OBJECTIVE: To evaluate if variation in the end-tidal CO DESIGN: Diagnostic accuracy study. SETTING: University hospital. ANIMALS: Twenty-seven dogs admitted for neutering. INTERVENTIONS: To obtain a balanced sample between fluid responder and nonresponder dogs, a 10-mL/kg lactated Ringer's solution was administered over 15 minutes to half of the population before the baseline measurements. All animals then received a fluid challenge of 10 mL/kg lactated Ringer's solution in 5 minutes. MEASUREMENTS AND MAIN RESULTS: The velocity-time integral of aortic blood flow (VTI CONCLUSIONS: The current study showed that, although minimal, ∆Petco