Concentration-dependent isoflurane effects on withdrawal reflexes in pigs and the role of the stimulation paradigm.
In this prospective two-phase experimental trial, 10 pigs were anaesthetized twice with isoflurane only. In the first phase, the individual minimum alveolar concentration (MAC) was determined and in the second phase the effects on withdrawal reflexes of increasing end-tidal isoflurane concentrations (from 1.6% to 2.8%) were assessed. Single, 10 and 60 repeated electrical stimulations were used to evoke withdrawal reflexes which were recorded and quantified by electromyography. Recruitment curves for reflex amplitude for increasing stimulation intensities and isoflurane concentrations were constructed. Isoflurane MAC was 1.9 ± 0.3%. Reflexes evoked by repeated stimulation were suppressed at isoflurane concentrations significantly higher than those which suppressed complex movements during MAC determination (P=0.014 and P=0.006 for 10 and 60 repeated stimuli respectively). Isoflurane up to 2.8% was still not able to abolish reflex activity evoked by repeated stimulations in all pigs. Single stimulation reflexes were suppressed at significantly lower concentrations than repeated stimulation reflexes (P=0.008 and P=0.004 for 10 and 60 repeated stimuli, respectively). Reflex amplitude was significantly correlated with isoflurane concentration (P<0.001, r=-0.85) independent of the individual MAC. The findings indicate that the level at which isoflurane suppresses withdrawal reflexes is dependent on the stimulation paradigm (single vs. repeated electrical stimulation), and there is limited value in expressing reflex withdrawal suppression in terms of MAC as purposeful and reflex movements are independently affected by isoflurane in individual animals.
Anaesthesiology Section, Department of Clinical Veterinary Sciences, VetSuisse Faculty, University of Berne, Switzerland. claudia.spadavecchia@vetsuisse.unibe.ch
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Haga
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Ranheim
D018685:Anesthetics, Inhalation / Q000008:administration & dosage*; D000818:Animals; D004305:Dose-Response Relationship, Drug; D004558:Electric Stimulation; D004576:Electromyography / Q000662:veterinary; D005260:Female; D005552:Forelimb / Q000502:physiology*; D007530:Isoflurane / Q000008:administration & dosage*; D008297:Male; D009043:Motor Activity / Q000187:drug effects*; D011446:Prospective Studies; D012018:Reflex / Q000187:drug effects*; D013552:Swine / Q000502:physiology*; D004561:Transcutaneous Electric Nerve Stimulation / Q000662:veterinary
D018685:Anesthetics, Inhalation; D007530:Isoflurane