BACKGROUND: The onset and offset of action of anesthetic gases might be delayed by respiratory changes and gas exchange alterations present in obese patients. In this study, we assessed the influence of obesity on the hysteresis between sevoflurane and its effect as measured by the bispectral index (BIS). Because the use of positive end-expiratory pressure (PEEP) in obese patients has improved gas exchange, we also assessed the influence of PEEP on hysteresis.
METHODS: Fifteen obese and 15 normal-weight patients, ASA physical status I and II, 20 to 50 years old, scheduled to undergo general anesthesia for elective laparoscopic surgery, were prospectively studied. Anesthesia was induced with propofol and maintained with sevoflurane and fentanyl. At the end of surgery and after stable BIS values of 60 to 65, the inspired concentration of sevoflurane was increased to 5 vol% for 5 minutes or until BIS was <40 and then decreased. Sevoflurane transitions were performed once in normal-weight subjects (without PEEP) and twice in obese patients (one without PEEP and one with a PEEP of 8 cm H2O). The hysteresis between sevoflurane end-tidal concentrations and BIS during these transition periods was modeled with an inhibitory Emax model using a population pharmacokinetic/pharmacodynamic (PK/PD) approach with NONMEM VI. A descriptive analysis of sevoflurane inspired and expired concentrations, BIS values, and time to reach different BIS end points was also used to compare the PK and PD characteristics.
RESULTS: All patients completed the study. The data were adequately fit with the PK/PD model. The hysteresis expressed as the effect-site elimination rate constant was not influenced by body mass index or PEEP (P > 0.05). Neither obesity nor PEEP showed any influence on the PK/PD descriptors.
CONCLUSIONS: Our results do not support the hypothesis that obesity prolongs induction or recovery times when sevoflurane, a poorly soluble anesthetic, is used to maintain anesthesia from 90 to 120 minutes.