Purpose: Sevoflurane, with a relative low blood-gas partition coefficient, is an ideal anesthetic to achieve rapid offset and recovery from general anesthesia. This study will determine the profiles of four concentration-time curves to characterize the pharmacokinetics of sevoflurane elimination.
Methods: Eight patients (aged 54-76 years) undergoing coronary arterial bypass grafting surgery were enrolled in this study. At the end of surgery, anesthetic gas and blood were sampled 20 min before and after stopping sevoflurane administration, with prior maintenance of a fixed 5% inspired sevoflurane (CIsev) in 6 L/min oxygen flow for 60 min before the cessation of sevoflurane administration for the subsequent 20 min elimination. An infrared analyzer was used to determine both CIsev and end-tidal sevoflurane (CEsev). The sevoflurane concentrations in the internal jugular-bulb (Jsev), arterial (Asev) and pulmonary arterial blood (PAsev) were analyzed by gas chromatography, and cardiac output was measured using an Opti-Q pulmonary artery catheter.
Results: A bi-exponential decay function was the best fit for the CEsev,Jsev, Asev, and PAsev time curves. There were two distinct components, the initial 5-min fast or distribution phase and the subsequent 15-min slow or elimination phase. Before cessation of the sevoflurane supplement, the step-down concentration of sevoflurane was listed in the following order: CIsev > CEsev > Asev ≧ Jsev > PAsev. During the elimination phase, the fastest decay occurred in CEsev, followed by Jsev, Asev and PAsev. Therefore, a reverse step-down pattern was observed (PAsev > Asev ≧ Jsev > CEsev) after 20 min. The ratio of Asev to CEsev was 89% at baseline before stopping sevoflurane administration, but the ratio of Asev to CEsev increased to 128% at the twentieth min of the sevoflurane elimination phase.
Conclusions: During elimination, the initial washout of sevoflurane from the functional residual capacity of the lungs was reflected in the fast component of the CEsev, Jsev, Asev, and PAsev time curves. In contrast, the slow component was dominated by the tangible effects of the physiological membrane barriers, such as the alveoli-pulmonary capillary and blood-brain barriers.