Cardiorespiratory and endocrine effects of endogenous opioid antagonism by naloxone in ponies anaesthetised with halothane.

The researchers investigated the impact of using opioid antagonist naloxone on the cardiorespiratory function and pituitary-adrenal activity of halothane-anaesthetised ponies, due to the concern of halothane enhancing the cardiorespiratory depression and risk of death in horses.

Research Design and Experimental Setup

• The study employed a cross-over design with six ponies anaesthetised on three separate occasions.
• Anaesthesia was initially induced using thiopentone and maintained with 1.2% halothane for a period of 2 hours.
• Immediately after induction, naloxone was either injected intravenously (at a dosage of 0.5 mg/kg as a bolus, followed by 0.25 mg/kg per hour for the next 2 hours), injected into the spinal canal, or replaced with saline as a control.

Measurements and Observations

• Pulse rate, respiratory rate, arterial blood gases, cardiac output and concentrations of plasma cortisol and adrenocorticotrophic hormone (ACTH) were measured.
• Observable in all test groups was the development of depression in the cardiorespiratory system, demonstrated through a 40% decrease in cardiac output.
• All test groups also demonstrated an increase in plasma cortisol levels. Plasma ACTH concentration, however, was notably higher in ponies treated with naloxone injections into the spinal canal.

Conclusions and Implications

• The research suggests that endogenous opioids could be inhibiting the secretion of ACTH, which in turn could be moderating the stress response to halothane anaesthesia in horses.
• This has implications for enhancing the safety of anaesthetised horses by managing and reducing the risk of cardiorespiratory depression and associated mortalities.