Bispectral EEG Index Monitoring of High-Dose Nitrous Oxide: METHODS

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30 Nov 2009

With the approval of The Ohio State University IRB Human Subjects Committee, 22 healthy volunteer dental students, both male and female, were recruited in pairs to participate in a single-blinded clinical protocol. A power analysis was performed on data from previous similar studies and indicated that an N (least significantnumber) greater than 15 should provide statistically and critically significant data (a = .05, p — .20 or less). Each participant completed a medical history and signed an investigational consent and a medical release. Potential subjects with any neurologic or psychiatric disease, relative or absolute contraindication to N20 or sevoflurane including pregnancy by patient self-report, and any recent prescription or illicit medication use were excluded. As an added precaution beyond usual accepted clinical practice for N20, all escorted subjects were NPO (nothing per os) for 8 hours and all research sessions were conducted in the oral surgery clinic with general anesthesia facilities and personnel available, including drugs and equipment for the treatment of malignant hyperthermia.

All subjects were placed in a semisupine position with feet slightly elevated in a standard dental chair and monitored by an automatic blood pressure monitor, pulse oximeter, skin temperature probe, and electrocardiograph (EKG) (Figure 2). Nitrous oxide and oxygen as well as sevoflurane were administered via an Accutron and Fraser Harlake Boyle System with an Ohmeda Sev-otec 5 Vaporizer nasal hood (Phoenix, Ariz), and scavenger through a semiopen, nonrebreathing, modified Magill system using individually adjusted total flows of 4-7 L/min in order to maintain a reasonably expanded reservoir bag. Inspired and expired ET (end tidal) concentrations of 02 and N20 or sevoflurane were continually measured by one airway gas analyzer (Millennia Model 3500, Invivo Research Inc, Orlando, Fla) and recorded at 5-minute intervals by the principal investigator. ET samples were collected via a nasal sampling cannula placed under the nasal hood, while inspired samples were taken immediately after supply gases left the sedation/anesthesia machine. cheap levitra professional

Figure 2. Research setup

Figure 2. Research setup.

A standard frontal-temporal montage was recorded using BIS sensor electrodes applied to the scalp-temple after mild cotton 2×2 abrasion, resulting in contact impedance under 5 kOhms, as recommended for BIS monitoring by Aspect Medical Systems. An Aspect A-1050 monitor visible only to an assistant investigator was used with default settings, including a 30-second smoothing time. BIS readings were recorded every 5 minutes for statistical evaluation, as were frontotemporal electromyography (EMG) logarithmic decibel values rounded to the nearest 5 db (30, 35, 40, 45, 50, 55). Blood pressure (BP), mean arterial pressure (MAP), heart rate (HR), oxygen saturation by pulse oximetry (Sp02), end tidal nasal hood C02 (ETC02), respiratory rate (RR), and temperature (Temp) were also recorded every 5 minutes by a second Millennia Model 3500, also visible only to an assistant investigator. Observer’s Assessment of Alertness/Sedation Scale (OAA/S; Table 1) readings were determined and recorded by the principal investigator every 5 minutes.

Table 1. Observer’s Assessment of Alertness/Sedation Scale

Facial Composite
Responsiveness Speech Expression Eyes Score
Responds readily to name spoken in Normal Normal Clear, no ptosis 5 (alert)
normal tone
Lethargic response to name spoken Mild slowing or thick- Mild relaxation Glazed or mild ptosis 4
in normal tone ening (less than half the eye)
Responds only after name is called Slurring or prominent Marked relaxation Glazed and marked pto 3
loudly and/or repeatedly slowing (slack jaw) sis (half the eye or
Responds only after mild prodding Few recognizable more) 2
or shaking words
Does not respond to mild prodding 1 (asleep)
or shaking

After at least 10 minutes of instrument calibrations and baseline measurements, each subject was given, by the principal investigator, at least 5 minutes of 100% 02. This was followed by sequentially increasing N20 concentrations at 15-minute intervals beginning at 30% concentration (30, 40, 50%, and, if clinically acceptable, 60 and 70% N20). At a second session, increasing concentrations of sevoflurane beginning at 0.2% concentration (0.2, 0.4, 0.6, 0.8%) in 02 were administered, always concluding with 15 minutes of 100% 02. An assistant investigator read and recorded all data except for the concentrations of N20, sevoflurane, and 02, as well as OAA/S scores, which were recorded by the principal investigator. The sequence of N20 and sevoflurane sessions was randomized. Any subject exhibiting signs and/or symptoms of oversedation during a sessionreceived a reduced dosage of inhaled agent or 100% 02. Subjects were shown pictures of simple objects for 5 seconds prior to increasing dosage levels. Subjects also heard test sounds (prerecorded words) to remember at the end of all but the first dosage level. Recall and recognition were tested after final recovery during the last 2 minutes of 100% 02 administration and via a 24-hour postsession questionnaire. For recall, subjects were asked if they remembered seeing any object pictures or hearing any words. For recognition, subjects were then shown composite pictures of 6 simple objects, including the 1 object used, and were asked to select any familiar item(s). Additionally, all subjects completed a poststudy questionnaire following their last session.
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Quantitative changes in BIS caused by gas inhalation dose-response were compared using nonparametric analysis of variance with repeated measures (Friedman’s test) and the Spearman correlation test, with P = .025 considered significant. Parametric data, including BP, Sp02, MAP, HR, RR, EMG, and Temp, were analyzed using paired and unpaired t tests, with P < .05 considered significant. Each subject participated in 2 randomized test sessions, 1 with N20 and 1 with sevoflurane. Additionally, each subject acted as his/her own control. Inspired concentrations and end tidal gas concentrations with a dental nasal hood were compared. Poststudy and 24-hour postsession questionnaires were evaluated and correlated with sedation data from this study (OAA/S, amnesia, BIS) and prestudy baseline group memory testing. Simple yes/no amnesia/memory nominal data were evaluated by chi-square likelihood ratios.

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