Our results demonstrated that BIS and PSI could both distinguish the steady state and recovery state of anesthesia in children with high prediction probability, although their correlations with sevoflurane concentration were relatively weak.
There is an inverse correlation between BIS values and end-tidal sevoflurane concentration in children, suggesting that EEG-based monitors can be applicable for monitoring anesthesia depth in this population. Our study also demonstrated an inverse correlation between sevoflurane concentration and BIS/PSI values. Relative to that for BIS values, the correlation between sevoflurane concentration and PSI values was poor in the present study. Although the reasons for this difference could not be determined with certainty, we also observed that fluctuations in PSI values tended to be greater than those in BIS values even at the steady state of inspiratory and expiratory sevoflurane concentration. This may explain why the correlation was weaker for PSI values than for BIS values.
A previous study reported that BIS-based methods are not reliable for assessing the depth of sevoflurane anesthesia in children under 2 years of age. Additional research has revealed that the performance of BIS or entropy improves as age increases. In the present study, all patients were over 3 years of age (mean age: 9.7 years), allowing us to avoid this limitation. Moreover, it is difficult to apply two different sensors to the small foreheads of children under 3 years of age.
Another study reported that elevated BIS values may indicate epileptoid patterns or EEG fast oscillations rather than an insufficient depth of hypnosis under high sevoflurane anesthesia. In this previous study, deep anesthesia was defined as a sevoflurane concentration over 4 vol%. In our study, the mean inspiratory/expiratory sevoflurane concentration during steady state was only 2.2/2.8 vol%, which is not considered high. In addition, our normal EEG findings indicated that the mean inspiratory or expiratory sevoflurane concentrations observed in the present study were within the safe range. Moreover, we did not include the induction period for analysis, which is common when using high concentrations of sevoflurane or bolus administration of propofol to avoid bias.
Several studies have compared the performance of different EEG-derived monitors and other monitors in the patients.[6, 14, 21, 22] Most of these studies reported comparable prediction probabilities for the different monitor types. In the present study, BIS and PSI yielded similar prediction probabilities for distinguishing the steady state from recovery state in children. This result is compatible with those of previous studies. In addition, basically, the present study was planned to compare the values of the two different devices, not to measure the absolute depth of anesthesia in children. Recent research has indicated that EEG parameters such as beta and delta band power are altered based on the depth of anesthesia in children and it would be helpful to develop the monitor with this finding. However, the algorithm of calculation of value of BIS was not unknown and basically developed with EEG of adults. Nevertheless, BIS might be the appropriate monitor for monitoring of depth of anesthesia for children with large number of researches to support this idea. In addition, the result of the present study demonstrated that the agreement and correlation of BIS and PSI was acceptable and it could lead the conclusion of usefulness of PSI in children.
In the present study, propofol administration was followed by inhalation of sevoflurane for the induction of anesthesia in pediatric population. Because BIS or PSI values may represent the mixed effects of these two drugs during the induction period, we did not analyze the correlation between BIS/PSI values and sevoflurane concentration. If sevoflurane had been inhaled from the beginning, BIS or PSI values may have been correlated with sevoflurane concentration.
At the same stable concentration of propofol, BIS values vary in children, relative to those observed in adults, and larger individual variations are observed during halothane or sevoflurane anesthesia in children. Despite these issues, several studies have utilized EEG-based methods to monitor the depth of anesthesia or hypnosis in children, as there is no gold standard for the pediatric population. The PSI is a different EEG-derived index that has been associated with a lack of pain upon attachment in patients. Although BIS and PSI are EEG-based indices, the algorithms used to manage the EEG data differ between the two monitors. Despite these differences, most previous studies have reported very good correlations between the two measures. Our results support the notion that BIS and PSI are comparable with regarding to agreement, correlation and prediction probability in children.
The present study possesses several limitations of note, including the arbitrary definition for the steady state and recovery state in clinical practice. However, this practice is common at our institution and reflects procedures applied in the pediatric population. In addition, we used propofol and sevoflurane for the induction of anesthesia. Therefore, we could not assess BIS and PSI values during the induction period. In addition, it was not easy to apply the BIS and PSI sensors in the alert state because due to fear among the included children.