Relationship
between the effect site concentration of propofol at loss of consciousness and
that adequate for maintenance of anesthesia
AUTHORS: Hiroo Yamanaka, M.D.*, Satoshi Hagihira,
M.D., Ph.D.*, Osamu Nagata, M.D.**, Makoto Ozaki, M.D.**, Takashi Mashimo,
M.D., Ph.D.*
* Department of Anesthesiology, Osaka University Graduate
School of Medicine, Osaka, Japan
** Department of Anesthesiology, Tokyo Women’s Medical
University, Tokyo, Japan
INTRODUCTION: Large interindividual variation of
sensitivity to propofol makes it difficult to adjust the optimal concentration
of propofol for each patient even using the target-controlled infusion (TCI)
system. We made a hypothesis that we could estimate the adequate concentration
of propofol for maintenance of anesthesia from the effect site concentration
(ESC) of propofol at loss of consciousness (LOC). We investigated the
relationship between the ESC of propofol at LOC (P_sleep) and adequate ESC for
maintenance of anesthesia determined by electroencephalographic monitoring
(P_maint). We also investigated the influence of fentanyl on this relation.
METHODS: After approval of local ethical committee and
obtained informed consent, we enrolled sixty patients (male and female, aged
53.2 ± 13.7 yr old; mean ± SD, American Society of Anesthesiologists physical
status I–II). EEG was monitor by BIS monitor (A-1050) and raw EEG data as well
as EEG derivatives were recorded on a computer using the software “BSA for BIS”
developed by one of the author (SH). And we used TCI system “ConGrase”
developed by one of the author (ON) in this study. Patients were divided into
three groups. At first fentanyl was infused with TCI system at target ESC for 0
ng/ml (group 1), 1.0 ng/ml (group 2) or 2.0 ng/ml (group 3). After we confirmed
the equilibrium of fentanyl ESC, we then started propofol infusion. ESC of
propofol was gradually increased using TCI system. We recorded the time at LOC
and ESC of propofol was increased till level of anesthesia was judged as enough
deep by EEG monitoring. The adequate level of anesthesia for maintenance was
defined the level when relative b ratio (RBR), which is one of sub-parameters for BIS calculation, became
the lowest. Generally RBR became the lowest, when the spindle wave became most
dominant. After data acquisition, we calculated the ESC of propofol at LOC
(P_sleep) and for maintenance of anesthesia (P_maint), and investigated the
relationship between P_sleep and P_maint.
RESULTS:
In all groups, P_sleep was significantly correlated with P_maint (r=0.88, 0.81,
0.83; group 1,2,3). Although fentanyl significantly decreased P_sleep (CP50:
1.81, 1.51, 1.04 [mg/ml]; group 1,2,3), it
had minimal influence on P_maint (CP50: 2.62, 2.45, 2.22 [mg/ml]; group 1,2,3).
CONCLUSION:
We showed that we could estimate the adequate maintenance dose of propofol from
ESC at LOC. Our study also indicated that it would be better to administar
fentanyl after LOC, because fentanyl altered P_sleep but not P_maint.