Objective—To determine the relationship between
bispectral index (BIS) and minimum alveolar concentration
(MAC) multiples of sevoflurane in cats.
Animals—8 domestic cats.
Procedure—Each cat was anesthetized twice with
sevoflurane. First, the MAC of sevoflurane for each
cat was determined by use of the tail clamp method.
Second, cats were anesthetized with sevoflurane at
each of 5 MAC multiples administered in random
order. Ventilation was controlled, and after a 15-
minute equilibration period at each MAC multiple of
sevoflurane, BIS data were collected for 5 minutes
and the median value of BIS calculated.
Results—The mean (± SD) MAC of sevoflurane was
3.3 ± 0.2%. The BIS values at 0.5 MAC could not be
recorded as a result of spontaneous movement in all
8 cats. The BIS values at 2.0 MAC were confounded
by burst suppression in all 8 cats. Over the range of
0.8 to 1.5 MAC, BIS values decreased significantly
with increasing end-tidal sevoflurane concentrations.
Mean (± SD) BIS measurements were 30 ± 3, 21 ± 3,
and 5 ± 2 at 0.8, 1.0, and 1.5 MAC, respectively.
Conclusions and Clinical Relevance—Values of BIS
are inversely and linearly related to end-tidal sevoflurane
concentrations in anesthetized cats, and BIS may be a
useful predictor of CNS depression in this species. The
consistently low BIS values recorded in this study suggest
that clinical BIS end points used to titrate anesthetic
agents in humans may not be applicable to cats.
( Am J Vet Res 2004;65:93–98)