Abstracts of Posters Presented at the International Anesthesia Research Society; 72nd Clinical and Scientific Congress; Orlando, FL; March 7-11, 1998: Ambulatory Anesthesia
Introduction: Recent results demonstrate the usefulness of BIS titration in a propofol/alfentanil/nitrous oxide anesthetic. Patients whose anesthesia was adjusted according to the BIS required less drug, awakened sooner, and were discharged from the PACU earlier. [1-3] The current study investigated BIS monitoring with propofol/isoflurane/nitrous oxide anesthesia.
Methods: Following IRB approval, surgeon approval and written informed consent, 80 elective adult surgical outpatients, were randomized to a Bispectral Index (BIS) guided or standard practice (SP) group. BIS guided patients had their anesthetic titrated to keep the BIS 50 - 65, when clinically possible. Clinicians titrated anesthetics of SP patients according to clinical signs. BIS was recorded but not available in the SP group. Anesthetists in both groups were to awaken their patients as quickly as possible.
Patients received midazolam pre-medication, propofol induction, and anesthesia maintenance of isoflurane (iso) with fentanyl analgesia and nitrous oxide (2L) in oxygen (1L). After intubation, neuromuscular blocking agents were administered if surgically indicated. During maintenance, BIS patients had their anesthetic agents adjusted to maintain BIS in the 50-65 range. Approximately 30 minutes prior to surgery end, anesthesia was reduced, as routinely practiced, to facilitate rapid recovery. In the BIS group, fentanyl doses were discontinued and isoflurane adjusted to achieve a BIS in the "lighter" 65-75 range. Approximately 10 minutes before surgery end, the isoflurane was discontinued, and the patient allowed to recover. N2 O was discontinued at the end of surgery for both groups. Upon wake up, analgesics were administered as required. A value from the printed BIS trend was recorded for each 15 minute interval. The analysis considered isoflurane use over the course of the protocol as well as the relationship between BIS, isoflurane concentration and recovery times.
Results: There were no demographic differences between the groups (p > 0.05). Divided into groups according to patient enrollment, the mean iso concentration was lower for the BIS than the SP group (0.74 +/- 0.22 vs. 0.87 +/- 0.26; p = 0.021). In both groups iso concentration variability increased in the 2nd quarter and decreased over time in remaining quarters. (Figure 1)
Pooling groups, the mean BIS value was inversely correlated with time to leave OR (Eligible for PACU) and PACU discharge (Ready for/Actual PACU 1 D/C). These times are computed from time of last agent off. Correlation of mean iso concentration with the same endpoints were not significant. (Table 1)
Discussion: Clinical utility of the BIS was previously demonstrated in a 302 patient trial, with propofol/alfentanil/nitrous anesthesia with similar group management.  Despite the small sample size (80), high number of attendings ([tilde operator]24) in combination with [tilde operator]35 CRNAs/ residents with 1-8 protocol cases per provider and varying end titration techniques, BIS guided patients had lower maintenance iso concentrations than SP patients. Mean maintenance BIS correlated better than mean end tidal iso concentration with recovery times (p < 0.05) and higher BIS is associated with earlier PACU discharge.
1. Anesth. 1996; (85)3A; A351
2. Anesth. 1996; (85) 3A; A1056
3. Anesth. 1996; (85) 3A; A468
4. Gan et al, Anesth. 1997 (in press)