The association of near-infrared spectroscopy-derived tissue oxygenation measurements with sepsis syndromes, organ dysfunction and mortality in emergency department patients with sepsis

Abstract

Introduction: Near-infrared spectroscopy (NIRS) noninvasively measures peripheral tissue oxygen saturation (StO₂). NIRS may be utilized along with a vascular occlusion test, in which limb blood flow is temporarily occluded and released, to quantify a tissue bed's rate of oxygen exchange during ischemia and recovery. The objective of this study was to test the hypothesis that NIRS-derived StO₂ measures (StO₂ initial, StO₂ occlusion and StO₂ recovery) identify patients who are in shock and at increased risk of organ dysfunction (Sequential Organ Failure Assessment (SOFA) score ≥ 2 at 24 hours) and dying in the hospital.

Methods: This prospective, observational study comprised a convenience sample of three cohorts of adult patients (age > 17 years) at three urban university emergency departments: (1) a septic shock cohort (systolic blood pressure < 90 after fluid challenge; the "SHOCK" cohort, n = 58), (2) a sepsis without shock cohort (the "SEPSIS" cohort, n = 60) and emergency department patients without infection (n = 50). We measured the StO₂ initial, StO₂ occlusion and StO₂ recovery slopes for all patients. Outcomes were sepsis syndrome severity, organ dysfunction (SOFA score at 24 hours) and in-hospital mortality.

Results: Among the 168 patients enrolled, mean initial StO₂ was lower in the SHOCK cohort than in the SEPSIS cohort (76% vs 81%), with an impaired occlusion slope (-10.2 and 5.2%/minute vs -13.1 and 4.4%/minute) and an impaired recovery slope (2.4 and 1.6%/second vs 3.9 and 1.7%/second) (P < 0.001 for all). The recovery slope was well-correlated with SOFA score at 24 hours (-0.35; P < 0.001), with a promising area under the curve (AUC) for mortality of 0.81. The occlusion slope correlation with SOFA score at 24 hours was 0.21 (P < 0.02), with a fair mortality AUC of 0.70. The initial StO₂ was significantly but less strongly correlated with SOFA score at 24 hours (-0.18; P < 0.04), with a poor mortality AUC of 0.56.

Conclusions: NIRS measurements for the StO₂ initial, StO₂ occlusion and StO₂ recovery slope were abnormal in patients with septic shock compared to sepsis patients. The recovery slope was most strongly associated with organ dysfunction and mortality. Further validation is warranted.

Trial registration: NCT01062685.

Figures

Figure 1

Tissue oxygen saturation vasoocclusive testing. The initial slope, occlusion slope and recovery slopes are shown. During the initial phase, the tissue oxygen saturation (StO2) level is monitored over time (initial). At occlusion, the tourniquet is programmed to insufflate a cuff to 50 mmHg above the patient's systolic blood pressure. The StO2 level is then monitored during the ischemic period to calculate the steady-state ischemic downslope (ischemic slope). This is postulated to represent a combination of oxygen content in the microcirculation and the metabolic demand of the tissues. Next the cuff is released so that blood flow may be reestablished. The tissue is then reperfused, with the rate representing the capacity to autoregulate and reperfuse the tissue exposed to regional ischemia (recovery slope). A patient with intact endothelial cell function, microcirculation and oxygenation capacity will reperfuse quickly, yielding a steep recovery slope, while patients in whom these mechanisms are impaired will have a shallower recovery slope.

Figure 2

Differences in initial, ischemic and recovery slopes stratified by sepsis severity. The boxplots are for initial tissue oxygen saturation (StO2). The top and bottom lines of the box are the 25th and 75th percentiles, respectively. The middle line is the median. The whiskers extend to the last data point within 1.5 quartile ranges of the box. The gray dots are the individual observed data points.

Figure 3

Differences in initial, ischemic and recovery slopes stratified by sepsis severity. Ischemic slope. The top and bottom lines of the box are the 25th and 75th percentiles. The middle line is the median. The whiskers extend to the last data point within 1.5 quartile ranges of the box. The gray dots are the individual observed data points.

Figure 4

Differences in initial, ischemic and recovery slopes stratified by sepsis severity. Recovery slope. The top and bottom lines of the box are the 25th and 75th percentiles. The middle line is the median. The whiskers extend to the last data point within 1.5 quartile ranges of the box. The gray dots are the individual observed data points.

Figure 5

Receiver operating characteristic curves for mortality. The receiver operating characteristic (ROC) curves for the initial StO2, ischemic slope, recovery slope and serum lactate measurements as predictors of in-hospital mortality are shown.

Figure 6

Receiver operating characteristic curves for Sequential Organ Failure Assessment scores ≥ 2. The receiver operating characteristic (ROC) curves for the initial tissue oxygen saturation, ischemic slope, recovery slope and serum lactate measurements as a predictor of Sequential Organ Failure Assessment scores > 2 at 24 hours are shown. The dashed line represents an AUC of 0.5 consistent with purely due to chance alone.