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AMBULATORY ANESTHESIOLOGY

Listening to Music During Anesthesia Does Not Reduce the Sevoflurane Concentration Needed to Maintain a Constant Bispectral Index

Peter Szmuk, MD^*, Nimrod Aroyo, Tiberiu Ezri, MD, Gleb Muzikant, MD^||, Marian Weisenberg, MD^||¶, and Daniel I. Sessler, MD

From the *Department of Anesthesia, University of Texas Southwestern Medical School and Children’s Medical Center at Dallas, Texas; Outcomes Research Consortium, The Cleveland Clinic, Cleveland, Ohio; Department of Anesthesia, Tel Aviv University, Israel; The Edith Wolfson Medical Center, Holon, Israel; ||Department of Anesthesia, The Edith Wolfson Medical Center, Holon, Israel; and ¶Sackler School of Medicine, Tel Aviv University, Israel.

Address correspondence to Peter Szmuk, MD, Department of Anesthesia, Children’s Medical Center, University of Texas Southwestern Medical School, Dallas, TX 75235. Address e-mail to pszmuk{at}gmail.com or peter.szmuk{at}UTSouthwestern.edu.

Abstract

BACKGROUND: Music reduces stress responses in awake subjects. However, there remains controversy about the role of music or therapeutic suggestions during general anesthesia and postoperative recovery. We thus tested the hypothesis that intraoperative exposure to soothing music reduces the end-tidal concentration of sevoflurane (ETSevo) necessary to maintain bispectral index (BIS) near 50 during laparoscopic surgery.

METHODS: Forty patients, aged 40–60 yrs, ASA I and II, undergoing laparoscopic hernias or cholecystectomy under general anesthesia were studied. All patients were connected to a BIS monitor. Anesthesia was induced with fentanyl 2 µg/kg, sevoflurane in oxygen, rocuronium (0.6 mg/kg), and maintained with sevoflurane in oxygen and 50% nitrous oxide, with an infusion of fentanyl (1 µg · kg^–1 · h^–1). Sevoflurane was titrated to maintain BIS near 50 throughout the procedure. Patients were randomly assigned to either listen to music or not.

RESULTS: The ETSevo necessary to maintain a BIS near 50 was virtually identical in patients who listened to music (1.29 ± 0.33%) and those who did not (1.27 ± 0.33%, P = 0.84). Patients who listened to music reported slightly less pain, but the difference was not statistically significant. Mean arterial blood pressure was slightly higher in patients who listened to music (101 ± 11 mm Hg) than in those who did not (94 ± 10 mm Hg, P = 0.040).

CONCLUSIONS: The end-tidal concentration of sevoflurane required to maintain BIS near 50 during laparoscopic cholecystectomy was virtually identical in patients exposed to music or not. Although previous work suggests that music reduces preoperative stress and may be useful during sedation, our results do not support the use of music during surgery.

Surgery is understandably stressful for most patients. Strategies to moderate the stress associated with surgery include preoperative assurance and relaxing medications. Auditory input modulates human stress responses1 and may thus be therapeutic in various circumstances,2,3 especially by reducing preoperative and postoperative anxiety and postoperative pain4–6 Consequently, Koch et al. proposed that the calming effects of music might ameliorate stress associated with unfamiliar environments, loss of control, and fear of disfigurement—all of which are common concerns in surgical patients.7

Although somewhat controversial,8–10 there is considerable evidence that listening to music improves physiological and emotional responses of patients awaiting surgery.11 For example, the calming effect of music augments the sedative effect of preoperative midazolam12 and patients listening to music require less midazolam during spinal anesthesia.13 Additionally, in patients who undergo urologic procedures with spinal anesthesia, intraoperative music decreases sedative and analgesic requirements.7

That soothing music might reduce stress and stress-related affective responses including pain in unanesthetized subjects is perhaps unsurprising.14 However, there is also evidence that listening to music, sounds, or therapeutic suggestions during general anesthesia may also improve postoperative recovery and reduce the need for analgesia.14–18 Other studies, though, were unable to confirm the benefits of intraoperative auditory stimuli.8–10,19 Furthermore, Migneault et al. found that music did not reduce the neuroendocrine response to stress or perioperative opioid consumption.9 The potential benefits of providing salutary auditory stimuli, such as music, during general anesthesia thus remain controversial.

Previous studies of intraoperative auditory stimuli have evaluated postoperative outcomes such as analgesic requirement. But if music, in fact, has the ability to moderate the stress response to surgery, it would seem likely to also reduce anesthetic requirements. The bispectral index (BIS) is widely used to quantify the hypnotic effects of anesthetics.20 The amount of a volatile anesthetic required to maintain a given BIS is thus a measure of anesthetic requirement.21 We therefore tested the hypothesis that intraoperative exposure to soothing music reduces the end-tidal concentration of sevoflurane (ET_Sevo) necessary to maintain BIS near 50 during laparoscopic surgery.

