title: Determination of the minimum alveolar concentration of isoflurane in Shetland ponies using constant current or constant voltage electrical stimulation.
abstract: OBJECTIVE
  To determine the minimum alveolar concentration (MAC) of isoflurane in Shetland ponies using a sequence of three different supramaximal noxious stimulations at each tested concentration of isoflurane rather than a single stimulation.
  
  
  METHODS
  Prospective, experimental trial.
  
  
  METHODS
  Seven 4-year-old, gelding Shetland ponies.
  
  
  METHODS
  The MAC of isoflurane was determined for each pony. Three different modes of electrical stimulation were applied consecutively (2 minute intervals): two using constant voltage (90 V) on the gingiva via needle- (CVneedle) or surface-electrodes (CVsurface) and one using constant current (CC; 40 mA) via surface electrodes applied to the skin over the digital nerve. The ability to clearly interpret the responses as positive, the latency of the evoked responses and the inter-electrode resistance were recorded for each stimulus.
  
  
  RESULTS
  Individual isoflurane MAC (%) values ranged from 0.60 to 1.17 with a mean (+/-SD) of 0.97 (+/-0.17). The responses were more clearly interpreted with CC, but did not reach statistical significance. The CVsurface mode produced responses with a longer delay. The CVneedle mode was accompanied by variable inter-electrode resistances resulting in uncontrolled stimulus intensity. At 0.9 MAC, the third stimulation induced more positive responses than the first stimulation, independent of the mode of stimulation used.
  
  
  CONCLUSIONS
  The MAC of isoflurane in the Shetland ponies was lower than expected with considerable variability among individuals. Constant current surface electrode stimulations were the most repeatable. A summation over the sequence of three supramaximal stimulations was observed around 0.9 MAC.
  
  
  CONCLUSIONS
  The possibility that Shetland ponies require less isoflurane than horses needs further investigation. Constant current surface-electrode stimulations were the most repeatable. Repetitive supramaximal stimuli may have evoked movements at isoflurane concentrations that provide immobility when single supramaximal stimulation was applied.
authors.affiliation: Department of Clinical Veterinary Sciences, Vetsuisse Faculty, University of Berne, Berne, Switzerland. olivier.levionnois@kkh.unibe.ch
authors.forename: Olivier L
authors.initials: OL
authors.lastname: Levionnois
authors.forename: Claudia
authors.initials: C
authors.lastname: Spadavecchia
authors.forename: Peter W
authors.initials: PW
authors.lastname: Kronen
authors.forename: Urs
authors.initials: U
authors.lastname: Schatzmann
mesh_terms: D000769:Anesthesia, Inhalation / Q000662:veterinary; D018685:Anesthetics, Inhalation / Q000008:administration & dosage* / Q000494:pharmacology; D000818:Animals; D004305:Dose-Response Relationship, Drug; D004558:Electric Stimulation; D006736:Horses / Q000502:physiology*; D007530:Isoflurane / Q000008:administration & dosage* / Q000494:pharmacology; D008297:Male; D017288:Pain Threshold / Q000187:drug effects*
chemical_list: D018685:Anesthetics, Inhalation; D007530:Isoflurane