6 Evaluation of the Shock Index for Out-of-Hospital Sepsis Recognition

Abstract

The purpose of this study was to assess the utility of sepsis suspicion and field vital signs, including calculation of SI and MSI in the out-of-hospital setting as a predictor for sepsis and sepsis-related outcomes including hyperlactatemia ≥2 mmol, as a marker of sepsis severity and a surrogate indicator for poor prognosis, and as a predictor for ICU admission and 28-day mortality. The authors present their real-world experience in a county EMS system evaluating the correlation of out-of-hospital shock index with hospital lactate, considered a marker for sepsis. This was a prospective observational study conducted as part of our county EMS system’s quality and research program. Our IRB-approved out-of-hospital research registry uses a comprehensive database and record-keeping system that prospectively identifies and collects relevant data for all potential sepsis cases. Patients with abnormal vital signs meeting SIRS criteria and either having suspected infection, or a shock index (defined as heart rate [HR]/systolic blood pressure [SBP] >1.0) or modified shock index [MSI] (defined as HR/mean arterial pressure <0.7 or > 1.3), or having high risk factors such as being from a nursing home or having human immunodeficiency virus, are called in as sepsis alerts (figure 1). Data were analyzed using JMP Pro 14.0. Contingency analyses with Fisher’s exact test and Pearson correlation were performed on serum lactate vs. sepsis diagnosis and in-hospital death, with relative risks calculated. A total of 1426 patients over the 24-month period were transported to five main receiving hospitals. The median age was 71 years, with an interquartile range of 60-80 years. The cohort was 45% female, 85% Caucasian, 14% Black, and 1% Hispanic. The initial shock index (SI1) ranged from 0.260 to 2.840 with a median of 1.11, and an IQR of 0.94 to 1.34. Sixty-eight percent had a “positive” SI1 (>1.0). The initial modified shock index ranged from 0.336 to 3.727, with a median of 1.465 and an IQR of 1.26 to 1.72. Seventy-four percent of the cohort had a “positive” MSI (<0.7, or >1.3). Three percent had MSI <0.7, while 71% had MSI > 1.3. 1321 patients (92%) had repeat vitals during transport, so a second SI was also calculated. The second SI (SI2) was elevated in 41%. All patients with elevated serum lactate were admitted to the hospital for sepsis. An elevated serum lactate was significantly associated with being diagnosed with sepsis (Pearson correlation, P<0.0001) with a relative risk of 38 (95% CI 20-72). Patients who were had an elevated SI were significantly more likely to be admitted for sepsis (P<0.0001, 95% CI 0.1038 to 0.2714). An elevated shock index mirrors an elevated lactate and as such appears to provide a reliable “invisible lactate” surrogate for use in the field.