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The objective of this study was to evaluate the relationship between tissue oxygen saturation (StO2) and oxygen delivery index (DO2I). Oxygen delivery index is product of two factors arterial oxygen content (CaO2) and cardiac index (CI). In this study the relationship between DO2I and StO2 was evaluated by manipulating both of these factors independently. In phase one of the study, CaO2 was altered by manipulating the fractional inspired oxygen (FiO2) concentration. Anesthetized dogs were evaluated at both high (0.40 and 0.95) and low (0.15 and 0.10) FiO2 sequences. In phase two of the study, CI was altered by manipulating the volemic state. Anesthetized dogs were evaluated at hypovolemic, normovolemic and hypervolemic states. In each phase dogs were instrumented for thermodilution cardiac index (CI) and sartorius muscle StO2. Data collected included hemoglobin concentration, heart rate (HR), MAP, CI, StO2. Arterial oxygen content and DO2I were calculated at each time point. Data analysis included Pearson's correlation and mixed model ANOVA (p < 0.05). In both phases one (r = 0.97; p = 0.0013) and two (r = 0.97; p = 0.005) there was a strong correlation between StO2 and DO2I. Under the conditions of this study, there was a strong correlation between StO2 and DO2I, suggesting that StO2 may be used to estimate the adequacy of oxygen delivery in dogs. / Master of Science