Response of arterial mechanical impedance to different concentrations of remifentanil during abdominal laparoscopic colectomy

A specific and reliable method for monitoring analgesia during general anesthesia is urgently required. Previously, we introduced a method that indicates arterial mechanical properties for estimating arterial wall stiffness (K). However, whether the response of K actually indicates changes in remifentanil target concentrations under continuous surgical stress, is unclear. Thus, we aimed to evaluate how K responds to different remifentanil target concentrations. This prospective study enrolled 30 patients who underwent laparoscopic colectomy. The patients received 3 different effect-site concentrations (2, 4, and 6 ng/ml) of remifentanil 3 times during the study period. The K values at 4-ng/ml administration were used as control values (K_control). K values at 6-ng/ml administration (K_6ng) and those at 2-ng/ml administration (K_2ng) were normalized by dividing them by the control values. The results showed that K responded to the changes in remifentanil concentration, significantly decreasing at 6-ng/ml remifentanil effect-site concentration and increasing at 2-ng/ml concentration. The stress response at low analgesia was higher than that at high analgesia, and, as predicted, normalized K_2ng was significantly greater than normalized K_6ng. In conclusion, we found that K is a sensitive stress response monitor and dynamically responds to changes in remifentanil concentration in invasive stimulation during laparoscopic colectomy.