Objective: To evaluate electroencephalographic (EEG) changes during ovariohysterectomy (OVH) in rats anaesthetized with halothane, and modification of the EEG changes by the co-administration of fentanyl, ketamine or thiopental.
Study design: Prospective, randomized, blinded controlled study.
Animals: Sixty adult female Sprague-Dawley rats.
Methods: Anaesthesia was induced and maintained with halothane [Fe'HAL 0.95 +/- 0.05%]. The electroencephalogram was recorded continuously from the left and right primary somatosensory cortices. Rats were randomly divided into four groups and a rapid IV infusion of physiological saline, thiopental, ketamine or fentanyl was administered. OVH started 10 minutes after drug administration. Blood samples, for assay of plasma drug concentrations, were collected 5 minutes after administration and at the end of surgery. Electroencephalograph descriptors median frequency (F50), spectral edge frequency 95% (F95) and total power (P(tot)), recorded during non-surgical baseline periods, were compared with those recorded during defined surgical periods; skin incision, right and left ovarian pedicle ligation; cervical ligation. Plasma drug concentrations were measured using high performance lipid chromatography.
Results: Although a large number of statistical differences in EEG data were observed, these generally represented a reduction in F50 or F95 throughout the experimental recording period and were similar between groups. A significant reduction in P(tot) occurred during ligation of the ovaries and cervix in the control group compared with other recording periods. The co-administration of ketamine, fentanyl and thiopental obtunded the reduction in P(tot).
Conclusions and clinical relevance: Electroencephalographic changes, in the control group, mimicked changes reported in other studies using the minimal anaesthesia model. However, the stability in F50 during the surgical period compared with the baseline period indicated that OVH is an unsuitable surgical stimulus to investigate EEG changes with noxious stimulation. This may be attributed to the relatively prolonged duration of this surgical procedure and the primarily visceral afferent sensory innervation of the genital tract.