Ketamine increases the amplitude of the 40-Hz auditory steady-state response in humans

The auditory middle latency response (AMLR) and the 40-Hz auditory steady-state
response (40-Hz ASSR) are evoked potentials which possibly arise from the same
generators in the primary auditory cortex. Both responses are attenuated by most
general anaesthetics. Ketamine, however, has been reported to have no effect on
the AMLR. Our aim was to evaluate the effects of ketamine on the 40-Hz ASSR.
Spectral analysis of the electroencephalogram (EEG) was also conducted to
independently examine the effects of ketamine. Ketamine 1.5 mg kg-1 was given to
12 patients for induction of general anaesthesia. Recordings of the 40-Hz ASSR
and EEG were obtained every minute from 3 min before administration of ketamine
to 5 min after injection, when the study was terminated. Similar recordings were
obtained in three control subjects under identical conditions except that no
medication was administered. Consciousness, defined as responsiveness to verbal
commands, was assessed before each recording. Ketamine caused an increase in the
amplitude of the 40-Hz ASSR (P < 0.01). Using published AMLR data, we conducted a
simulation experiment that suggested that the effect of ketamine on the AMLR can
explain its effects on the amplitude of the 40-Hz ASSR. There was a pronounced
increase in relative theta (3.9-7.9 Hz) EEG power and a decrease in relative
alpha (8.0-12.8 Hz) power (P < 0.001). These changes were not observed in the
control group. Ketamine produced unconsciousness until the end of the study in
five patients and transient unconsciousness in five patients. Two patients did
not lose consciousness after administration of ketamine. The 40-Hz ASSR and EEG
revealed no consistent differences between conscious and unconscious patients. No
relationship could be demonstrated between the increase in amplitude of the 40-Hz
ASSR or of relative theta power (the hallmark of ketamine effect) and loss of
responsiveness to commands. We conclude that ketamine, unlike other anaesthetics,
increases the amplitude of the 40-Hz ASSR.