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

Roel W De Boer,John M Karemaker

doi:10.3389/fnins.2019.00694

Roel W De Boer, John M Karemaker

Open Access

https://doi.org/10.3389/fnins.2019.00694

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Abstract

IntroductionBaroreflex sensitivity (BRS) is often presented as a single number, but it is actually a frequency-dependent phenomenon whose value changes constantly due to internal and external stimuli. The standing posture, for instance, necessitates a changeover from vagal to sympathetic predominance for cardiovascular control. We present a wavelet cross-spectral analysis of blood pressure (BP) and interbeat interval (IBI) recordings in the search for variations in gain and phase between these signals. Additionally, we show how the lag in sympathetic response dictates BP-to-IBI phase relations.MethodsRecordings in supine and head-up tilted (HUT) position, obtained earlier in 10 healthy subjects (4f/6m, aged 27–47 years) were used. BP and IBI were measured from the continuous finger pressure (by Finometer). The cross-wavelet analysis produced time- and frequency dependent gain (wBRS, wavelet derived BRS) and phase, using the MATLAB® wavelet toolbox. We also applied the wBRS method to model-generated BP- and IBI-data with known interrelations to test the results of this analysis technique. Finally, wBRS values were compared with the xBRS-approach, which is a time domain method for continuous BRS estimation in a sliding 10-s window.ResultsIn resting supine conditions, wBRS fluctuates; more at respiratory frequencies than in the 0.1 Hz band. After HUT, wBRS at the respiratory frequency decreases from average 22.7 to 8.5 ms/mmHg, phase between BP and IBI increases from −30° to −54°; in the sympathetic 0.1 Hz range these numbers are 13.3→6.3 ms/mmHg and −54°→−59°. The values found by xBRS are intermediate between wBRS-resp and wBRS-0.1 Hz. The Appendix shows that for the simulated data the BRS and phase values as found by the wavelet technique can be explained from vector additions of vagal and sympathetic BRS contributions.DiscussionDuring supine rest parasympathetic control of heart rate dominates BRS; after HUT this is diminished and less effective. Due to the reaction times of the autonomic effectors, the phase relations between the signals depend on the relative contribution of the sympathetics, which explains the larger phase shift.ConclusionCross wavelet analysis allows to follow fast BRS changes in time and frequency, while the computed phase relations help understand sympathetic participation.

Highlights

Baroreflex sensitivity (BRS) is often presented as a single number, but it is a frequency-dependent phenomenon whose value changes constantly due to internal and external stimuli
In this study we used the interbeat interval (IBI) data and the derived systolic pressures from the continuous finger blood pressure data (Finometer, BMI-TNO, Netherlands; sample frequency 200 Hz, start of an IBI is set by the firmware at a point that corresponds to the very start of the systolic upslope as determined by a proprietary algorithm)
During the head-up tilted (HUT)-period, both the value and the variability of the IBI decrease, while little change is seen in the blood pressure values

Summary

Introduction

Baroreflex sensitivity (BRS) is often presented as a single number, but it is a frequency-dependent phenomenon whose value changes constantly due to internal and external stimuli. Investigators derived BRS estimators from spontaneous fluctuations in pressure and heart-rate for subjects in resting conditions, either using time-domain techniques such as sequential methods (Bertinieri et al, 1985; Parati et al, 1988), or applying spectral approaches (De Boer et al, 1986; Robbe et al, 1987; Parati et al, 1995) Most of these techniques return a single BRS-value over a period of time, time-varying BRS-values have been considered, both for time-domain (Westerhof et al, 2004; Eckberg and Kuusela, 2005; Wesseling et al, 2017) and frequency domain (e.g., Li et al, 2018) approaches. The value of the BRS will change continuously, due to internal and external stimuli, of which a change of posture is a very strong one, as it necessitates activation of the sympathetic nervous outflow to the vasculature and concomitantly to the heart, thereby increasing heart rate and contractility, that effect is not strictly necessary as is proven in patients who have a cardiac transplant (Rudas et al, 1993)

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