Reversal of acepromazine-induced hemodynamic alterations by norepinephrine in standing horses

Acepromazine (ACP) is a phenothiazine commonly used to sedate horses. Additionally, ACP exerts strong anti-inflammatory effects, which might have a therapeutic potential in horses suffering from systemic inflammatory response syndrome (SIRS). However, the ACP-induced vasodilation precludes its use in horses with SIRS-related cardiovascular compromise.

The objective of this study was to test if the hemodynamic effects of the administration of 0,1 mg/kg of ACP could be counteracted by an intravenous infusion of norepinephrine (NOR) at 1ug/kg/min in healthy horses.

In 5 healthy adult horses, a 15 minutes NOR IV infusion was administered 45 minutes after an injection of 0,1 mg/kg of ACP IV. The systolic arterial blood pressure (SAP) was non-invasively measured by Doppler sphingometry at the tail. Hemodynamics of the median artery of the left forelimb were studied using Doppler ultrasonography, through calculation of the vessel’s surface (SURF), diameter (DIAM), circumference (CIRC), and peak systolic velocity (PSV), end diastolic velocity (EDV), mean velocity (MV), volumetric flow (VF) and resistivity index (RI) of the flow. Both SAP and Doppler parameters were determined at regular intervals during the entire study.

ACP induced a hypotension and a vasodilatation, that were evidenced by a significant rise of the SURF, DIAM, CIRC, PSV, EDV, MV and VF and reduction of the SAP and RI. During NOR infusion, all these ACP-induced hemodynamic changes were reversed.

These findings suggest that a continuous IV NOR infusion at 1ug/kg/min is able to revert ACP-induced hypotension and vasodilation in healthy adult horses.