Authors: Kavitha Kongara, Lorna Johnson, Nikki J Kells, Craig B Johnson, Venkata SR Dukkipati and Sheryl L Mitchinson
The aim of this study was to investigate the effect of opioids on electroencephalogram (EEG) indices of nociception in cats undergoing castration. Cats were randomly assigned to receive one of the four treatments (n=8); 0.2 mg/kg morphine, 0.005 mg/ kg fentanyl, 0.01 mg/kg buprenorphine or 0.2 mg/kg butorphanol, administered subcutaneously (SC) at the time of pre-anesthetic medication. Anesthesia was induced with intravenous propofol and maintained with halothane in oxygen. EEG was recorded continuously in a three electrode montage. Median frequency (F50), total power (PTOT) and 95% spectral edge frequency (F95) derived from the EEG power spectra recorded prior to skin incision (baseline) were compared with those recorded during the ligation of the spermatic cords of both testicles. During the ligation of testicle 1, the mean F50 of cats that received buprenorphine and butorphanol was significantly (p<0.05) higher, compared with baseline values. During the ligation of testicle 2, the cats in the butorphanol and fentanyl groups showed significantly (p<0.05) higher F50 values, compared with that of the morphine group as well as with their respective values during the ligation of testicle 1. Ptot values decreased significantly (p<0.05) in all the treatment groups (excluding morphine after the removal of testicle 2), compared with baseline values. Morphine treated cats had significantly (p<0.05) higher Ptot values than cats in the buprenorphine and fentanyl groups during the removal of both testicles. The F95 of the EEG did not differ between the two groups during the ligation of either testicle (p>0.05). These results indicate that opioid analgesics, acting at different opioid receptors with variable affinity, produce changes in the EEG responses that reflect their anti-nociceptive efficacy. This study demonstrates the usefulness of the EEG as a valid tool for evaluating analgesic efficacy in cats, as shown in other species of animals in previous studies.
Keywords: Electroencephalogram, EEG, cat, castration, analgesia, nociception, opiods, morphine, fentanyl, butorphanol, buprenorphine