Study design and overall approach
We conducted a prospective, multicenter, observational study of a convenience sample of patients who presented to the ED of one of three large, urban, tertiary care facilities. Three specific cohorts of patients (septic shock (the "SHOCK" cohort), sepsis without shock (the "SEPSIS" cohort) and uninfected controls) were assembled, encompassing a spectrum of sepsis severity. We collected pertinent demographic and clinical covariates as well as initial StO2% and NIRS-derived measurements in response to VOT testing. Then we analyzed the association and predictive ability of the NIRS measurements with our outcomes of interest. The study was approved by the ethics committees of each of the hospitals.
Assembly of cohorts
Three distinct cohorts of patients were enrolled. The SHOCK cohort had to meet the American College of Chest Physicians/Society of Critical Care Medicine criteria for septic shock, specifically (1) suspected infection, (2) fulfillment of two or more of the criteria for systemic inflammatory response syndrome (SIRS) (temperature > 100.4°F or < 96.8°F, heart rate > 90 beats/minute, respiratory rate > 20 breaths/minute or partial pressure of carbon dioxide < 32 mmHg, white blood cell count > 12,000/μL or < 4,000/μL or > 10% bands) and (3) hypotension despite adequate fluid resuscitation (systolic blood pressure (SBP) < 90 mmHg after 20 mL/kg crystalloids) [7]. The SEPSIS cohort had to meet the inclusion criteria of suspected infection, two or more SIRS criteria (see above) and no refractory hypotension. The third cohort comprised uninfected ED control patients who met the criteria of no suspected infection, no SIRS criteria met and no evidence of hypoperfusion. The control patients were age-matched (by decade) as well as sex- and race-matched to the SHOCK cohort.
A common set of exclusion criteria were applied to all patient cohorts, which included any of the following: age < 18 years, pregnancy, established "Do Not Resuscitate" orders prior to enrollment, acute traumatic or burn injury (primary diagnosis), acute cerebrovascular event (primary diagnosis), acute coronary syndrome (primary diagnosis), acute pulmonary edema (primary diagnosis), cardiac dysrhythmia (primary diagnosis), acute and active gastrointestinal bleeding (primary diagnosis), acute drug overdose (primary diagnosis), requirement for immediate surgery and inability to obtain written informed consent. Clinical management at each institution is in agreement with the Surviving Sepsis Campaign guidelines.
Demographic and clinical covariates
We collected demographic variables (age, sex and race), comorbidities (cerebrovascular disease, chemotherapy, congestive heart failure, chronic obstructive pulmonary disease, dementia, diabetes, HIV or AIDS, Hodgkin's disease, intravenous drug use, leukemia, liver disease, myocardial infarction, non-Hodgkin's lymphoma, peripheral vascular disease, renal disease, splenectomy, steroid use, ulcer disease, transplant, presence of any malignancy and residence in a nursing home), vital signs data (temperature, blood pressure, heart rate, respiratory rate and oxygen saturation) and pertinent laboratory data (serum lactate, complete blood count, chemistry panels, markers of coagulation and liver function tests).
NIRS assessment
The InSpectra StO2 Tissue Oxygenation Monitor (model 650; Hutchinson Technology, Hutchinson, MN, USA) with probes spaced at 15 mm was utilized to obtain StO2 measurements. The measurements were taken at the thenar eminence during the resuscitation phase. Following a minimum initial five-minute stabilization period, we assessed the initial StO2 measurement and then performed a VOT procedure using an automated tourniquet (Delfi Tourniquet System; Delfi Medical Innovations, Inc, Vancouver, BC, Canada), which was insufflated to 50 mmHg over the patient's SBP for a period of three minutes. After three minutes, the cuff was quickly removed. The subsequent StO2 tracing was analyzed offline to record the following NIRS-derived metrics (see Figure 1) (1) StO2 initial, the baseline StO2 recorded after a five-minute stabilization period; (2) StO2 occlusion, the steady-state rate of occlusion (StO2%/second), represented by the descending slope during the ischemic period; and (3) StO2 recovery, the steady-state recovery slope during the reoxygenation phase after the tourniquet was released. The StO2 measurements were imported into a Microsoft Excel software file (Microsoft Corporation, Redmond, WA, USA), and the slopes were derived by (1) drawing a best-fit line for the steady-state slope for the respective metric and (2) calculating the slope.
Outcomes
We examined the association of StO2 parameters in relation to three patient-oriented outcomes: (1) presence of shock, as defined above, assessed at the time of enrollment; (2) in-hospital mortality, defined as vital signs status at hospital discharge; and (3) organ dysfunction at 24 hours assessed on the basis of the SOFA scores calculated at the time of enrollment and 24 hours later [6]. Consistent with prior publications, we defined organ dysfunction as a SOFA score ≥ 2, which was our primary outcome of interest. The use of a threshold SOFA score ≥ 2 for an ill patient has previously been established [8–10]. All patients in the control group with missing SOFA scores at 24 hours (discharge was the primary reason for missing data) were assumed to have a SOFA score < 2. For patients enrolled with a history of chronic renal insufficiency or end-stage renal disease, the renal SOFA score was not included in the total SOFA score.
Data analysis
Descriptive statistics (means, standard deviations, medians or proportions with percentiles) were reported for demographics, clinical characteristics, vital signs and laboratory values stratified by the three cohorts. We compared mean (or median) values for the StO2 parameters of interest (initial, ischemic slope and reperfusion slope) of the three different groups using the Wilcoxon two-sample test. We used a Bonferroni correction to address multiple testing; thus, for this analysis, Cronbach's α < 0.017 was considered significant. Next, we compared the StO2 parameters of interest for the outcomes of in-hospital mortality and SOFA scores ≥ 2 at 24 hours. To assess the diagnostic accuracy for the StO2 parameters as a predictor of outcomes, we constructed receiver operating characteristic curves (ROCs) and calculated the area under the curve (AUC) along with the 95% confidence intervals (95% CIs). We used multivariate logistic regression models to obtain adjusted estimates for age, serum lactate and SBP and to identify the StO2 parameters and adjustor variables with the strongest independent associations with outcomes by using a stepwise backward elimination technique with forward examination of parameters eliminated after final model selection. Throughout the analysis we used serum lactate level as a comparison predictor.
Sample size calculation
Our study was powered on the ability of our anticipated best StO2 readout (StO2 recovery slope) to discriminate the SEPSIS cohort from the SHOCK cohort. Based on previous studies, assuming mean changes in slope of 2.3 ± 1.3 for the SHOCK group and 3.2 ± 1.4 for the SEPSIS group, with a power of 90% and Cronbach's α set at 0.05, we calculated that approximately 60 patients per group were needed [11]. We also enrolled 50 uninfected controls as comparators for comparisons between controls and the sepsis groups.