Determination of the minimum alveolar concentration of isoflurane in Shetland ponies using constant current or constant voltage electrical stimulation.

This research investigated the minimum amount of isoflurane—an anesthetic—needed in Shetland ponies, using different methods of electrical stimulation. It indicated that ponies may need less isoflurane than larger horses, and found that repetitive stimulation could cause movement even at doses that would keep the animal immobile under single stimulation.

Research Method

• The experiment involved seven 4-year-old Shetland ponies.
• To determine the minimum alveolar concentration (MAC) of isoflurane—anesthetics gas—for each pony, the researchers used three different modes of electrical stimulation.
• The methods of stimulation included two that employed constant voltage (at 90 V) on the gingiva through either needle- (CVneedle) or surface-electrodes (CVsurface), and one that used a constant current (CC; 40 mA) with surface electrodes on the skin above the digital nerve.
• The stimulations were applied consecutively at 2-minute intervals, and the reaction, response time, and resistance between electrodes was recorded.

Results

• The MAC values of isoflurane for the Shetland ponies were between 0.60% and 1.17%, with an average of 0.97% (+/-0.17).
• Responses were most clear when using constant current (CC), but the difference was not statistically significant.
• Surface-electrode stimulation (CVsurface) took a longer time to produce a response.
• The resistance between the electrodes in needle-stimulation (CVneedle) varied, which meant the intensity of the stimulus couldn’t be controlled effectively.
• At an isoflurane concentration of 0.9 MAC, the third stimulation produced more movement than the first one, regardless of the stimulation method used.

Conclusions

• The MAC of isoflurane in the ponies tested was lower than the researchers suspected, with a significant variation between individuals.
• Of the forms of stimulation, using a constant current and surface electrodes was found to be the most repeatable.
• A succession of three supramaximal stimulations seemed to generate a cumulative effect around the 0.9 MAC mark.
• The finding that Shetland ponies potentially require less isoflurane than regular horses introduces the need for further investigation.
• The data suggest that repeated supramaximal stimuli can cause movements at isoflurane concentrations that would normally render the subject immobile when a single supramaximal stimulation is applied.