Effects of two different fluid rates on tissue oxygenation and haemodynamic stability in anaesthetised horses

Background Perianaesthetic fluid therapy can influence oxygen delivery and tissue oxygenation, yet the optimal fluid rate in horses remains undefined. Objectives To compare two fluid strategies on peripheral tissue oxygenation (StO 2 ) and cardiorespiratory variables in isoflurane‐anaesthetised horses. Study Design Randomised, controlled, experimental design with two independent groups. Methods Sixteen euvolemic adult horses undergoing coeliotomy were anaesthetised with xylazine, ketamine, and midazolam, maintained on isoflurane with lidocaine infusion, and randomised to lactated Ringer's solution at 2.5 mL/kg/h (maintenance; MNT) or 40 mL/kg/h (high fluids; HF) for 120 min. StO 2 was measured at the extensor carpi radialis muscle using near‐infrared spectroscopy. Heart rate, mean arterial pressure (MAP), central venous pressure (CVP), cardiac output (CO), serum lactate, and urine output were recorded at baseline, 60, and 120 min and analysed using mixed‐effects linear regression and Bland–Altman repeated‐measures correlation. Results StO 2 increased over time in both groups ( p ≤ 0.018) and weakly correlated with MAP ( ρ = 0.36, p = 0.0498) and lactate ( ρ = 0.36, p = 0.05), but not with ScvO 2 or PvO 2 ( p > 0.88). HF had significantly higher median MAP (80 vs. 76 and 79 vs. 72 mmHg), CVP (7.5 vs. 2.7 and 17.0 vs. 4.0 mmHg), serum lactate concentrations at 60 and 120 min, respectively (3.0 vs. 1.4; 2.8 vs. 1.3 mmol/L) and total urine output (0.72 vs. 0.37; mL/kg/h) than MNT (all p < 0.001). CO increased over time in MNT but not HF. No differences were detected in StO 2 , CO, ScvO 2 , or PvO 2 between groups. Main Limitations Small sample size and short observation window. Conclusions High‐rate fluid therapy increased MAP, CVP, and urine output not StO 2 or CO, and was associated with higher serum lactate. Tissue oxygenation improved over time regardless of fluid rate, suggesting limited benefit of aggressive crystalloid administration in anaesthetised euvolemic horses.