The effect of dobutamine and bolus crystalloid fluids on the cardiovascular function of isoflurane‐anaesthetised horses

REASONS FOR PERFORMING STUDY: Cardiac output does not always increase with dobutamine administration in anaesthetised horses and information on peripheral perfusion is lacking. OBJECTIVES: To determine the effect of intravenous (i.v.) dobutamine infusion with and without a concurrent 20 mL/kg bodyweight (bwt) bolus of crystalloid fluids on the cardiovascular function of acepromazine premedicated, hypotensive, isoflurane‐anaesthetised horses. STUDY DESIGN: Randomised, cross‐over experiment. METHODS: A total of 6 horses aged 5–13 years, weighing 464–578 kg were premedicated with acepromazine 0.02 mg/kg bwt and then sedated with xylazine 0.8 mg/kg bwt i.v. Anaesthesia was induced with ketamine 2.2 mg/kg bwt and diazepam 0.08 mg/kg bwt i.v. and maintained with isoflurane, adjusted to achieve a target mean arterial pressure (MAP) (60 mmHg ± 5%) 60 min post‐induction of anaesthesia (T0). One of 2 treatments was then given. In treatment D, dobutamine was initially infused at 0.5 μg/kg bwt/min and adjusted to achieve a target MAP (80 mmHg ± 5%) within 30 min of infusion initiation. In treatment D+F dobutamine was administered as described for treatment D, with 20 mL/kg bwt Hartmann's solution infused i.v. over 20 min. Cardiac index (CI), haemoglobin concentration ([Hb]), arterial oxygen content (CaO₂), oxygen delivery index (DO₂I) and bilateral femoral arterial blood flow (FBF) were recorded at T0, 30 min following dobutamine initiation (T1) and 15 min following dobutamine cessation (T2). Data were analysed using a mixed‐effect linear model (P<0.05 considered significant). RESULTS: A significant increase in DO₂I (P = 0.008, T0/T1), CaO₂ (P = 0.0002, T0/T1) and [Hb] (P<0.0001, T0/T1) and in CaO₂ (P = 0.0005, T1/T2) and [Hb] (P = 0.002,T1/T2) occurred during treatment D. A significant increase in FBF (P = 0.005, upper limb; P = 0.042 lower limb, T0/T1) occurred during treatment D+F. Significant differences between treatments were recorded at T1 ([Hb] P = 0.0001, CaO₂ P = 0.0003) and T2 ([Hb] P = 0.013). There was no change in CI during either treatment. CONCLUSIONS: The increase in FBF seen with co‐administration of fluids and dobutamine may provide a beneficial effect on muscle compared with the use of dobutamine alone.