Do common anaesthetic protocols influence cerebral oxygenation?

Abstract

Near-infrared-spectroscopy (NIRS; Jobsis 1977) has been used in humans to measure regional cerebral oxygen saturation (rsO2), decreases of which can be indicative of a critical cerebral O2 supply (Kirkpatrick et al. 1998). This study investigated changes in rsO2, the redox status of cytochrome a/a3, and the cerebral total hemoglobin content using NIRS during different sedative and anaesthetic techniques in dogs. Eighty four foxhounds and 32 beagles aged 12.9 ± 13.4 (6.3–94.4) (±SD (range)) months and weighing 24.8 ± 6.6 (2.7–43) kg were divided randomly into four groups. Group AM received 0.1 mg kg−1 acepromazine and 0.5 mg kg−1 l-methadone/fenpipramid (l-Polamivet) IV, group DM 0.5 mg kg−1 diazepam and 0.5 mg kg−1 l-Polamivet IV, and group MM 40 µg kg−1 medetomidine and 0.5 mg kg−1 l-Polamivet IV. Group P was anaesthetized (7 mg kg−1) and maintained (0.3 mg kg−1 minute−1) with propofol. Thirty minutes after sedation groups AM, DM and MM received 8 µg kg−1 naloxone IV, and at 35 minutes group MM received 200 µg kg−1 atipamezole IV. NRIS was monitored continuously (recording interval, 1 second). Blood gases were analysed before and every 5 minutes after sedation or induction, and at 2 and 7 minutes post-naloxone injection or end of propofol infusion. Descriptive and comparative statistics (global: Kruskal–Wallis test, level of significance α = 0.05, group comparison: Mann–Whitney test, α = 0.0083), and Spearman's correlation were determined using the statistical software SPSS. Before drug administration rsO2 was 65 ± 7% (n = 109). One minute after drug administration significant differences between groups were found (Table 1). There was a significant increase of about 8% in rsO2 over time in the p group, whereas groups AM, DM and MM showed a significant decrease with the greatest reduction of 20% in group MM. One minute after naloxone IV pre-induction values were rapidly attained in groups AM (65.2 ± 6%, n = 31) and DM (65.4 ± 5.4%, n = 24) while in group MM (52.1 ± 10.8%, n = 27) pre-induction values of rsO2 were not reached until 1 minute after atipamezole IV (62.9 ± 7.4%). Periodic breathing (MM) was mirrored by variations of rsO2. There were no significant changes or intergroup differences in the redox state of cytochrome a/a3 or of total cerebral hemoglobin content indicating cerebral blood volume (Wyatt et al. 1990). The 20% decrease in rsO2 in group MM is in the range regarded as indicative of a critical cerebral O2 supply in humans (Kirkpatrick et al. 1998). The effects of naloxone and atipamezole and the correlation with blood gas analysis demonstrate an influence of respiratory depression on the vascular cerebral O2 status whereas the lack of cytochrome a/a3 reduction indicates an undisturbed cellular O2 supply (Jobsis 1977). In spite of some limitations in terms of applicability (Alef 2002). NIRS seems to enable the influence of clinical routines on cerebral oxygenation to be monitored although further validation of NIRS in the dog and the cat is required.