Guaranteeing adequate depth of general anesthesia is an essential task for the anesthesiologist (1). Approaches for monitoring the depth of anesthesia and the transition from being awake to being anesthetized are of increasing interest. Most of those approaches are based on information derived from the electroencephalogram (EEG) or parts of it, such as acoustic evoked potentials (AEP). Two of the main approaches are the bispectral index (BIS) (Aspect Medical Systems, Newton, MA) and the A-line autoregressive index (AAI) (Danmeter, Odense, Denmark) (2–7).
In contrast to the more established BIS which is derived from the raw EEG, the AAI is based on changes in AEP provoked by repeated application of clicks to the monitored person by headphones (7).
These monitoring systems are not only affected by anesthetics. Recently, BIS was shown to be influenced by different factors, such as neuromuscular blockade or by pressure applied on the acupuncture Extra 1 point (EP) in volunteers (8,9).
The aim of this study was to evaluate whether acupressure influences the AAI monitoring in the same way as it does BIS (9).
AAI monitoring is based on the processing of mid-latency AEP (7,10–12). This is in contrast to BIS monitoring which is derived from the raw EEG signal (13). Mid-latency AEP are the EEG response 10–50 ms after predefined auditory stimuli. As signal for the AAI monitoring, a bilateral click sound of 65-dB sound pressure level intensity, 2-ms duration, and 9-Hz repetition rate is delivered through a pair of headphones and detected by the applied electrodes. Mid-latency AEP signals are filtered from the EEG by autoregression with exogenous input (ARX) modeling in order to eliminate spontaneous EEG and electromyography activity. The ARX method is applied to shorten the extraction time and therefore the reaction time of the system and was originally used for creating stable images from infrared cameras for night vision (14). Changes in mid-latency AEP amplitudes and latency are transformed into the A-line ARX index. Similar to the BIS, a dimensionless index is generated with the end-points 0 and 100 (=awake). Processing time for the AAI is reported to be 30 s for the first signal and 6 s for further monitoring.
After obtaining approval from the local ethics committee, 15 consenting adult volunteers of both sexes participated in the study. Exclusion criteria were ASA physical status ≥III, taking sedatives or analgesics, experience with traditional Chinese medicine, known central nervous or cerebrovascular diseases, known hearing deficits, and being an anesthesiologist.
The volunteers were lying comfortably in the supine position in a quiet environment. At this point, each volunteer was asked to quantify her or his stress level using a visual analog scale (VSS) from 0 (=no stress at all) to 100 (=highest imaginable stress level) (9).
After preparation of the skin with fine sandpaper and an EEG electrode paste for better skin-electrode coupling, three silver-silver chloride electrodes (Danmeter) were placed according to the AAI instruction manual (A-line monitoring, version 1.4; Danmeter) as follows: one electrode at the median forehead (positive), one laterally at the right side of the forehead (reference), and one at the right mastoid bone (negative). Then, the impedance of the electrodes was checked and the A-line headphones were applied to the volunteers.
Each volunteer received pressure on the acupuncture EP on one day, and on a control point on another day in a randomized manner by the same investigator using the index finger.
The acupuncture EP is located midway between the medial ends of the eyebrows at the root of the nose. The control point was defined 2 cm lateral and horizontal from the lateral end of the left eyebrow (9).
After a 5-min period of stable AAI values, pressure was applied on the appropriate point by the index finger for 10 min while performing small circular movements 20–25 min−1. Afterward, a 5-min period was allowed to achieve stable AAI values again.
When the measurements were completed, each volunteer was asked to quantify her or his level of stress, again using the VSS from 0 to 10.
Data Assessment and Presentation
The following data were assessed: VSS values before and after interventions, AAI values before application of pressure, every 60 s during application of pressure, and 5 min after pressure was released. Data were recorded from the AAI monitor by a study nurse with the monitor positioned in a way that neither the volunteer nor the person performing the acupressure could see the display. Data are presented as median (range).
VSS values before and after interventions were compared by Wilcoxon’s signed rank test. AAI values before application of pressure were compared with corresponding values after 10 min of pressure and to corresponding values 5 min after pressure was released, respectively, again using Wilcoxon’s signed rank test. The gap between AAI values before and after 10 min of pressure was calculated (Δ AAI), as was the gap between VSS values before and after the interventions (Δ VSS). Δ AAI and Δ VSS from volunteers in whom pressure on the EP was performed first were compared with gaps from volunteers in whom pressure on the control point was performed first by Wilcoxon’s signed rank test. AAI and VSS values before pressure on the EP were compared with corresponding values before pressure on the control point by Wilcoxon’s signed rank test. Bonferroni correction was performed to compensate for the possible effects of repeated testing. Simple regression analysis was performed to evaluate correlation between AAI values recorded in the same volunteer before measurements on different days, and between preintervention AAI and VSS values, respectively. A P value < 0.05 was considered statistically significant for all tests.
