Electromagnetic Emission of High-Energy Extracorporeal Shockwave Lithotripsy to the Shoulder does not Cause Disruption of Bispectral Index Monitoring of Propofol or Sevoflurane Anesthesia

Abstract

To the Editor: Hemmerling et al. (1,2) in a recent series of case reports appearing in Anesthesia & Analgesia displayed disruptions of bispectral index (BIS) monitoring (Aspect Medical Systems Newton, MA) with a shoulder shaver device (1) and with the use of an otorhinolaryngology electromagnetic positioning system (2). We examined the effect on BIS monitoring of an electromagnetic emitter of a high-energy extracorporeal shockwave therapy (ESWT) lithotripter applied to the shoulder. After Graz Medical University ethics committee approval, and written informed consent, 6 adult patients underwent shoulder ESWT lithotripsy for symptomatic rotator cuff calcareous tendinopathy under propofol general anesthesia (group 1, 3 patients) or sevoflurane-nitrous oxide (N2O) (group 2, 3 patients). BIS “Quattro” sensors were placed on patients’ foreheads and connected to a BIS XP monitor (version 3.4). Data were captured from the Serial Port onto a laptop computer every 5 s, after verifying a signal quality index of >95% and electrodes impedance of <5 kΩ. The smoothing window was set at 30 s. Anesthesia was induced with fentanyl 1.5 μg/kg and propofol 2–3 mg/kg, followed by rocuronium 600 μg/kg to facilitate tracheal intubation. Anesthesia was maintained with either propofol 100–150 μgkg−1min−1 and remifentanil 0.1–0.3 μgkg−1min−1 (group 1) or sevoflurane 1 MAC with 60% N2O in oxygen (group 2). All patients underwent electromagnetic induction of high-energy shockwave lithotripsy to the shoulder (Minilith SL1; Storz Medical, Kreuzlingen, Switzerland). The energy flux density in the therapy focus was 0.5 mJ/mm2 of a total of 2000–3000 impulses. In all 6 patients, BIS tracings indicated a signal quality index of >95%. With induction of anesthesia the mean ± sd BIS value declined to 28.4 ± 2.7. Anesthesia was maintained at BIS 36.7 ± 3.1 before the start of ESWT. The mean BIS value during ESWT was 37.2 ± 2.7, whereas the BIS value in the 10 min after the switching the ESWT off remained at 37.9 ± 3.4 before starting to increase with termination of anesthesia (Fig. 1). During the anesthesia maintenance phase, using the paired Student’s t-test there was no statistically significant difference between mean BIS values before and during (P = 0.2384) ESWT or between during and after (P = 0.2198) switching the ESWT off. There was no significant difference in electromyelogram values before, during, and after ESWT.Figure 1.: Mean Bispectral Index (BIS) and electromyography (EMG) during extracorporeal shockwave therapy (ESWT) in six patients.The literature reports demonstrating artifactual increases in BIS give the impression that electric devices applied to the shoulder “similar” to the shoulder shaver (1), or devices generating an electromagnetic field (2), are likely to result in erroneous BIS readings. In the former case, BIS suddenly increased from 40 to 60 during the activity of endoscopic shoulder shaver oscillations and decreased equally abruptly after the end of the use of the shaver device (1). Our modest study suggests that the new BIS algorithm is properly shielded against shoulder vibrations caused by the electromagnetic shock wave emitter. In the latter report, switching a three-dimensional otorhinolaryngology electromagnetic positioning device increased the BIS from 40 to 60–90 despite increased anesthetic drug doses, during both propofol and sevoflurane maintenance of anesthesia. We failed to find any suggestion that outside electromagnetic field generated during ESWT altered the BIS. This may reflect further refinement of the BIS to exclude such artifacts or may reflect a difference in the electromagnetic fields between that generated by the ESWT device compared with the electromagnetic positioning device. Despite these negative findings, the electroencephalogram (EEG) waveform is easily altered by artifacts. The possibility of artifact should always be considered when evaluating the information obtained from computerized EEG monitors. Ashraf A. Dahaba, MD, MSc, PhD Helmar Bornemann, MD Department of Anaesthesiology and Intensive Care Medicine [email protected] Peter H. Rehak, PhD Biomedical Engineering and Computing Unit of the Department of Surgery Helfried Metzler, MD Department of Anaesthesiology and Intensive Care Medicine Graz Medical University Graz, Austria