The effects of isoflurane minimum alveolar concentration on withdrawal reflex activity evoked by repeated transcutaneous electrical stimulation in ponies.

This research examined how different doses of isoflurane, an anesthetic, affect the reflexes of ponies during and after anaesthesia. The study applied repeated electrical stimulations to a nerve in the ponies’ legs and measured the reflex responses in different muscles.

Study Objective

• The primary objective was to determine the effect of isoflurane at about the minimum alveolar concentration (MAC), which is the smallest amount of anesthetic in the lungs necessary to prevent a reaction to surgical incision in 50% of patients, on the temporal summation (TS) of reflex activity in ponies.

Methodology

• To achieve this, the investigators induced TS by applying repeated electrical stimulations to the digital nerve of the left forelimb of seven ponies.
• This stimulation was done at a rate of 5Hz for 2 seconds.
• The researchers recorded the resulting electromyographic activity from the deltoid and common digital extensor muscles, two muscles involved in limb movement.
• They assessed the TS thresholds and the response amplitude to increasing intensities of stimulations during anaesthesia at 0.85, 0.95 and 1.05 times the individual MAC.
• This assessment was also done after anaesthesia while the animals were standing.

Results and Findings

• Under isoflurane anesthesia, it was noted that TS thresholds significantly increased in a concentration-dependent manner. This means, the higher the concentration of isoflurane, the higher the TS threshold.
• Also, for each MAC level of isoflurane, the responses significantly increased for increasing stimulation intensities.
• The study observed a concentration-dependent depression of evoked reflexes with a reduction in the slopes of the stimulus-response function for both muscles. That is, there’s a decrease in the reflex responses as the amount of isoflurane increases.

Significance of the Study

• This research provided a model that could be used to describe and quantify the effects of anesthetics on spinal sensory-motor processing in ponies.
• It helps scientists to understand how anesthesia affects the nervous system and motor responses of large animals, and could have applications in veterinary anesthesia practices in future.