Coherence and Coupling Functions Reveal Microvascular Impairment in Treated Hypertension

Abstract

The complex interactions that give rise to heart rate variability (HRV) involve coupled physiological oscillators operating over a wide range of different frequencies and length-scales. Based on the premise that interactions are key to the functioning of complex systems, the time-dependent deterministic coupling parameters underlying cardiac, respiratory and vascular regulation have been investigated at both the central and microvascular levels. Hypertension was considered as an example of a globally altered state of the complex dynamics of the cardiovascular system. Its effects were established through analysis of simultaneous recordings of the electrocardiogram (ECG), respiratory effort, and microvascular blood flow [by laser Doppler flowmetry (LDF)]. The signals were analyzed by methods developed to capture time-dependent dynamics, including the wavelet transform, wavelet-based phase coherence, non-linear mode decomposition, and dynamical Bayesian inference, all of which can encompass the inherent frequency and coupling variability of living systems. Phases of oscillatory modes corresponding to the cardiac (around 1.0 Hz), respiratory (around 0.25 Hz), and vascular myogenic activities (around 0.1 Hz) were extracted and combined into two coupled networks describing the central and peripheral systems, respectively. The corresponding spectral powers and coupling functions were computed. The same measurements and analyses were performed for three groups of subjects: healthy young (Y group, 24.4 ± 3.4 y), healthy aged (A group, 71.1 ± 6.6 y), and aged treated hypertensive patients (ATH group, 70.3 ± 6.7 y). It was established that the degree of coherence between low-frequency oscillations near 0.1 Hz in blood flow and in HRV time series differs markedly between the groups, declining with age and nearly disappearing in treated hypertension. Comparing the two healthy groups it was found that the couplings to the cardiac rhythm from both respiration and vascular myogenic activity decrease significantly in aging. Comparing the data from A and ATH groups it was found that the coupling from the vascular myogenic activity is significantly weaker in treated hypertension subjects, implying that the mechanisms of microcirculation are not completely restored by current anti-hypertension medications.

A novel MRI analysis for assessment of microvascular vasomodulation in low-perfusion skeletal muscle

“…A final comment pertains to the study of microvascular dynamics. There is ample literature on the oscillatory nature of microvascular blood flow, with the majority employing optical perfusion methods with a very high temporal sampling rate 16,17 . With such rapid sampling, one can observe a wide frequency range associated with cardiac, respiratory, or myogenic origins.…”

“…With such rapid sampling, one can observe a wide frequency range associated with cardiac, respiratory, or myogenic origins. Sophisticated extraction of these distinct frequency components is possible through wavelet analysis and non-linear mode decomposition 17 . These analysis methods may potentially be valuable for analyzing our MRI measurements; to do so, we would need to increase our temporal resolution to exceed at minimum 10 seconds and ideally approach 1 second.…”

Blood and lymph circulation under normal conditions and in circulatory hypox

“…Laser Doppler flowmetry (LDF) is a modern non-invasive method for evaluating microcirculation. Registered Doppler frequency shift of the reflected signal is proportional to the speed of movement of the microvasculatory particles [3,6,11,14].…”

Alterations in the coupling functions between cerebral oxyhaemoglobin and arterial blood pressure signals in post-stroke subjects

“…This method is suitable for the treatment of nonstationary time series, which often exist in physiological signals, such as BP and delta [HbO 2 ] signals. Previous studies have evaluated the effective coupling interactions based on coupling functions among different physiological indices, such as neuronal, cardiorespiratory, cardiac, respiratory, and vascular regulation[ 34 , 35 ]. However, the effective coupling interaction between the cerebral oscillations (i.e.…”