To the Editor:
In the article by Choo et al .1on skin conductance fluctuations (SCFs) and postoperative pain in children, the conclusions are different compared with those of the article by Hullett et al .2on skin conductance as a measure of postoperative pain, even though the authors use the same technology on the same population of patients. How come? The SCFs that are studied mirror the bursts in the skin sympathetic nerves. The bursts in the skin sympathetic nerves are more specific and sensitive for monitoring pain and noxious stimuli than blood pressure and heart rate because they are not influenced by temperature changes or changes in microcirculation and because acetyl choline acts on the muscarine receptors.3It reacts within 1–2 s.3Moreover, patients/volunteers without pain/noxious stimuli and other stressors have a low variation between individuals regarding SCFs per second.3In awake patients, it is well-known that pain4,5and other emotional stressors (e.g ., vomiting, nausea, and intellectual tasks, such as explaining and teaching children how a pain score works [used by Choo et al .]),1,6,7may influence the SCFs per second when monitoring pain. Therefore, correlation tests, such as those that Choo et al . have performed, should not be used in the postoperative setting to study pain by SCFs per second if the patients are not controlled for stressors other than pain. Cutoff values to discover the level of pain (i.e ., no or mild, moderate, and severe pain) should be used instead.2,8–11The cutoff value to discover moderate and severe pain of 0.1 SCFs/s when using a 15-s analyzing window gave a sensitivity to discover moderate and severe pain of 90% and a specificity of approximately 65–70%.2,9To use a cutoff value based on optimized sensitivity and specificity, as Choo et al . performed in their study, does not make sense as long as the specificity to pain is known to be weak in awake patients. Therefore, it would make more sense to use cutoff values to show whether the skin conductance method can predict no/mild or severe pain with high specificity because moderate pain will most likely be mixed with the other stressors (fig. 3 in the article by Choo et al .). Moreover, the analyzing window is important. The nature of postoperative acute pain is often short lasting (i.e ., lasting only a few seconds) and occurs during movement. When using pain and anxiety scores, they are often the result of the maximum score in the time window analyzed. If the SCFs per second increase during acute pain, lasting for a few seconds, this increase will be averaged when an analyzing window of 60 s is used. These are exactly the findings from Choo et al . (fig. 2 in their article): during no/mild pain (few SCFs per second are expected, left part of the figure), a 15-s analyzing window gave fewer SCFs per second compared with a 60-s analyzing window. Moreover, during severe pain (high SCFs per second are expected, right part of the figure), a 15-s analyzing window gave higher SCFs per second compared with the 60-s analyzing window. Therefore, it is difficult to understand why Choo et al . chose and recommended a 60-s analyzing window. Interestingly, Hullett et al . used an analyzing window of 15 s and a cutoff value of 0.13 SCFs/s to discover moderate and severe pain in children; the sensitivity for discovering pain was 90%, and the specificity was 64%.2The predictive value for discovering no or mild pain, with a cutoff of 0.13 SCFs/s, was 97%.2These results indicate how skin conductance technology can be used: physicians and nurses obtain an indication for when to ask patients about their pain status. It is important to know when to ask patients about their pain status, especially in the United States, where it is mandatory to monitor pain (Joint Commission on Accreditation of Health Care Organizations).3With the high predictive value of discovering no and mild pain (97%), the skin conductance monitor may possibly help to at least give less analgesia to patients with no pain and facilitate work in the hospitals when the physicians and nurses know when to ask patients about their pain status.
It would be interesting if Choo et al .1reanalyzed their data and used a 15-s analyzing window, as Hullett et al . did,2and cutoff values for the calculation of sensitivity and specificity of 0.13 SCFs/s (to discover moderate and severe pain).2It would be important to reproduce the results of Hullett et al . to know whether the SCFs per second may facilitate the way to monitor pain in children. Moreover, important clinical knowledge could have been discovered if 0.28 and 0.33 SCFs/s were used as cutoff values to discover severe pain, based on the findings from Choo et al . (fig. 3 in their article). Moreover, Choo et al . should also find the predicative values for no/mild and severe pain based on the cutoff values 0.13, 0.28, and 0.33 SCFs/s. These results would have been helpful to know whether the Skin Conductance Algesimeter index, SCFs per second, is useful in children postoperatively to discover no or mild pain and acute severe postoperative pain with high specificity. It would then probably be in agreement with the conclusions from the articles by Choo et al . and Hullett et al .; in addition, new important clinical information would be added from the article by Choo et al .
University of Oslo, Oslo, Norway, and CEO, Med-Storm Innovation, Oslo, Norway. firstname.lastname@example.org.