Electrocardiographic Electrodes Provide the Same Results as Expensive Special Sensors in the Routine Monitoring of Anesthetic Depth

The Bispectral Index (BIS) is a mathematically derived electroencephalographic (EEG) derivative that has been introduced to monitor depth of anesthesia (1,2). The A-2000 BIS monitoring system (Aspect Medical Systems, Inc., Newton, MA) is currently the only commercially available system to monitor depth of anesthesia. In several studies, its propensity to optimize the use of hypnotics to maintain and achieve a certain depth of anesthesia has been described (3,4). Some studies have even proposed that the routine use of the monitoring system can decrease awareness (1,5), an increasing factor in malpractice claims. The cost-benefit calculations for BIS monitoring suffer from the fact that like its predecessor, the 1000-A BIS monitor, the A-2000 BIS monitoring system demands the use of expensive, special electrodes (6). Although the application of the single-use BIS sensor is very comfortable and easy to use, its high price of approximately $10–20 US prevents many anesthesiologists from using it. Furthermore, whereas the former model of the monitor (1000-A BIS monitor; Aspect Medical Systems, Inc.) used standardized connectors, which allowed the use of other electrodes such as electrocardiogram (ECG), the new monitoring system makes this very difficult because of special connectors that match the equivalent connector at the proximal BIS sensor site. The purpose of this prospective study was to compare BIS values derived from the original BIS sensor with BIS values derived from commercially available ECG electrodes. This comparison was made possible by designing and manufacturing a connector allowing the use of ECG electrodes. Methods The original BIS sensor consists of a long piece of plastic with three integrated electrodes. The BIS sensor is connected with the patient interface cable (PIC) via special pin connectors. It is important that the original connectors are firmly linked together; if this is not the case, the A-2000 BIS monitoring system will not proceed to impedance testing and monitoring is impossible. We therefore used a small proximal part of the original sensor and attached very fine wires (diameter, 0.48 mm) on top of the corresponding built-in wires of the sensor. The new custom made connector was used to connect the PIC with three wires, at which end standard crocodile connectors were connected to commercially available Meditrace ECG electrodes (Graphic Controls, Buffalo, NY). The A-2000 BIS monitoring system self test recognized the connector. After approval of the IRB and informed consent, 12 patients (4 men, 8 women; mean age, 52 ± 18 yrs) undergoing general anesthesia (mean duration: 45 ± 16 min) were included in the study. Anesthesia was induced with propofol 1–1.5 mg/kg, fentanyl 10 μg/kg, and intubation was facilitated by succinylcholine 1 mg/kg. Anesthesia was maintained by sevoflurane 1–1.5 MAC in a gas mixture containing 30% oxygen and 70% nitrous oxide with controlled ventilation. After routine defatting of the skin using alcohol swabs, the two sets of electrodes were glued to the forehead and temporal facial area and pressed onto the skin for at least 5 s (Fig. 1). BIS values were collected using two A-2000 BIS monitoring systems, one using the original BIS sensor and the other connected with the Meditrace ECG electrodes.Figure 1: Location of the Bispectral Index monitoring sensor (S) and the Meditrace electrocardiogram electrodes (E) on the forehead and the temporal area. The same locations were used in all patients.Impedance values were recorded at all three electrode sites for both electrode types. BIS values were collected every 30 s during anesthesia. Impedance values were compared using paired Student’s t-test. P < 0.05 was considered as showing a significant difference. The agreement between BIS values derived by the two electrode sets was evaluated by Bland-Altman testing. Data are presented as mean ± sd. Results BIS values could be obtained in all patients with both electrode sets. Impedance was significantly lower with the BIS sensor with a mean impedance of 1.5 ± 0.3 kΩ versus 5.4 ± 2 kΩ for the Meditrace ECG electrodes (P < 0.05). The mean BIS value during anesthesia of all patients was 37.3 ± 6 and 38.7 ± 6 using BIS sensor and Meditrace ECG electrodes, respectively, with a mean of 82 ± 7 recordings of BIS values. The mean bias (precision) between BIS values obtained using BIS sensor and the BIS values obtained using the Meditrace ECG electrodes was −1.34 (2.88) with narrow limits of agreement between −7.1 and + 4.4 (Fig. 2). In most patients, the mean BIS values obtained using BIS sensor were lower than the BIS values obtained using the Meditrace ECG electrodes with a range of bias between −3.1 and +3.1 for all patients.Figure 2: Bland-Altman plot of agreement between Bispectral Index (BIS) values derived from the BIS sensor and derived from the Meditrace electrocardiogram (ECG) electrodes: mean bias of −1.34 and limits of agreement between the two electrodes of −7.1 and + 4.4 (BIS sensor versus Meditrace ECG electrodes).Discussion Our results show that after routine skin alcohol swab pretreatment, commercial ECG electrodes can be used for monitoring depth of anesthesia. BIS values derived using these electrodes or the original BIS sensor can be used interchangeably. Usually BIS values between 40 and 60 are regarded as providing sufficient anesthesia (7), and follow-up of the trend during anesthesia is very important. Therefore a mean bias of −1.34 and limits of agreement between the two electrodes −7.1 and +4.4 seem quite acceptable. One previous study has presented a comparison between BIS values obtained by ECG electrodes and those obtained by the special adhesive electrodes (Zipprep, Aspect Medical System) with the former Aspect 1000-A EEG monitor. In that study, skin preparation with abrasion paste (rather than with alcohol swab) provided monitoring with pregelled ECG electrodes that showed good agreement with the original Zipprep electrodes (8). Seitsonen et al. (8) give the higher skin impedance of ECG electrodes after only alcohol pretreatment as a reason for the difference of the BIS values with Zipprep electrodes because skin-electrode impedances below 10 kΩ are generally considered adequate for EEG monitoring. This was not achieved with only alcohol swab treatment. Although our study shows higher impedance for the ECG electrodes in comparison to the sensor, in no patient did the impedance exceed 7.5 kΩ. Our study shows that commercially available pregelled ECG electrodes after routine alcohol skin preparation at a cost of approximately 10 cents US per electrode and BIS sensor at a cost of $10–20 US (both single use) can be used interchangeably to monitor BIS values with acceptable limits of agreement between the two sets of electrodes. If the high costs of the original sensors prevent the routine use of BIS monitoring in a hospital setting, ECG electrodes can provide a cheaper alternative. This might lead to a new calculation of cost-benefit ratios and should make this useful tool accessible to everybody.

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