DISCUSSION
To our best knowledge, this study is one of the few researches that
attempt to define BIS values during normal sleep in the sleep laboratory
while performing PSG for obstructive sleep apnea syndrome (OSAS)
diagnosis. Although the patients included in this prospective study were
patients suspected to have OSAS; moderate and severe OSAS patients were
not included to study to provide a population closer to normal. By this
way, a study population with mildly distorted sleep architecture was
created.
Only the BIS and sleep stage values in the sleep period that were longer
than 2 minutes were taken into account (stable sleep stage) after the
end of second minute.
Although the BIS devices are not calibrated to identify the natural
sleep stages, there are few studies in the literature investigating the
BIS values during normal sleep with conflicting results. In 1999, Slergh
et al(6) claimed that; changes in the depth of natural
sleep are reflected sensitively by changes in the BIS and BIS may be a
simple indicator of the depth of sleep. But their study group was
consisted of only 5 subjects. Benini et al(5) also
concluded that BIS is sensitive in reflecting the changes on the EEG
trace that accompany various stages of natural sleep. To be more
specific, they found a progressive decrease in BIS values as the stages
of sleep becomes deeper, in their study performed with 15 children.
Contrarily, Neuwenhuijs D et al (11) reported that the
processed EEG measurements (both BIS and spectral edge frequency) have a
limited ability to estimate classical sleep stages as a result of their
work with 10 patients.
It has been shown that, BIS can identify stage N3 with satisfactory
sensitivity and specificity, but it is unable to distinguish light sleep
stage (N1) from REM, sleep or wakefulness (W). (3)Signal quality index is used in different scientific areas to explain
some characteristics of determined signals.Therefore the defined SQI is
related with signal strength.The SQI is ranged between 0 and 100% (best
signal quality). Mohammad et al suggested that BIS signals should be
read in relation to the current signal quality index (SQI) and
impedance. (3) However there is no validated threshold
for the quality of the BIS signal and it is not completely clear how the
SQI is calculated and what should be an ideal value.
In our study, we first investigated the BIS scores for SQI values of
50-70, 70-90 and over 90 and we found that there was no statistically
significant difference between average BIS values of 70-90 and over 90
SQI values, but there was significant difference between 50-70 and
50-70/over 90 SQI values. So we included only the BIS values with an
index of SQI over 70 to study.
According to our study, BIS values decrease progressively as the stages
of sleep becomes deeper. Mean BIS values were 87.70, 83.94, 74.41, 56.60
and 74.90 For W, N1, N2, N3 and R3 stages respectively (table 4).
Furthermore, BIS values were sensitive in reflecting the stages of
natural sleep except REM and N2; REM and stage N2 BIS values were
similar. One possible explanation of proximity of the index scores in N2
and REM stages may be the inability of the available BIS monitor to
distinguish the brain’s electrical activiy in these two stages. Our
results are incompatible with some of the previous studies, which
reported BIS is not 100% specific in predicting intraoperative
awakening and recall( 2). In their study, Duarte et al
also claimed that the model of BIS monitor could influence the
interpretation of the results. We found a statistically significant
relationship between BIS scores and sleep stages, which may due to use
of a higher model of BIS monitor.
All of the currently available monitors need different period of times
to calculate and update the index in response to changes in the depth of
anesthesia (2). The time to update BIS records can
range from 14 to 155 seconds. For this reason, to acquire more reliable
results, we evaluated only the data obtained after the second minute of
stable stages.
BIS monitoring may be a practical method to identify sleep
stages.Because in oder to recognize sleep stages,BIS is a numerical
method that everyone can evaluate in stead of EEG which can only be
assesed with a specific training. Which BIS values should be used for
sleep endoscopy is not exactly known yet. As a result of their research
Babar-Craig *12* et all reported that : average BIS
values ranging from 50 to 61 is appropriate for sleep nasendoscopy. Our
results suggest that, the 50-70 BIS values correspond to the deep stages
of sleep (N3), and it is well known fact that respiratory events are
less common in N3 phase of sleep (13). Thus, we
recommend that sleep endoscopy should be done at 70-80 BIS values, which
corresponds to N1 and N2 phases of sleep where respiratory events are
more frequently seen.
In conclusion, according to us BIS is a usefool tool for distinguishing
of normal sleep stages except the R2 REM stages and BIS values decreases
as the sleep stages become deeper.