AbstractObjectiveTo evaluate initial tissue hemoglobin oxygen saturation (StO2) in dogs presenting to an emergency room (ER) for acute hemorrhage.DesignProspective, observational study.SettingUniversity veterinary teaching hospital.AnimalsThirty‐eight dogs with acute hemorrhage were enrolled between July 2009 and October 2010. Seventy‐eight normal dogs from a previous observational study were included to represent healthy controls (“no shock”).InterventionsTissue oxygen saturation measurement was obtained at enrollment on dogs presented to the ER for acute hemorrhage. Baseline clinicopathologic (CBC, serum biochemical profile, prothrombin time, and activated partial thromboplastin time) and physiologic (plasma lactate concentration, venous blood gas, blood pressure, and hemoglobin oxygen saturation by pulse oximetry) data were recorded from all patients with hemorrhage. An ER clinician blinded to the StO2 value guided patient management. Patient survival to discharge from the hospital in the study group was recorded. Once data collection was complete, 3 emergency and critical care clinicians blinded to the StO2 data retrospectively classified patients into 1 of 4 shock categories (no shock, mild, moderate, or severe shock).Measurements and Main ResultsThe historical group of healthy dogs had higher StO2 concentrations compared to the dogs classified with shock at all 3 levels (mild, moderate, and severe, P = 0.0006, <0.0001, and 0.0018, respectively); however, there was no statistical difference in StO2 between the levels of shock. A cut‐off StO2 value of 87.6% identified a patient as having shock (area under the curve: 0.824, 95% confidence interval 0.749, 0.899).ConclusionsDogs with hemorrhagic shock have lower StO2 than a population of healthy dogs.