Evaluating physiological coherence of microvascular hemodynamics between brain and skeletal muscle using high-resolution near-infrared spectroscopy

Abstract

Physiological coherence between organ-specific microvascular hemodynamics is a fundamental yet underexplored aspect of human systemic blood flow regulation, particularly in systemic vascular diseases such as critical illness. Current critical care monitoring predominantly relies on systemic macro-hemodynamic parameters, which fail to capture the heterogeneity and dynamic relationships of microvascular perfusion across organs like brain and skeletal muscle. This knowledge gap is especially evident regarding the temporal and frequency-specific coherence of microvascular signals between these organs, limiting the development of targeted, organ-specific therapies. To address this, we developed a novel methodological framework to quantify physiological coherence between brain and skeletal muscle microcirculation using high-resolution Near-Infrared Spectroscopy (hr-NIRS) and advanced wavelet-based signal processing. This study employed continuous, non-invasive monitoring in ICU patients (n=40) and healthy controls (n=15), simultaneously acquiring total hemoglobin (HbT) from the frontal cortex and brachioradialis, along with systemic blood pressure. Time-frequency and time-phasic relationships were characterized using wavelet coherence and semblance analyses, assessing both the strength and synchrony of microvascular coupling across cardiac and microvascular frequency bands. Key findings include: (i) ICU patients exhibited significantly reduced coherence duration and phase synchrony between brain and skeletal muscle microcirculation in the cardiac frequency band compared to healthy controls; (ii) ICU non-survivors demonstrated excessive synchronization between skeletal muscle microcirculation and systemic blood pressure in the microvascular and cardiac frequency bands compared to ICU survivors. These results establish, for the first time, that dynamic physiological coherence between brain and skeletal muscle microcirculation is significantly altered in critical illness, with distinct patterns associated with patient outcomes. Integrating wavelet-based coherence metrics into bedside monitoring offers a promising direction for the development of novel biomarkers to guide personalized therapeutic strategies and risk stratification in the ICU.