Trendelenburg positioning with or without norepinephrine infusion for haemodynamic support in anaesthetised hypovolemic horses

Abstract Background Intraoperative haemorrhage during equine anaesthesia is a rare but potentially fatal complication requiring prompt haemodynamic support. Postural manoeuvres such as Trendelenburg (TL) positioning and vasopressor support augment venous return and improve cardiac output (CO), yet their efficacy in anaesthetised hypovolemic horses remains unstudied. Objectives To evaluate haemodynamic effects of TL positioning during normovolemia and acute haemorrhagic hypovolemia, and to determine whether concurrent norepinephrine (NE) infusion enhances cardiovascular stabilisation. Study Design In vivo experiments. Methods Six dorsally recumbent isoflurane‐anaesthetised horses were studied under normovolemia, acute haemorrhage‐induced hypovolemia, and hypovolemia with NE infusion. Haemodynamic variables, including CO, mean arterial pressure (MAP), central venous pressure (CVP), systemic vascular resistance (SVR), mean pulmonary arterial pressure (mPAP), and heart rate (HR), were recorded at multiple timepoints before, during, and after TL positioning (30° incline). Norepinephrine was administered at 0.1 μg/kg/min to restore MAP above 80 mmHg. Results Trendelenburg positioning during normovolemia resulted in modest, transient increases in CO (~28%), MAP (~22%), CVP, and mPAP ( p < 0.05), with no change in HR and SVR. Haemorrhagic hypovolemia significantly reduced all haemodynamic indices ( p < 0.01) except HR and SVR. TL positioning partially reversed these effects (increase in CO ~40% and MAP ~67%; p < 0.05). Norepinephrine infusion alone during hypovolemia significantly increased CO, MAP, and SVR, with additional improvement when combined with TL positioning (increase in CO ~75%; p < 0.05), suggesting enhanced effects. Main Limitations Small sample size and use of healthy experimental animals limit translational generalisability of the findings to clinical populations. Conclusions TL positioning transiently improves haemodynamics in anaesthetised horses, with more pronounced benefits during hypovolemia. The addition of NE infusion enhances this effect, supporting their combination as a potential physiologic rescue strategy in hypovolemic equine patients.