A Reliable Index for Peripheral Microcirculation Perfusion Monitoring: The Resistance Index (RI) of Nail Bed Capillaries as Monitored by Ultrasound

Objective: Microcirculatory dysfunction is a critical determinant of organ failure in shock, yet validated tools for real-time microcirculation assessment remain scarce. This study aimed to evaluate the predictive value of resistance indices (RIs) of nail bed capillaries measured by ultrasound for detecting hypoperfusion in shock patients. Design: Prospective single-center cohort study. Patients: Critically ill patients ( n = 62) admitted to the intensive care unit (ICU) of West China Hospital (April 2023–April 2024) were stratified into pre-shock and classical shock groups based on Sepsis-3 and the Society for Cardiovascular Angiography and Interventions (SCAI) SHOCK criteria. Methods: Ultra-high-frequency ultrasound (37 MHz probe) measured RIs of three vascular beds: proper digital palmar artery (PDPA), nail bed capillaries (NBC), and snuffbox artery (SBA). Diagnostic performance was evaluated via receiver operating characteristic (ROC) analysis. Results: NBC RI demonstrated superior discriminative capacity for hypoperfusion compared to PDPA RI and SBA RI (area under the curve [AUC]: 0.82 for bilateral little fingers; 95% confidence interval [CI]: 0.71–0.93). No significant correlation was observed between the resistance index (RI) of nail bed capillaries and lactate levels. Conclusion: NBC RI might be a sensitive, non-invasive marker for early microcirculatory impairment in shock. Integration of NBC RI into multimodal monitoring protocols may enhance personalized resuscitation strategies.