Analgesia: Pain Medicine: Brief Report
Two auditory impulses with a difference in frequency between 1 and 30 Hz, when presented simultaneously to both ears, can result in a perception of the difference between the 2 tones as a single auditory binaural beat.1–3 Such binaural beats have been described to be brainstem responses that originate in the superior olivary nucleus of each cerebral hemisphere and are thought to cause hemispheric synchronization.1,2 This modality is promoted as a treatment for stress, anxiety, pain control, and other conditions (http://www.monroeinstitue.org). Kliempt et al.3 reported that subjects who were exposed to Hemisync® sounds under general anesthesia consumed significantly less intraoperative fentanyl. Lewis et al.4 confirmed these findings in a group of patients undergoing bariatric surgery who were treated with Hemisync sounds. In contrast, Dabu-Bondoc et al.5 found that Hemisync sounds do not reduce the hypnotic component of an anesthetic state in a group of patients undergoing outpatient surgery. None of these previous studies, however, examined the effects of Hemisync sounds on postoperative pain and analgesic requirements. As such, we designed the following randomized controlled study to also examine the effect of Hemisync on postoperative pain and analgesic requirements.
Subjects included outpatients aged 18–65 years, ASA physical status I–II, undergoing outpatient surgery requiring general anesthesia. IRB approval was obtained and each patient consented. Subjects with a history of consuming psychiatric medications or hearing impairment were excluded. On the day of surgery, demographic data and trait anxiety (State Trait Anxiety Index) were obtained from each participant. The State Trait Anxiety Index is a widely used and well-validated self-report anxiety assessment instrument.6
Subjects were randomized into 3 groups, and all subjects were given headsets. The treatment group received Hemisync tapes marketed as “surgical support,” the music group received a music tape of their choice, and the control (placebo) group received a blank cassette tape that produced white noise when played. In the preoperative holding area, all subjects received the designated intervention for 30 min via a headset. None of the subjects was offered any sedative premedication. The anesthesiologist and the postanesthesia care unit (PACU) caregiver were blinded to group assignment of each patient. A blank cassette tape was used for the control group, and all tapes looked similar. Headsets from all groups of participants were removed before the patients entered the operating room and restarted after the induction of anesthesia for all participants. Headsets were discontinued at the conclusion of surgery, patients were allowed to recover in the PACU, and data collection was continued.
In the operating room, anesthetic technique was standardized for all patients, and a bispectral index (BIS) monitor (Aspect Medical Systems, Natick, MA) was applied on all patients. General anesthesia was induced with propofol (1 mg/kg), followed by a single dose of fentanyl (1 μg/kg). After observing clinical responses and allowing for BIS equilibration (60 s), additional incremental doses of propofol (20–30 mg) were given to reach a BIS value of 40–60. Vecuronium (0.1 mg/kg) was given as required to facilitate endotracheal intubation. A laryngeal mask airway was used for procedures not requiring endotracheal intubation. Anesthesia was maintained with oxygen/nitrous oxide (1:2 L/min), and a propofol infusion 100–200 μg/min, adjusted to maintain BIS in the 40–60 range. A heart rate and/or arterial blood pressure beyond 20% of baseline were treated with fentanyl in 25-μg increments. To control for the confounding effects of various types of surgeries and different surgeons, patients were matched to control for specific surgical procedure and surgeon. In the PACU, patients were given no fentanyl when the visual analog scale (VAS) score was 0–3, 25 μg of fentanyl for a VAS score of 4–6, and 50 μg of fentanyl for a VAS score of 7–10. If patients required >200 μg of fentanyl, analgesia was then supplemented with morphine in increments of 1–2 mg until the pain score reduced to 0–3. Amount of analgesics used, incidence of nausea/vomiting, oxygen desaturation, and pain (VAS) scores on arrival and every 10–20 min thereafter were measured in the PACU and then 24 h postoperatively. An independent observer who was blinded to group assignment collected data in the PACU. Subjects were all prescribed Percocet for pain after discharge. Subjects were followed up via a phone call 24 h after surgery.
The primary end point of this study was perioperative analgesic consumption. A sample size of 20 subjects in each group (yielding a total number of 60) provided 80% power to detect an effect size difference in consumption of 0.4 among groups, with an α of 0.05. The secondary end points included postoperative measurement of pain (VAS) scores. All analgesics consumed (morphine, fentanyl, and Percocet) were converted to IV morphine milligram equivalents. Comparisons among groups were analyzed with χ2 test for categorical data and 1-way analysis of variance for continuous data. Data are presented as mean ± sd. P < 0.05 was considered significant. Analysis was conducted using SPSS statistical software (SPSS, Chicago, IL).
