Clinical outcomes and peripheral tissue oxygen saturation monitoring of the knee region by near-infrared spectroscopy in circulatory shock: a prospective observational cohort study

Background: In circulatory shock, tissue hypoperfusion leads to adverse outcomes. We hypothesized that peripheral tissue oxygen saturation (StO₂), measured with near-infrared spectroscopy (NIRS), could provide a non-invasive method for assessing tissue hypoperfusion and predicting pending organ dysfunction and mortality. Methods: ASSESS-SHOCK was a prospective, observational study enrolling circulatory shock patients from April 2019 to May 2023 in three intensive care units (ICU). Adult patients fulfilling the criteria for circulatory shock within 24 h of ICU admission were eligible. Patients underwent continuous 48 h StO₂ (INVOS™) monitoring of the knee region. To express the burden of tissue hypoperfusion we calculated mean StO₂ and areas below predefined StO₂ thresholds. The primary outcome, change in Sequential Organ Failure Assessment (SOFA) score, was dichotomized to improvement or non-improvement in SOFA score from enrollment to day 7 or ICU discharge. Death within 7 days was considered as SOFA non-improvement. 90-day mortality was among the secondary outcomes. Results: We included 256 patients. Due to several reasons, including the COVID-19 pandemic, the patient sample was not consecutive. The median of 48-h mean StO₂ was 68.3% (interquartile range [IQR] 57.5–74.1) in SOFA-improvers (n = 171), compared to 63.5% (IQR 52.7–70.8, p = 0.020) in non-improvers (n = 85), and 68.7% (IQR 58.2–74.5) in 90-day survivors, versus 60.9% (IQR 49.5–67.1, p < 0.001) in non-survivors. There were no statistically significant differences in the areas below predefined StO₂ thresholds between the SOFA-improvers and non-improvers but all the areas were larger in 90-day non-survivors. The 90-day mortality was 27.0% (n = 69). In multivariable analyses 48-h mean StO₂ was associated with 90-day mortality (Odds ratio [OR] 0.97, 95% confidence interval [CI 95%] 0.94–1.00, p = 0.047) but not with SOFA change. The association with mortality was, however, no longer significant in multivariable analyses after exclusion of the last 6 hours of StO₂ registration in the patients (n = 29) who died during the 48 h registration (OR 0.97, CI 95% 0.94–1.00, p = 0.062). Conclusions: Lower peripheral StO₂ was associated with 90-day mortality in critically ill patients with circulatory shock but not with persisting or worsening organ dysfunction. NIRS shows promise as a non-invasive monitor of tissue perfusion in circulatory shock. Trial registration: ClinicalTrials.gov Identifier: NCT03814564, registered 15 January 2019.