Microvascular monitoring in the ICU using high-resolution near-infrared spectroscopy

Abstract

Circulatory shock, particularly septic shock, is a life-threatening condition characterized by impaired tissue perfusion and a high risk of organ failure. Conventional intensive care monitoring focuses on macro-hemodynamic parameters such as mean arterial pressure (MAP), which may not reflect the state of microvascular perfusion. High-resolution near-infrared spectroscopy (hr-NIRS) enables continuous, non-invasive monitoring of tissue hemoglobin dynamics, offering potential insight into regional microvascular function. This thesis presents two complementary analyses using hr-NIRS in critically ill patients: (1) derivation of optimal MAP (MAPopt) from skeletal muscle autoregulation signals, and (2) wavelet-based quantification of microvascular and cardiac power from NIRS signals. The study included 15 mechanically ventilated ICU patients (MAPopt study) requiring vasopressors, and 43 critically ill patients (wavelet study), along with 14 healthy volunteers recruited from a parallel study. MAPopt was derived using correlation indices between arterial pressure and NIRS signals in both skeletal muscle and brain. Muscle-derived MAPopt values moderately correlated with cerebral MAPopt (r = 0.76, p < 0.001), demonstrating the feasibility of peripheral autoregulation assessment. Notably, most muscle MAPopt values were below the standard clinical threshold of 65 mmHg, suggesting that conventional MAP targets may overestimate optimal perfusion pressure in some patients. Wavelet analysis demonstrated that microvascular power was significantly lower in ICU patients compared to healthy controls (p < 0.001), while cardiac power did not differ significantly. Subgroup analyses revealed no statistically significant differences in microvascular or cardiac power between survivors and non-survivors or between septic and non-septic patients. However, patients receiving vasopressors on day 1 exhibited significantly reduced microvascular power compared to those not receiving pressors (p < 0.05), particularly early in their ICU stay. These findings support the feasibility of deriving MAPopt from skeletal muscle and highlight the value of wavelet-based microvascular metrics in distinguishing physiologic states. Together, these techniques offer a promising framework for individualized hemodynamic monitoring and may enhance resuscitation strategies in critical care.