Demographic and personality characteristics did not differ among the 3 groups and are presented in Table 1. Similarly, perioperative propofol consumption adjusted for weight and length of procedure did not differ among the 3 groups (Table 2). Analysis showed that the Hemisync group required significantly less fentanyl during the anesthetic procedure compared with the music or control group (Table 2, P = 0.046). Post hoc analysis showed that the difference was significantly lower between the Hemisync and the music group (P = 0.024) and between the Hemisync and the control group (P = 0.045).
Analgesic consumption was similar among the 3 groups both in the PACU and during the first 24 h postoperatively (P = not significant). Pain VAS scores at 1 h in the PACU and at 24 h after surgery were significantly lower in the treatment group compared with the music and control groups (P = 0.02 and P = 0.005, respectively). Discharge time from the PACU was also lesser in the Hemisync group compared with the other 2 groups (P = 0.048). The 3 groups were similar in terms of incidence of nausea and vomiting, oxygen desaturation, recall, and patient satisfaction. Finally, intraoperative average heart rate and arterial blood pressure did not differ significantly among the 3 groups (P = not significant).
Consistent with previous studies,3,4 our study demonstrates that listening to Hemisync sounds results in decreased use of intraoperative analgesics in surgical patients who undergo general anesthesia. This study, however, differed from previous studies3–5 because it also explored postoperative analgesic requirements, and we provided Hemisync music both before and during general anesthesia. The reader should note that in previous studies, subjects listened to Hemisync sounds only while under general anesthesia. We found that although analgesic consumption in the PACU was similar among the 3 study groups, analgesic consumption after discharge from the PACU was about 30% lower in the Hemisync group compared with the control and music groups, respectively, but post hoc comparisons among groups did not show consistent statistical difference. The negative effect in analgesic consumption in the PACU may have been due to the following. First, approximately 25% of the subjects underwent relatively minor and short surgical procedures, such as hysteroscopy and tubal ligation, which generally required relatively minimal postoperative analgesia or even no analgesia at all. This could have potentially diluted the positive effect of the intervention. Second, this could be a case of a Type II statistical error. That is, a false negative effect secondary to a sample size that is not large enough. Although we have calculated an a priori sample size, this calculation may have been based on a better than expected effect size.
Pain (VAS) scores were significantly decreased at 1 h (T60) after arrival in the PACU and at 24 h after discharge. These results were consistent with delay in manifestation of effect in analgesic consumption. The consistency of effect in 2 parameters seems to be strongly suggestive of the potential positive effect of listening to Hemisync on postoperative pain requirements. Why there was a delay in effect by the intervention also remains to be further investigated.
In conclusion, under the condition of this study, we found that listening to Hemisync sounds before and during surgery results in decreased consumption of intraoperative analgesics as well as lower postoperative pain scores and earlier discharge from the PACU. This consistency of effect in 2 parameters seems to be strongly suggestive of the potential positive effect of listening to Hemisync on postoperative pain requirements.
1. Smith JC, Marsh JT, Brown WS. Far-field recorded frequency-following responses: evidence for the locus of brainstem sources. Electroencephalogr Clin Neurophysiol 1975;39:465–72
2. Atwater F. The Hemi-Sync process. The Monroe Institute, 1997
3. Kliempt P, Ruta D, Ogston S, Landeck A, Martay K. Hemispheric-synchronisation during anaesthesia: a double-blind randomized trial using audiotapes for intra-operative nociception control. Anaesthesia 1999;54:769–73
4. Lewis AK, Osborn IP, Roth R. The effect of hemispheric synchronization on intraoperative analgesia. Anesth Analg 2004;98:533–6
5. Dabu-Bondoc S, Drummond-Lewis J, Gaal D, McGinn M, Caldwell-Andrews AA, Kain ZN. Hemispheric synchronized sounds and intraoperative anesthetic requirements. Anesth Analg 2003;97:772–5
© 2010 International Anesthesia Research Society
6. Spielberger CD. Manual for the State-Trait Anxiety Inventory. Palo Alto, CA: Consulting Psychologists Press, 1983