.

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  FIGURE 4. Comparison of the computed xBRS (dotted line) with the wavelet BRS in the LF range (drawn red line) and the HF range (black). Data are for the subject of Figure 2. The HUT period is indicated.

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  FIGURE 5. Scatterplots of xBRS vs. the wavelet BRS in the (A) LF and (B) HF range. The black dots indicate data points in supine position, and red dots are for HUT. The black line of identity is shown.

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  FIGURE 6. Simulated IBI (A) and SAP (B) data as described in section “Materials and Methods.” In the simulation, during the “HUT”-period (25 to 28.5 min) the mean IBI is decreased from 1.0 to 0.7 s and the vagal influence is reduced.

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  FIGURE 7. Wavelet analysis of IBI and SAP variability and resulting BRS for simulated data. The layout of this figure is identical to Figure 3. The colors in the upper panels show the spectral power for each frequency and for each point of time, (A) for IBI and (B) for SAP. (C) shows the wavelet coherence r2 (color) and phase (arrows) between SAP and IBI. In (D), the BRS is calculated from the data in (A,B). In (E,F), the mean spectral power in the LF band (red line) and in HF band (black line) is shown for IBI and SAP variability. (G,H) show the mean phase between SAP and IBI and the mean BRS for both frequency bands. A negative phase implies SAP-variations leading IBI-variations. The imposed frequencies are 0.1 and 0.25 Hz but due to our beat-to-beat approach the apparent frequencies are slightly higher during the supine period, and lower during the (shorter) HUT period.

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  FIGURE 8. Vector addition of the frequency-dependent vagal and sympathetic BRS contributions for simulated data. The black vector indicates the vagal BRS contribution, coinciding in phase with the SAP variation. The blue vectors are the sympathetic BRS contributions, varying in size and direction, as suggested by Figure 1. The sum of vagal and sympathetic contributions determines the total BRS-vector (red), which controls the IBI-variations, the phase of which differs in general from the SAP phase. (A,B) are for the simulated supine conditions, with frequency 0.1 and 0.25 Hz, respectively. The orientation and size of the dashed vector in (A) is explained in detail in the text. (C,D) are for the HUT condition, with faster IBI and less vagal contribution. The figure shows that a variety of phase angles and BRS magnitudes can result from the combination of vagal and sympathetic effects.

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  FIGURE 9. Bode plot of BRS for simulated supine and HUT data. The size (A) and the phase angle (B) of the BRS vector are given as a function of frequency, both for the supine (drawn line) and the HUT conditions (dotted line).

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  FIGURE 10. Vector addition of vagal and sympathetic BRS contributions for the 0.16 Hz frequency, both for the supine (A) and the HUT condition (B). Around this frequency the Bode plot (Figure 9B) shows for the HUT condition a phase angle of ±180°. Note the antiphase between SAP and IBI variations in (B).