Differences in Quantitated Electroencephalographic Variables During Surgical Stimulation of Horses Anesthetized With Isoflurane

The effects of noxious surgical stimulation on the electroencephalogram (EEG) in 15 horses anesthetized with isoflurane were evaluated during orthopedic (group 1) and soft tissue (group 2) procedures. The quantitative EEG variables theta/delta ratio (T/D), alpha/delta ratio (A/D), beta/delta ratio (B/D), median power frequency (MED), and 80% spectral edge frequency (SEF 80) recorded during surgeries at 1.7% end-tidal concentration of isoflurane (ETiso) were compared with values from five nonstimulated control horses anesthetized at 1.7% ETiso. The EEG variables T/D, A/D, MED, and SEF 80 from surgically stimulated horses were significantly higher compared with controls. These differences in measured EEG variables were accompanied by a significantly lower relative power in the delta frequency band and a concomitant significantly higher alpha activity. Because the A/D ratio, MED, and SEF 80 in surgically stimulated horses were significantly higher than in nonstimulated control horses these measured EEG variables may provide a valuable tool for identification of nociceptive transmission in isoflurane anesthetized horses.