Relationship between skin microvascular blood flow and capillary refill time in critically ill patients

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Capillary refill time (CRT) and skin blood flow (SBF) have been reported to be strong predictors of mortality in critically ill patients. However, the relationship between both parameters remains unclear. We conducted a prospective observational study in a tertiary teaching hospital. All patients older than 18 years admitted in the intensive care unit (ICU) with circulatory failure and a measurable CRT were included. We assessed index SBF by laser doppler flowmetry and CRT on the fingertip, at T0 (Within the first 48 h from admission) and T1 (4 to 6 h later). Correlation was computed using Spearman or Pearson’s formula. During a 2-month period , 50 patients were included, 54% were admitted for sepsis. At baseline median CRT was 2.0 [1.1–3.9] seconds and median SBF was 46 [20–184] PU. At baseline SBF strongly correlated with CRT (R 2 = 0.89; p < 0.0001, curvilinear relationship), this correlation was maintained whether patients were septic or not (R 2 = 0.94; p = 0.0013; R 2 = 0.87; p < 0.0001, respectively), and whether they received norepinephrine or not (R 2 = 0.97; p = 0.0035; R 2 = 0.92; p < 0.0001, respectively). Between T0 and T1, changes in SBF also significantly correlated with changes in CRT (R 2 = 0.34; p < 0.0001). SBF was related to tissue perfusion parameters such as arterial lactate level ( p = 0.02), whilst no correlation was found with cardiac output. In addition, only survivors significantly improved their SBF between T0 and T1. SBF was a powerful predictor of day-28 mortality as the AUROC at T0 was 85% [95% IC [76–91]] and at T1 90% [95% IC [78–100]]. We have shown that index CRT and SBF were correlated, providing evidence that CRT is a reliable marker of microvascular blood flow. Trial registration Comité de protection des personnes Ouest II N° 2023-A02046-39.

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