Skin conductance for monitoring of acute pain in adult postoperative patients: influence of electrode surface area and sampling time.

Aim of this prospective randomized study was to determine the influence of the electrode surface area and sampling time on the accuracy of the number of fluctuations in skin conductance per second to distinguish different states of acute pain. These methodological issues have been previously suggested as an explanation for contradictory data related to the accuracy of the skin conductance monitor. A total of 541 pain ratings on a numeric rating scale (0-10) were obtained from 120 adult postoperative patients. The number of fluctuations in skin conductance per second was recorded using two different electrode types (surface area 254 vs. 474 mm(2)) and sampling times (7.5 vs. 30 s). A longer sampling time did result in higher values for the number of fluctuations in skin conductance per second, though without improving its accuracy to distinguish different states of pain. However, the latter was found improved when the smaller surface area electrodes were used. A combination of small surface area electrodes and a 30 s sampling time resulted in the highest area under the curve in the receiver operating curve analysis of the method to identify states of moderate to severe pain (numeric rating scale > 3): 0.68 vs. e.g. 0.55 [data from all patients combined]). We conclude that the type of electrodes used but only to a lesser degree the sampling time influence the accuracy of the number of fluctuations in skin conductance per second to identify states of moderate or severe postoperative pain.