Fifteen volunteers completed the study protocol (10 women, 5 men). Median age of the volunteers was 33 (21–58) yr.
AAI values before interventions ranged from 25 to 99 (median 71).
There was a statistically significant difference between VSS values before and after pressure on the EP (P = 0.0066), but not on the control point. AAI values steadily decreased during the experiment, becoming statistically significant at the end of the tenth minute, with pressure on the EP (P = 0.0044), but not with pressure on the control point.
There were no differences in VSS and AAI values before the two interventions. VSS and AAI values before, after 10 min of pressure, and 5 min after release of pressure are summarized in Table 1. There was close correlation between AAI recorded before measurements in the same volunteer on different days (r 2 = 0.785; Fig. 1). Five minutes after release of pressure in both points, there was no difference in AAI values compared with initial values. The time course of AAI values during the interventions is shown in Figure 2.
No statistically significant difference was found by comparison of Δ AAI, and Δ VSS, respectively, in volunteers who received pressure on the EP first, to volunteers in whom pressure was applied on the control point on the first day.
There was no correlation between AAI and VSS values before the interventions (r = 0.020).
In this study, the influence of acupressure on the AAI and stress level was evaluated.
The main findings were that (a) there was a wide range of AAI values in unsedated adult volunteers, and that (b) acupressure with pressure applied on the acupuncture EP but not on a control point significantly decreased the recorded AAI value and stress level.
The AAI is reported to be more sensitive to the transition from consciousness to unconsciousness than the more widespread BIS (15). Like the BIS, the AAI is a dimensionless index from 0 to 100 (3). Whereas 100 indicates “fully awake,” there is a large range of normal values in unsedated volunteers and patients. In addition, the actual threshold value for “being adequately anesthetized” remains to be defined.
Our results show a wide range of values in awake adult volunteers. This implies further need for definition of normal values for interindividual comparisons and as a baseline from which decreasing values truly represent increasing depth of sedation. The close correlation for values recorded in the same volunteer on different days (r 2 = 0.785) indicates that different persons show different but constant AAI values. This would make the AAI monitoring useful for monitoring of intraindividual trends rather than for interindividual comparisons. Schmidt et al. (7) reported initial AAI values of 78 ± 14 for premedicated awake patients and a value of 43 ± 21 for unconsciousness. Kreuer et al. (4) reported AAI values of 85 (55–99), Litvan et al. (10) of 74.9 (±13.3) for premedicated awake patients. These values underline the need for further definition of threshold values, as shown by our data.
Our results further show a reduction in stress levels by pressure applied on the acupuncture EP. In contrast, stress levels tended to decrease by pressure applied on a sham point but this reduction did not reach statistical significance. Acupressure was shown to reduce BIS values and stress levels in unsedated volunteers (9). It was also reported to be a valuable technique for prehospital analgesia after minor trauma and for decreasing anxiety during prehospital transports (16,17). However, the mechanisms by which acupuncture and acupressure influence physiological reactions remain unclear and the positive effects of acupressure during anesthesia are not undisputed (18).
These are the following implications of our results: (a) monitoring of level of consciousness by monitoring changes in EEG is not solely influenced by anesthetics. Neuromuscular blockade was shown to decrease BIS values as well (8), acupuncture decreases BIS values (9) and as shown by our results, AAI values. More data concerning normal values, threshold values, and confounding factors are needed to improve the understanding of anesthesia-associated EEG changes. (b) Acupressure deserves attention for potentially being a noninvasive, easy to apply alternative for reduction of stress and anxiety.
Some limitations to our study have to be considered. The findings of Fassoulaki et al. (9) were criticized for the choice of control procedure, as were other acupuncture studies (19,20). The problem remains that, as there are no defined effects and criteria for acupuncture points, every control point may turn out to be another acupuncture point with unknown effects. We chose the same control procedure as did Fassoulaki et al. in order to compare the effects on BIS they found with the effects on AAI.
In conclusion, there was a wide range of AAI values in awake adult persons, making it difficult to define normal values. Acupressure applied on the acupuncture EP should be further investigated as a noninvasive method potentially decreasing stress levels.
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