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

The research article focuses on understanding the stimulation of adrenocortical activity during halothane anaesthesia in ponies. Two groups of ponies were anaesthetized and had their blood pressure maintained through different methods, and the reactions of their bodies regarding key physiological measures were compared to determine safer and more effective practices.

Overview of the Experiment

• Two groups of six ponies were premedicated with acepromazine and subsequently anaesthetized with thiopentone and treated with halothane in oxygen for 120 minutes.
• One group, referred to as the H group, had their mean arterial blood pressure (MABP) sustained near preanaesthetic levels using the plasma replacer Haemaccel. In the second group (DH), a lower dose of Haemaccel was combined with an infusion of the drug dobutamine to maintain blood pressure.
• Measurements of various physiological indicators were conducted before, during, and after anaesthesia, including MABP, blood gases, pulse and respiratory rates, and cortisol, ACTH, glucose, and lactate levels in the blood.

Results and Analysis

• Ponies in both groups developed conditions of high blood oxygen, high blood carbon dioxide (which can lead to respiratory acidosis), and increased pulse rate. The H group showed a more significant increase in pulse rate.
• The amount of red blood cells (haematocrit) reduced by 50% in the H group and by 20% in the DH group.
• There were no significant changes in cortisol and ACTH levels during anaesthesia in either group, with the DH group showing a lower area under the time curve (AUC(0-140)), which implies a lower overall hormone response.
• Plasma glucose and lactate levels remained stable throughout the experiment.
• Post anaesthesia, all ponies in group H experienced watery nasal discharge, and one pony passed away due to endotoxaemia, a condition that implicates bacterial toxins in the bloodstream.

Conclusions

• The research demonstrated that adrenocortical response to halothane anaesthesia in ponies could be reduced by manipulating arterial blood pressure, either through plasma expansion or inotrope infusion.
• The combination of low-dose Haemaccel with dobutamine (used in group DH) was found to be safer and more practical due to lower indications of distress in ponies and fewer severe adverse effects post anaesthesia.
• The study hypothesizes that although hypotension may not be the sole stimulus to adrenocortical activity during halothane anaesthesia, it can contribute, most likely through effects on tissue perfusion.