Endocrine and metabolic responses to plasma volume expansion during halothane anesthesia in ponies

The study was designed to contribute to identification of the stimulus to adrenocortical activity during halothane anaesthesia in equidae. Two groups of six ponies were premedicated with acepromazine before induction of anaesthesia with thiopentone and maintenance for 120 min with halothane in oxygen. In group H Haemaccel modified gelatine plasma replacer was infused (48+/-13 mL/kg) to maintain mean arterial blood pressure (MABP) close to preanaesthetic values. In group DH, blood pressure was maintained close to preanaesthetic levels with a lower dose of Haemaccel (10 mL/kg) combined with an infusion of dobutamine. Measurements were made before anaesthesia, at 20 min intervals during anaesthesia and 20 and 120 min after anaesthesia. MABP and blood gases, pulse and respiratory rates were measured, and blood was withdrawn for assay of cortisol, adrenocorticotrophic hormone (ACTH), glucose and lactate. Ponies in both groups became hyperoxic, hypercapnic and developed a respiratory acidosis; pulse rate increased in both groups but this was more marked in group H. Haematocrit decreased by 50% in H and by 20% in DH. Cortisol and ACTH did not change significantly during anaesthesia in either group and the area under the time curve (AUC(0-140)) was lower in the DH group. Plasma glucose and lactate remained stable. After the H treatment all ponies had a watery nasal discharge and one pony died from endotoxaemia. This investigation demonstrated that the adrenocortical response to halothane anaesthesia in ponies can be ameliorated by manipulation of ABP using plasma expansion with or without inotrope infusion; however, low dose Haemaccel with dobutamine was safer and more practical. It is suggested that, although hypotension is not the sole stimulus to adrenocortical activity during halothane anaesthesia, it may contribute, probably through an effect on tissue perfusion.