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AMBULATORY ANESTHESIA

The Effect of Noise on the Bispectral Index During Propofol Sedation

Dae Woo Kim, MD PhD^*, Ho Yeong Kil, MD PhD, and Paul F. White, PhD MD, FANZCA

*Department of Anesthesiology, St. Vincent Hospital, College of Medicine, The Catholic University of Korea, Seoul, Korea; Department of Anesthesiology, School of Medicine, Hallym University, Seoul, Korea; Department of Anesthesiology and Pain Management, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas

Address correspondence to Paul F. White, PhD, MD, FANZCA, Department of Anesthesiology & Pain Management, University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd, F2.208, Dallas, TX 75390-9068. Address e-mail to paul.white{at}utsouthwestern.edu

	Abstract

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Because noise in the operating room has been alleged to interfere with the ability to sedate patients before surgery, we evaluated the effect of noise on the Bispectral index (BIS) value during propofol sedation. Thirty unpremedicated patients were studied before the start of surgery while receiving propofol sedation on two separate occasions according to a randomized, crossover protocol design. After achieving a stable baseline BIS value of either 75 or 80 with a target-controlled infusion of propofol, an external sound source administered noise at 50, 80, 110, and 120 dB. The changes in the BIS value were recorded over a 1-min interval at each noise level. In the BIS 75 group, increasing levels of noise did not significantly alter the BIS value. However, in the BIS 80 group, the BIS values at 80, 110, and 120 dB were significantly higher compared to the value at 50 dB. In conclusion, experimental noise increases the BIS and appears to have a greater effect on the BIS value at "lighter" levels of propofol sedation.

IMPLICATIONS: Experimental noise levels can increase the Bispectral index (BIS) values during propofol sedation in the operating room. However, the magnitude of the BIS response is influenced by the depth of sedation.

	Introduction

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Noise levels in the operating room (OR) range from 55 to 86 dB, depending on the type of surgery being performed (1). With technological advances in anesthesiology, an ever-increasing number of auditory alarms are also sounding during surgery (2). It has been alleged that noise in the OR may interfere with the ability of anesthesia providers to achieve a stable level of sedation for patients undergoing surgical procedures with local anesthesia and IV sedation as part of a monitored anesthesia care (MAC) technique (3).

Given the increasing use of MAC techniques as cost-effective alternatives to both general and regional anesthesia (4,5), it is important to understand the effect of noise on the ability to maintain a stable level of sedation during MAC. Fluctuations in the electroencephalographic (EEG) Bispectral index (BIS) value have been alleged to reflect changes in the clinical state of sedation and hypnosis (6). Furthermore, the BIS has been previously shown to correlate with the depth of both midazolam- (7) and propofol- (8) induced sedation. Therefore, we designed a study to assess the effect of experimental noise on BIS values at two different levels of propofol sedation. We hypothesized that noise would increase the BIS value at a stable level of sedation.

	Methods

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After obtaining IRB approval and written, informed consent, 30 healthy unpremedicated adult outpatients (age 34 ± 11 yr, weight 63 ± 4 kg) were studied according to a randomized, cross-over protocol design. All study patients had been scheduled to undergo two consecutive local anesthetic-based otolaryngologic procedures (e.g., nasal septal surgery or endoscopic sinus surgery) 2–3 days apart. The effect of experimental noise on the BIS value was studied at two different levels of propofol sedation. Patients initially assigned to the BIS 80 group were assigned to the BIS 75 group for the second procedure and vice versa.

An otoscopic examination was performed prior to a standardized hearing test to remove any occlusive cerumen. All study patients underwent pure tone audiometry testing to exclude those with preexisting hearing deficits. Patients with a sound detectability threshold 15 dB at any frequency were considered abnormal and excluded from participating in this study. Other exclusionary criteria included neurological or psychiatric diseases, unstable cardiovascular diseases, clinically significant liver or renal diseases, and a history of drug abuse or current use of centrally acting medications.

A preliminary survey of noise levels was performed in the OR designated for these surgical procedures. The ambient (background) noise levels were measured with a sound level meter (Model LD824; Larson-Davis Inc., Provo, UT) for a 5-min interval during induction of sedation using the same equipment used during the sedation study. The recording instrument was positioned near the patients’ head with the microphone at the same level as the patient’s ear. Baseline recordings of EEG-BIS values were also obtained during this 5-min interval using a BIS monitor (Model A-2000, version 3; Aspect Medical Systems Inc., Natick, MA).

