Needle Electrodes Can Be Used for Bispectral Index Monitoring of Sedation in Burn Patients

Although the bispectral index (BIS) is often used to monitor depth of sedation in ventilated intensive care patients (1–3), the A-2000 BIS monitoring system (Aspect Medical Systems, Inc, Newton, MA) is currently the only commercially available system, but other companies are about to enter the market. All commercial systems rely on recording electroencephalographic signals from the patient’s head or forehead using special electrodes glued on the skin. However, burn patients often have facial burns, which makes the use of these superficial electrodes impossible. Furthermore, the A-2000 BIS monitoring system uses a special patient interface cable whose connector only allows the use of the company’s own sensor. We monitored depth of sedation in burn patients with the A-2000 BIS monitoring system using subdermal needle electrodes and the original BIS sensor and determined the limits of agreement between the two types of electrodes. Methods The original BIS sensor is made of a long piece of plastic with three integrated electrodes. It is connected with the patient interface cable 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 therefore, monitoring is impossible. We 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 (4). The new custom made connector was used to connect the patient interface cable with 3 wires, at which end standard crocodile connectors were connected to commercially available subdermal needle electrodes (size, 22-gauge; length, 2 cm; Inomed Company, Teningen, Germany). In a prospective audit, BIS values derived using the two different types of electrodes were compared in intubated burn patients. All 7 male patients (mean age, 38 yr; range, 18–54) were admitted to the burn unit with burn injury and were ventilated and analgo-sedated using a continuous infusion of morphine and lorazepam. No patient suffered from frontal facial burn injury, which would prevent the proper attachment of the BIS sensor. After defatting of the skin using alcohol swabs, the BIS sensor was glued to the forehead and temporal facial area and pressed onto the skin for at least 5 s; the 3 subdermal electrodes were inserted under the skin proximal to the corresponding 3 electrode sites (Fig. 1). BIS values were collected using two A-2000 BIS monitoring systems, one using the original BIS sensor and the other connected to the subdermal electrodes.Figure 1: Position of the bispectral index (BIS) sensor (S) and the subdermal needle electrodes (N) on the forehead and the temporal area. The same location was used in all patients.BIS values were collected every 30 s during the first period of 20 min and after leaving both electrode types in place for 12 h during a second period of 20 min. Impedance values were recorded at all three electrode sites for both electrode types before each recording period. During both test periods, no additional analgesic or sedative was applied. Power for seven patients was calculated as more than 80% (the second run of BIS comparisons was not included in the power calculation), with an estimated difference of BIS values of 20% between the 2 methods. Impedance values were compared between the initial impedance and the impedance after 12 h and between the two electrode types using rank sum test. P < 0.05 was considered as showing a significant difference. The bias and limits of agreement between BIS values derived by the two electrode sets were evaluated using Bland-Altman testing. Data are presented as mean ± sd. Results BIS values could be obtained in all patients with both electrode sets. Impedance was not significantly different between the BIS sensor and the needle electrodes at 1.8 ± 0.7 and 1.7 ± 0.6 kOhm, respectively. There was no difference between those initial impedances and the impedances recorded 12 h later at 2 ± 0.7 and 1.8 ± 0.6, respectively, for both electrode types. For all patients, 561 BIS sets were recorded. Table 1 gives the mean BIS (sd) per patient and study period. Figure 2 shows the variation in BIS values during a 20-min period for a representative patient. The bias (precision) between BIS values obtained using needle electrodes and the BIS values obtained using the BIS sensor was −1 (5.2) with narrow limits of agreement between −11.4 and +9.4 (Fig. 3).Table 1: Mean BIS per Patient and Study PeriodFigure 2: Bispectral index (BIS) variation for both electrodes (subdermal electrodes ▪; BIS sensor ▴) for a representative patient during a 20-min period.Figure 3: Agreement between bispectral index (BIS) values derived from subdermal electrodes and theBIS™ sensor: mean bias of −1 and limits of agreement between the two electrodes of −11.4 and +9.4 (subdermal needle versus BIS sensor). Discussion BIS values derived from subdermal needle electrodes and BIS values derived from the original sensor electrode can be used interchangeably. The mean bias and the limits of agreement between the two BIS values are acceptable because of the wide range of BIS reflecting a sufficient level of sedation. Depending on the sedative used, BIS can range between 48 and 76 (mean BIS95 to avoid recall for propofol, isoflurane, midazolam, or combined), as reported in one study (5). In these sedated and ventilated patients, BIS values for each type of electrode varied considerably between each recording point despite the fact that the degree of sedation was not changed during each 20-minute period. We did not find different impedance values for the subdermal needle electrodes, and the original sensor and impedance levels remained unchanged after 12 hours. In a previous study (4), BIS values derived from commercial electrocardiogram electrodes and the original sensor were compared; significantly higher impedance levels with electrocardiogram electrodes were found without affecting the interchangeability of the BIS values. Although BIS monitoring of sedation has been widely used in clinical routine, there are no studies on the use of this useful monitoring tool in burn patients, perhaps because facial burns are a common feature in inhalation injury in burn patients (6). Most ventilated and sedated burn patients would therefore suffer from facial burns of various degrees, which makes the application of superficial, glued-on electrodes impossible. Although only patients without burns to the frontal head were included in this study (to make it possible to compare both types of electrodes), needle electrodes can easily be applied even in patients with extensive facial burns (Fig. 4). The custom made connector makes it possible to connect subdermal needle electrodes to the BIS monitor.Figure 4: Use of subdermal electrodes in a patient with extensive burns to the frontal head.Our study shows that subdermal needle electrodes and BIS sensor can be used interchangeably to monitor BIS values with acceptable limits of agreement between the two sets of electrodes. The use of subdermal needle electrodes could make BIS monitoring accessible for patients with facial burns where the original sensor cannot be used.

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