Pituitary-adrenocortical effects of methoxamine infusion on halothane anaesthetised ponies.

This research examines how maintaining normal blood pressure using methoxamine infusion could impact the stress response in ponies anesthetized with halothane. The findings suggest that maintaining normal tension might attenuate the stress response induced by the anaesthesia, but does not prevent it entirely.

Research Context

• The study was conducted to determine the effect of methoxamine, a medicine used to treat hypotension (low blood pressure) during anaesthesia, in horses (ponies).
• The hypothesis was that maintaining normal blood pressure could potentially mitigate the stress response typically observed during anaesthesia with halothane, a general anaesthetic.

Research Methodology

• Six ponies were premedicated with acepromazine and anaesthetized with thiopentone.
• Horses were then kept under anaesthesia for 120 minutes with halothane.
• Maintaining normocapnia (a normal concentration of carbon dioxide in the blood) was ensured by regulating their ventilation.
• The infusion of either methoxamine or saline was administered throughout the anaesthesia.
• Blood samples were periodically taken to measure cortisol and ACTH, hormones related to stress response.

Research Findings

• A normal mean arterial blood pressure was maintained in ponies that received methoxamine, whereas those that received saline developed hypotension.
• Cardiac output, the volume of blood the heart pumps per minute, decreased in both groups, but more so among the methoxamine group.
• Ponies on methoxamine showed a more considerable rise in systemic vascular resistance, essentially the ‘friction’ that blood encounters while flowing through blood vessels.
• Plasma cortisol levels increased in both groups, albeit more slowly in the methoxamine-infused ponies. There was no change in plasma ACTH in either group.
• A notable observation was that the maintenance of normal blood pressure, despite increasing vascular resistance, could attenuate but not fully prevent the adrenocortical response to anaesthesia.

Conclusion

• The researchers concluded that factors such as diminished cardiac output and potential compromise to tissue perfusion may provide as significant a trigger to the stress response as hypotension during anaesthesia does.
• The stress response observed in horses to halothane anaesthesia appears to be multifactorial and not solely reliant on the maintenance of normal blood pressure.