A propofol target-controlled infusion (TCI) was initiated at an initial target concentration (C_T) of 2 µg/mL IV using a Diprifusor^TM (Master TCI; Fresenius Vial, SA, Brezins, France) in the flash mode until the BIS value decreased to 80 (BIS 80 group) or to 75 (BIS 75 group) under "background" OR noise conditions (Table 1). The TCI set point was adjusted to maintain the targeted BIS value for a 5-min interval prior to applying the external sound source. The elapsed time and propofol dosage requirements to achieve the target BIS value were recorded. A modified observers’ assessment of alertness/sedation (OAA/S) scale (1 = asleep/unresponsive to 5 = awake/alert) (9) was used to quantify the sedation level before initiating the noise testing. Hemodynamic variables (e.g., systolic and diastolic blood pressure and heart rate) were also recorded at 1-min intervals throughout the study period.

Data were analyzed by paired Student’s t-test and analysis of variance followed by a post hoc test as appropriate using SPSS (Davis, CA) software. Data are expressed as mean values ± SD or median values, with P values < 0.05 considered statistically significant.

	Results

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The baseline EEG-BIS value, OAA/S score, and background OR noise levels were similar in the two treatment groups (Table 1). Compared with the values for the BIS 80 group, the effect site propofol concentration was larger and the elapsed time to achieve the targeted BIS value was longer in the BIS 75 group. However, these differences were not statistically significant. In addition, the systolic and diastolic blood pressure values were only significantly decreased (versus baseline values) in the BIS 75 group (Table 1).

In the BIS 80 group, the BIS values at 80, 110, and 120 dB were significantly increased compared with the value at 50 dB (Fig. 1). Additionally, the BIS values at 110 and 120 dB were significantly increased compared with the value at 80 dB in the BIS 80 group. In contrast, the BIS values were not significantly increased with increasing noise levels from 50 to 120 dB in the BIS 75 group (Fig. 1).

View larger version (24K): [in this window] [in a new window]   Figure 1. Effect of experimental noise on Bispectral index (BIS) values in the two propofol sedation groups. Values are mean ± SD. *P < 0.05 compared with BIS at 50 dB; = P < 0.05 compared with BIS at 50 and 80 dB in the BIS 80 group.

	Discussion

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Shapiro and Berland (1) reported that noise levels in the ORs ranged from 55 to 86 dB. We found that BIS values were increased at noise levels of 80 dB and above in the less deeply sedated (BIS 80) group. The experimental noise levels applied during propofol sedation in the current study did not appear to elicit a clinically apparent arousal response. However, it was not possible to assess the OAA/S score at the time the experimental noise stimulus was applied without disturbing the patient.

The BIS values of sedated patients in the intensive care unit increase during nursing interventions and physiotherapy (12). Although these authors suggested that arousal resulting from mechanical stimulation or ambient noise was responsible for the increase in the BIS values, they were unable to demonstrate that insertion of earplugs had any effect on the BIS values of intensive care unit patients sedated with a midazolam infusion (13). Because the effect of noise on the BIS value appears to be dependent on the level of sedation, the use of earplugs might have been more effective if the intensive care unit patients had been less deeply sedated.

The EEG-BIS is a multivariate measure of the depressant effect of sedative drugs on the central nervous system (14). Because the effect of noise has been shown to be dependent on the anesthetic depth (15), it is not surprising that the effect of noise on the EEG-BIS is influenced by the level of sedation. These data also suggest that experimental noise may elicit an arousal response at light levels of sedation, and it theoretically could increase the potential for intraoperative recall. However, further studies are required to evaluate this controversial issue.

The primary shortcoming of our study design relates to the fact that the experimental ("non-OR") noise was applied to only one ear during the testing period; "dichotic" noise may be more disturbing to the patient than the usual binaural presentation. Unfortunately, the audiometer used as our external sound source could be applied to only one ear. In order to minimize the confounding effect of random OR noise, "white band" noise was applied simultaneously to the opposite ear at the same sound pressure levels. Because the same external sound pressures were applied simultaneously to both ears during the testing intervals, the noise presentation was not true dichotic noise.

In conclusion, experimental noise can alter the EEG-BIS value during MAC sedation with propofol. Although this effect was only apparent at "lighter" levels of propofol-induced sedation, these data are consistent with the clinical perception that OR noise may elicit an arousal response.

	Acknowledgments

 
Supported, in part, by the Catholic University of Korea, St. Vincent Hospital and the White Mountain Institute of Los Altos (Dr. P. F. White, President).

	References

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