Feasibility study for the administration of remifentanil based on the difference between response entropy and state entropy

Background. Facial electromyography (FEMG) may have utility in the assessment of nociception during surgery. The difference between state entropy (SE) and response entropy (RE) is an indirect measure of FEMG. This study assesses an automated algorithm for remifentanil administration that is based on maintaining an entropy difference (ED) that is less than an upper boundary condition and greater than a lower boundary condition. Methods. The algorithm was constructed with a development set (n=40), and then automated and studied with a validation set (n=20) of patients undergoing anterior cruciate ligament repair. The percentage of time that the ED was maintained between the two boundary conditions was determined. Remifentanil and propofol predicted effect-site concentrations (Ce) were determined at surgical milestones and, after drug discontinuation, the time to response to verbal stimulation and orientation was measured. Results. The median (25th-75th percentile) per cent of time that the ED was recorded between the boundary conditions was 99.3% (98.1-99.8%). Predicted propofol (mu g ml(-1)) and remifentanil (ng ml(-1)) Ce (SD), respectively, were 3.5 and 4.0 at induction, 1.9 (0.8) and 7.2 (3.7) at the end of surgery, and 1.1 (0.5) and 3.2 (2.2) at eye opening. The median time to eye opening and orientation was 3.8 and 6.8 min, respectively. Conclusion. This feasibility study supports the concept that remifentanil may be delivered using an algorithm that maintains the difference between SE and RE between the upper and lower boundary condition.