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

Perianaesthetic fluid therapy can influence oxygen delivery and tissue oxygenation, yet the optimal fluid rate in horses remains undefined.

To compare two fluid strategies on peripheral tissue oxygenation (StO₂) and cardiorespiratory variables in isoflurane-anaesthetised horses.

Randomised, controlled, experimental design with two independent groups.

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. StO₂ 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.

StO₂ 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 ScvO₂ or PvO₂ (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 StO₂, CO, ScvO₂, or PvO₂ between groups.

Small sample size and short observation window.

High-rate fluid therapy increased MAP, CVP, and urine output not StO₂ 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.