Our results demonstrated that BIS and PSI could both distinguish the maintenance state and recovery state of anesthesia in children with high prediction probability, although the correlations of BIS or PSI values with sevoflurane concentration were relatively weak.
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.[18] Additional research has revealed that the performance of BIS or entropy improves as age increases.[19] 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.
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.[20] Our study also demonstrated an inverse correlation between sevoflurane concentration and BIS/PSI values shown in figure 1. 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 maintenance state. The averaging time of BIS or PSI with the obtained EEG data were 15 sec or 20 sec, respectively by manufacturer’s default. Moreover, PSI values are shown after the manipulation of removal of artifact automatically. Therefore, it is not easy to compare the values directly. In addition, when the sensor was detached, PSI value started from 100 unlike BIS. Therefore, the values of PSI could give the bias to the result. This may explain why the correlation was weaker for PSI values than for BIS values.
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%.[21] In our study, the mean inspiratory/expiratory sevoflurane concentration during maintenance state was only 2.2/2.8 vol%, which is not considered high. 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. Therefore, the mean inspiratory or expiratory sevoflurane concentrations observed in the present study were within the safe range to avoid the epileptoid pattern of EEG.
Several studies have compared the performance of different EEG-derived monitors and other monitors in the patients.[6, 14, 22, 23] 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 maintenance 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.[24] 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. Considering the usefulness of BIS monitor in children, PSI might be an alternative appropriate monitor for BIS from the result of this study because the agreement between BIS and PSI was relatively good in pediatric population in spite of different recommendation range of BIS or PSI for anesthesia.
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 propofol and sevoflurane 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 the whole range of sevoflurane concentration. However, this study was designed to compare the BIS and PSI in the same patient, propofol effect was equal to both monitors.
At the same stable concentration of propofol, BIS values vary in children, relative to those observed in adults,[25] and larger individual variations are observed during halothane or sevoflurane anesthesia in children.[26] 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.
Remifentanil was infused continuously during the study period with the rate of 0.1-0.2 ug/kg/min. Although the remifentanil would affect the electroencephalogram, it was believed that the infusion of remifentanil did not affect the result of the present study because we applied the BIS and PSI simultaneously in the same patient.
In this study, the correlation was relatively weak. We could explain this result by the maintenance state containing high inspiratory sevoflurane concentration before reaching the equilibrium of sevoflurane. If we refined the maintenance state, the correlation might be stronger. In addition, the BIS or PSI values were lower during the recovery state (median 62 vs 52, table 2) than the usual awake state because definition of recovery state was the period from the cessation of sevoflurane to extubation. Sevoflurane was not fully washed out during the recovery state and it would affect the BIS or PSI values. Moreover, the recruited patients were children who were the high-risk population of emergence delirium and it was exceedingly difficult to apply the BIS or PSI sensor after extubation in practice and we could not obtain the data.
Limitations
The present study possesses several limitations of note, including the arbitrary definition for the maintenance state and recovery state in clinical practice. However, this practice is common at our institution and reflects procedures applied in the pediatric population. Secondly, 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. Thirdly, this study was performed in a single center. Finally, the 2 sensors were applied simultaneously and one of sensors was inevitably applied in inappropriate position and this might become a bias of the result. Lastly, BIS and PSI sensors were applied together and one of the sensors was inevitably attached in inappropriate position. However, the relationship between the sensors was same in the recruited patients and it did not give the bias for the result.