Minimal alveolar concentration of sevoflurane for maintaining bispectral index below 50 in morbidly obese patients

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Abstract

Background

Morbid obesity is associated with important differences in pharmacokinetics and pharmacodynamics. The aim of this study was to determine minimum alveolar concentration of sevoflurane for maintaining bispectral index (BIS) below 50 (MACBIS50) in morbidly obese patients undergoing bariatric surgery using the Continual Reassessment Method (CRM) method.

Methods

Twenty-four morbidly obese patients (body mass index 40–70 kg/m2) were enrolled in our study. Twenty minutes following pre-medication with fentanyl 100 μg, general anaesthesia was induced using propofol 2 mg/kg and cisatracurium 2 mg/kg to facilitate tracheal intubation. The lowest BIS score was recorded following induction. Thereafter, when BIS began to increase > 60, maintenance of anaesthesia was started with a pre-determined end-tidal sevoflurane concentration (ETSevo) and maintained for 10 min followed by 1-min assessment of BIS taken at 10-s intervals to determine the ETSevo. The ETSevo leading to a probability close to 80% success was calculated using the CRM, and the MACBIS50 leading to 50% success was calculated by fitting the data to a dose-probability sigmoid curve, respectively.

Results

The ETSevo able to maintain BIS value below 50 was 1.8% in 67% [95% confidence interval (CI) 0.44–0.86] and higher in the remaining 33% of the patients and the ETSevo leading to a BIS value below 50 in 50% of the patients (MACBIS50) was 1.6 ± 0.10%.

Conclusions

The calculated values (1.8% and 1.6%) were higher than that previously reported in normal adult patients (0.97%; 95% CI 0.89–1.1%) and less than that reported in children (2.8%; 95% CI 2.7–3.1%).

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