Pituitary-adrenocortical effects of methoxamine infusion on halothane anaesthetised ponies

This study investigated whether maintenance of normotension using a methoxamine infusion would prevent activation of the stress response during halothane anaesthesia in ponies. After acepromazine premedication, anaesthesia was induced with thiopentone in six ponies, and maintained for 120 minutes with halothane in oxygen and ventilation to normocapnia. Methoxamine (M, 0.013+/-0.005 mg kg(-1) min(-1)) or saline (C) was infused throughout anaesthesia. Sequential blood samples were taken for cortisol and ACTH assay. During anaesthesia normotension (mean arterial blood pressure 101-124 mm Hg) was maintained in M, whilst hypotension developed in C (nadir 71+/-12 mm Hg). Cardiac output decreased in both groups but to a greater extent in M. Systemic vascular resistance increased more in M (max 2015+/-650 dyne.s cm(-5)) than C (max 939+/-285 dyne.s cm(-5)). Plasma cortisol increased in both groups but more slowly in M. Plasma ACTH did not change in either group. Maintenance of normotension via increased systemic vascular resistance did not prevent, but may have attentuated, the adrenocortical response to anaesthesia. Reduced cardiac output and presumed compromise to tissue perfusion appear as important a stimulus as hypotension. The equine stress response to halothane anaesthesia is likely to be multifactorial.