Electrical Activity of the Brain in Tortoises During Brumation Monitored with Bispectral Index (BIS)
Bispectral index (BIS) (BIS A-2000XP, Aspect Medical Systems), a monitor validated in human patients, was used to document brumation in tortoises. The bispectral analysis is a statistical technique that evaluates interfrequency phase relations of the electroencephalogram (EEG) and combines them into a single index of electrical activity with a dimensionless variable from 0 (cortical silence) to 100 (maximal cortical activation). BIS measurements have been empirically obtained in anesthesized horses,4 pigs,8 goats,1 dogs,3 cats,6 rabbits,7 and dolphins.5
Ten adult Hermann’s tortoises (Testudo graeca) were, as part of their routine husbandry, allowed to brumate for 4 mo. BIS values and heart rate were recorded before and after the brumation period (>17°C at room temperature), during the 2 wk of adaptation period at 7°C, and every 2 wk (total of seven measurements per animal) during the hibernation period at 3–4°C in a high relative humidity hibernaculum. Measurements were taken continuously for 10 min per animal. The tortoises showed the same cerebral activation (BIS values between 90 and 100) before and after brumation and during the adaptation period. During brumation, the BIS score showed a pattern of deep suppression (20–30), interrupted at 0.5 to 2.0 min intervals by bursts of high cerebral cortex activation. This mechanism to maintain the brain alive during a hypometabolic state has been described during brumation in anoxia-tolerant freshwater turtles (Trachemys and Chrysemys).2,9
We conclude that during brumation, land tortoises display similar brain activation pattern as freshwater turtles. The results indicate possible use of the BIS Monitor as instrument to monitor specific brain activity of brumating tortoises.
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