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BACKGROUND: Sepsis is a leading cause of mortality among critically ill patients and is associated with both systemic inflammation and up-regulation of coagulation. In the translational sub-study of the HALO (Heparin AnticoaguLation to improve Outcomes in septic shock) pilot trial, we evaluated the mechanisms by which unfractionated heparin (UFH) may reduce inflammation and coagulation in patients with septic shock. METHODS: In this multicenter pilot randomized trial of 69 patients diagnosed with septic shock, we evaluated the feasibility of administering therapeutic dose intravenous UFH (18 IU/kg/hr) compared to thromboprophylactic subcutaneous dalteparin (5000 IU daily). Blood samples were collected on days 1 (baseline prior to study infusion), 2, 3, 5, and 7. We evaluated coagulation using assays for protein C, activated protein C, thrombin-antithrombin (TAT), thrombin generation, clot lysis, plasminogen activator inhibitor-1 (PAI-1) and cell-free DNA (cf-DNA). Systematic inflammation was evaluated by measuring inflammatory cytokines (interleukin (IL)-6, IL-8, IL-10, and IL-17). RESULTS: The mean age of the study population was 61 years, of whom 43% were male. Thirty two patients (46%) were randomized to receive unfractionated heparin while 37 (54%) received dalteparin. The baseline mean aggregate Acute Physiology and Chronic Health Evaluation II (APACHE II) score was 25 ± 7.8, and Multiple Organ Dysfunction Score (MODS) 5.6 ± 2.38. Baseline laboratory testing (coagulation assays and inflammatory cytokines) was not statistically different between UFH vs. LMWH treated patients. On day 2, the median clot lysis time in UFH-treated patients compared to those receiving dalteparin was significantly decreased [6630 (IQR 0 - 14156) seconds vs. 9615 (IQR 8209 - 11018) seconds; p = 0.008] (Figure 1). UFH administration was associated with increased protein C levels [66.4% of normal (IQR 9.9 - 122.9) vs. 41.1% of normal (IQR 4.8 - 77.4); p = 0.02], and reduced thrombin generation of 0 (IQR 0 - 1725) units/min vs. 3393 (IQR 0 - 8519) units/min; p CONCLUSION: In patients diagnosed with septic shock, IV UFH reduces thrombin generation, shortens clot lysis time and improves endogenous protein C levels compared to dalteparin administered for thromboprophylaxis. Analyses for samples obtained on days 3, 5 and 7 are ongoing. These preliminary data provide a biologic rational for the use of heparin in sepsis. Figure 1. Differences in clot lysis, protein C and thrombin generation in patients treated with UFH vs. LMWH. UFH is associated with reduced thrombin generation, improved Protein C levels, and reduced clot lysis time. Figure 1. Differences in clot lysis, protein C and thrombin generation in patients treated with UFH vs. LMWH. UFH is associated with reduced thrombin generation, improved Protein C levels, and reduced clot lysis time. Disclosures No relevant conflicts of interest to declare.