PITUITARY-ADRENOCORTICAL EFFECTS OF METHOXAMINE INFUSION ON HALOTHANE-ANESTHETIZED PONIES

This study investigated whether maintenance of normotension using a me thoxamine infusion would prevent activation of the stress response dur ing halothane anaesthesia in ponies. After acepromazine premedication, anaesthesia was induced with thiopentone in six ponies, and maintaine d for 120 minutes with halothane in oxygen and ventilation to normocap nia. Methoxamine (M, 0.013 +/- 0.005 mg kg(-1) min(-1)) or saline (C) was infused throughout anaesthesia. Sequential blood samples were take n for cortisol and ACTH assay. During anaesthesia normotension (mean a rterial blood pressure 101-124 mm Hg) was maintained in M, whilst hypo tension developed in C (nadir 71 +/- 12 mm Hg). Cardiac output decreas ed in both groups but to a greater extent in M. Systemic vascular resi stance increased more in M (max 2015 +/- 650 dyne.s cm(-5)) than C (ma x 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. Mai ntenance of normotension via increased systemic vascular resistance di d not prevent, but may have attentuated, the adrenocortical response t o anaesthesia. Reduced cardiac output and presumed compromise to tissu e perfusion appear as important a stimulus as hypotension. The equine stress response to halothane anaesthesia is likely to be multifactoria l.