Evaluation of administration of isoflurane at approximately the minimum alveolar concentration on depression of a nociceptive withdrawal reflex evoked by transcutaneous electrical stimulation in ponies.

The research article discusses a study aimed at finding the effects of the anesthetic, isoflurane, at minimum alveolar concentration (MAC), on the nociceptive withdrawal reflex (NWR) in ponies, and its potential use for quantifying anesthetic potency.

Introduction and Methods

• The researchers used seven healthy adult Shetland ponies as their subjects. The first phase of the study focused on determining the individual MAC (iMAC) for isoflurane for each pony.
• Once the iMAC was confirmed, the effects of isoflurane at different levels including 0.85, 0.95, and 1.05 iMAC, on the NWR were then assessed. The NWR threshold was defined electromyographically.
• Testing involved stimulating the digital nerve for the common digital extensor and deltoid muscles at various electrical stimulations (3, 5, 10, 20, 30, and 40 mA). The resulting activity was recorded and analyzed.
• The NWR threshold was then examined in the ponies once they were awake and no longer under anesthesia.

Results

• The results indicated a mean MAC of isoflurane to be 1.0 +/- 0.2%. Subsequently, it was observed that NWR thresholds for both the deltoid and the common digital extensor muscles increased significantly, in a concentration-dependent manner, during anesthesia. However, they decreased when the ponies were awake.
• Significantly higher thresholds were recorded for the deltoid muscle when compared to the common digital extensor muscle in anesthetized ponies.
• It was noticed that as the stimulus intensities were increased from 3 to 40 mA, the amplitudes of the reflex responses from both muscles also increased in a systematic manner.
• A concentration-dependent depression of evoked reflexes was detected as the slopes of the stimulus-response functions reduced.

Conclusion and Implications

• The research concluded that changes in sensory-motor processing induced by the anesthesia in ponies can be detected via an assessment of NWR while using isoflurane at an approximate concentration of 1.0 MAC.
• Such methods would enable researchers and veterinary practitioners to compare the effects of inhaled anesthetics or combinations on spinal processing in equids, thus, presenting a method to quantify anesthetic potency.