METHODS

After Institutional review board approval and obtaining patients’ informed consent, 40 patients were included in this randomized, double-blind, controlled trial. Patients aged 40–60 yrs, ASA Physical Status I and II, undergoing laparoscopic hernias or cholecystectomy under general anesthesia were included. Patients were excluded from participation if they were scheduled to receive regional or combined regional and general anesthesia, had partial or complete deafness, were mentally impaired, used analgesics daily, or were known alcoholics or users of illicit drugs.

Protocol
Patients were not premedicated, but chronic medications (e.g., antihypertensives) were given the morning of surgery. All patients were connected to a BIS monitor (Aspect Medical Systems, Newton, MA). Anesthesia was induced by inhalation of sevoflurane in 100% oxygen until the end-tidal concentration reached 6%. Fentanyl (2 µg/kg) and rocuronium (0.6 mg/kg) were added, and then the trachea was intubated. Additional rocuronium was administered as clinically appropriate at the discretion of the anesthesia provider. Anesthesia was maintained with sevoflurane in oxygen and 50% nitrous oxide (N₂O), with an infusion of fentanyl (1 µg · kg^–1 · h^–1). Sevoflurane was titrated to maintain BIS near 50 throughout the procedure. The sevoflurane concentration was adjusted as necessary to maintain BIS near 50 (45–55). Our general strategy was to increase the inspired sevoflurane concentration 0.1% for each unit that BIS increased above 50, and to comparably decrease sevoflurane administration when BIS was <50.

After tracheal intubation, patients’ lungs were mechanically ventilated with a tidal volume of 7 mL/Kg to maintain an end-tidal carbon dioxide partial pressure between 30 and 35 mm Hg. The peritoneum was inflated to a pressure of 15 cm H₂O. Core temperature, as measured by a distal esophageal probe, was maintained near 36°C with forced-air.

Patients were randomly assigned either to hear music or not. Patients in both groups thus received headphones before induction of anesthesia and wore headphones throughout surgery. Allocation was based on computer-generated codes that were maintained in sequentially numbered opaque envelopes until after induction of anesthesia. Patients assigned to music were allowed to choose the style, but their choices were restricted to classical, pop-rock, or Israeli music. The sound-system volume was adjusted to the comfort level of individual patients using a random tape before induction of anesthesia. Designated sounds (music or no music) started 5 min after induction of anesthesia and continued until sevoflurane administration was stopped. All operating room personnel, including the anesthesiologist, were blinded to group allocation.

Hypertension (systolic blood pressure 190 mm Hg and/or diastolic blood pressure 110 mm Hg) with or without tachycardia (heart rate>90) was treated by increasing the sevoflurane concentration if the BIS exceeded 50 or boluses of 10 mg of esmolol if the BIS was 50. Bradycardia <50 bpm with normal arterial blood pressure was not treated, while bradycardia with hypotension was treated with boluses of ephedrine (5 mg/bolus) and atropine (0.5 mg) if one dose of ephedrine was ineffective.

Sevoflurane was stopped after skin closure. Patients’ tracheas were subsequently extubated, and the patients transferred to the recovery unit.

Postoperatively in the postanesthesia care unit, if the pain visual analog scale was 3, patients received boluses of 20 mg IV meperidine every 10 min, up to 1 mg/kg. Opioid analgesia was supplemented by administration of 2 g IV dipyrone.

Measurements
Morphometric and demographic characteristics were recorded, along with the duration of anesthesia. End-tidal sevoflurane partial pressure required to maintain BIS near 50 was determined by an AS/3 Anesthesia Monitor (Datex-Engstrom Instrumentarium, Helsinki, Finland). Starting after intubation, BIS and ETSevo were recorded every 5 min until extubation. For both the BIS and ETSevo, at each measurement time point, the three consecutive values read on the screen were recorded and these values were averaged to give a single value which was recorded for each.

Heart rate, mean arterial blood pressure, and end-tidal Pco₂ and N₂O concentrations were measured at 5-min intervals from intubation until extubation, at skin incision, and after abdominal inflation. Core temperature was measured in the distal esophagus. Arterial blood pressure was measured oscillometrically. Episodes of hypertension and intraoperative movement were also recorded. Rocuronium and fentanyl doses were recorded, as were the fraction of patients requiring ephedrine.

Time to eye opening was considered to be the time from sevoflurane cessation until patients opened their eyes in response to command. Patients reported their pain 15 min after surgery using a visual analog pain scale (a 10-cm scale where 0 cm = no pain and 10 cm = worst pain imaginable).

