Effects of plasma expansion on the pituitary-adrenocortical response to halothane anaesthesia in sheep

The study aimed to investigate the stimulus to adrenocortical activity that is induced by halothane anaesthesia. Groups of 7 sheep were anaesthetised with thiopentone and halothane (TH) or acepromazine, thiopentone and halothane (ATH). During 120 min of anaesthesia hypotension was prevented (mean arterial blood pressure kept at pre-anaesthetic level) by infusion of a modified gelatine plasma replacer given to effect (0.34–1.1 litres with TH and 1.1–3.1 litres with ATH). Pulse rate, arterial blood pressure and gases were measured and sequential samples withdrawn for analysis of plasma cortisol, adrenocorticotrophic hormone (ACTH), arginine vasopressin (AVP), glucose and lactate. Heart rate increased in the ATH but not the TH group. All sheep were well oxygenated but developed hypercapnia and respiratory acidosis. In both groups, cortisol increased more than 2-fold 20 min after the end of anaesthesia but there were no significant changes in ACTH. AVP was measured in the TH group only and increased 3-fold at the end of anaesthesia. Glucose and lactate remained stable except for lactate in the TH group which decreased during anaesthesia. These data indicate that hypotension is a major component of the stimulus inducing adrenocortical activity during halothane anaesthesia. However, maintenance of normotension did not entirely depress the response; halothane itself or decreased perfusion may also contribute.