The effects of several inspired oxygen fractions (FiO2) on the bispectral index in spontaneously breathing dogs submitted to continuous infusion of propofol were evaluated. Eight adult mongrel dogs were used. Each animal was submitted to five anesthesias. In each procedure, the patient was allowed to breathe a different FiO2, thereby resulting in five groups, namely: G100 (FiO2 = 1), G80 (FiO2 = 0.8), G60 (FiO2 =0.6), G40 (FiO2 = 0.4), and G20 (FiO2 = 0.21). To induce anesthesia, propofol was given until the animals allowed orotracheal intubation, followed by immediate continuous infusion of drug. The initial measurement (M0) was recorded 30 minutes after the infusion of propofol onset. Additional recordings were performed at 15-minute intervals during 60 minutes (M15, M30, M45, and M60). No significant differences on BIS parameters were recorded. Regarding arterial partial pressure of carbon dioxide (PaCO2), the mean of G100 was greater than G20 at M30, whereas arterial partial pressure of oxygen (PaO2) varied according to the changes in oxygen. The mean intervals of BIS were: for G100, from 68 to 62; for G80, from 71 to 58; for G60, from 72 to 62; for G40 from 76 to 68; and for G20, from 77 to 68. In conclusion, different FiO2 does not impair BIS parameters. However, it is suggested that BIS was able to detect changes in the balance of cerebral blood flow, which was a result of changes in respiratory dynamic by the use of several inspired fractions.
dog; oxygen inspired fraction; propofol; bispectral index
