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.
OBJECTIVE
To investigate effects of isoflurane at approximately the minimum alveolar concentration (MAC) on the nociceptive withdrawal reflex (NWR) of the forelimb of ponies as a method for quantifying anesthetic potency.


METHODS
7 healthy adult Shetland ponies.


METHODS
Individual MAC (iMAC) for isoflurane was determined for each pony. Then, effects of isoflurane administered at 0.85, 0.95, and 1.05 iMAC on the NWR were assessed. At each concentration, the NWR threshold was defined electromyographically for the common digital extensor and deltoid muscles by stimulating the digital nerve; additional electrical stimulations (3, 5, 10, 20, 30, and 40 mA) were delivered, and the evoked activity was recorded and analyzed. After the end of anesthesia, the NWR threshold was assessed in standing ponies.


RESULTS
Mean +/- SD MAC of isoflurane was 1.0 +/- 0.2%. The NWR thresholds for both muscles increased significantly in a concentration-dependent manner during anesthesia, whereas they decreased in awake ponies. Significantly higher thresholds were found for the deltoid muscle, compared with thresholds for the common digital extensor muscle, in anesthetized ponies. At each iMAC tested, amplitudes of the reflex responses from both muscles increased as stimulus intensities increased from 3 to 40 mA. A concentration-dependent depression of evoked reflexes with reduction in slopes of the stimulus-response functions was detected.


CONCLUSIONS
Anesthetic-induced changes in sensory-motor processing in ponies anesthetized with isoflurane at concentrations of approximately 1.0 MAC can be detected by assessment of NWR. This method will permit comparison of effects of inhaled anesthetics or anesthetic combinations on spinal processing in equids.
Spadavecchia
Claudia
C
Anesthesiology Section, Department of Clinical Veterinary Sciences, Vetsuisse Faculty, University of Berne, Langassstrasse 124, 3012 Berne, Switzerland.
Levionnois
Olivier
O
Kronen
Peter W
PW
Leandri
Massimo
M
Spadavecchia
Luciano
L
Schatzmann
Urs
U
D000769:Anesthesia, Inhalation / Q000662:veterinary; D018685:Anesthetics, Inhalation / Q000008:administration & dosage* / Q000494:pharmacology* / Q000627:therapeutic use; D000818:Animals; D004558:Electric Stimulation; D006736:Horses; D007530:Isoflurane / Q000008:administration & dosage* / Q000494:pharmacology* / Q000627:therapeutic use; D009619:Nociceptors / Q000187:drug effects / Q000502:physiology; D010146:Pain / Q000188:drug therapy*; D017288:Pain Threshold / Q000187:drug effects / Q000502:physiology; D012018:Reflex / Q000187:drug effects*
D018685:Anesthetics, Inhalation; D007530:Isoflurane