Statistical Analysis
A sample-size estimate was performed for an unpaired, two-tailed t-test, assuming equal variances for the two groups. Considering a 20% difference in ETSevo (approximately 0.3%) to be clinically important, a sample size of 40 patients (20 patients per group) provided 80% power assuming a mean ETSevo of 1.4 with a standard deviation of 0.28 in each group.

Our primary outcome was the ETSevo required to maintain a BIS near 50. Values were averaged over time within each patient, excluding the first and last 15 min. These values were then averaged within each treatment group. Sevoflurane partial pressures in each group were compared using two-tailed, unpaired t-tests.

Demographic and morphometric data for the two groups were compared using unpaired, two-tailed t-tests for continuous data and ² for categorical data. Appropriate nonparametric tests were substituted when continuous results were not normally distributed. Data are presented as means ± sd for continuous data and counts for categorical data; P < 0.05 was considered statistically significant.

RESULTS

Morphometric and demographic characteristics and potential confounding factors were similar in patients who listened to music or a tape of operating room sounds during surgery (Table 1).

The major outcomes of the study are shown in Table 2. Most importantly, the ETSevo necessary to maintain BIS near 50 was virtually identical in patients who listened to music (1.29 ± 0.33%) and those who listened to routine operating room noise (1.27 ± 0.33%, P = 0.84). These results provided an 80% power to detect a difference of 0.3% in ETSevo between the two study groups.

Patients who listened to music reported slightly less pain, but the difference was not statistically significant. Mean arterial blood pressure was slightly higher in patients who listened to music (101 ± 11 mm Hg) than in those who did not (94 ± 10 mm Hg, P = 0.040). Postoperative analgesic requirements were similar in each group.

DISCUSSION

Our results do not confirm our hypothesis that music decreases the ETSevo necessary to maintain the BIS value near 50 in patients undergoing laparoscopic surgery. This result conflicts with previous studies showing that listening to music, sounds, or therapeutic suggestions during general anesthesia improves postoperative recovery and reduces analgesic requirements 14–18 and is consistent with others showing no benefit.8–10,19

Explicit memory of auditory stimuli, such as words, stories, poems, and music, is rare during general anesthesia22 and probably does not occur at MAC fractions exceeding 0.6.23,24 Implicit memory testing is much more complicated, and results vary widely depending on the test and anesthesia method used. Some studies show that 0.4–0.45 MAC isoflurane abolishes both explicit and implicit memory.25 However, even in the absence of postoperative recall, processing of auditory stimuli still occurs during general anesthesia.26 Additionally, brainstem auditory- evoked response waveforms are relatively resistant to both IV and inhaled drugs. Even large doses of fentanyl or isoflurane, for example, do not increase response latency.27 There is thus considerable potential for auditory stimuli to moderate the neurocognitive responses to surgery.

Nonetheless, potentially salutary effects of intraoperative music seem most likely at relatively light planes of anesthesia, although two studies report beneficial effects of intraoperative music on postoperative pain at up to one MAC.9,14 As in other studies,9 we avoided deep anesthesia and instead titrated BIS to near 50 which corresponded to slightly more than one MAC, considering the additive effects of sevoflurane and N₂O. But even at this modest anesthetic level, we were unable to demonstrate that music reduced anesthetic requirement. Although it remains possible that music would be helpful at yet lighter anesthetic levels, the risk of awareness precludes routine use of lower concentrations.28

Volume of the music was adjusted preoperatively to patient comfort. However, we do not know if the volume was adequate during anesthesia and it thus remains possible that higher volumes may have been effective. Instrumental music with slow, flowing rhythms that duplicate a heart rate of 60–80 bpm is generally thought to be relaxing.29 Pain reduction through relaxing music has been demonstrated by using different musical styles such as soothing music by synthesizer, harp, piano, or orchestra, slow jazz,30–32 synthesizer music accompanied by sounds of sea waves,15 and classical music.33 Although we offered our patients a choice of three different musical styles, it remains possible—although perhaps unlikely—that different types of music might have proven more effective. It is also possible that music reduces anesthetic requirement only in patients who are especially anxious preoperatively. However, we did not administer anxiety tests to the patients or evaluate stress hormone concentrations and were thus unable to evaluate this potential confounding factor.

In conclusion, the partial pressure of sevoflurane required to maintain BIS at 50 during laparoscopic surgery was virtually identical in patients exposed to intraoperative music or not. Although previous work suggests that it reduces preoperative stress and may be useful during sedation, our results do not support the use of music during surgery.

Footnotes

Supported by NIH Grant GM 061655 (Bethesda, MD) and the Joseph Drown Foundation (Los Angeles, CA).

Accepted date for publication March 5, 2008.

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