Monitoring of sympathetic tone to assess postoperative pain: skin conductance vs surgical stress index.

The number of fluctuations in skin conductance per second has been described as a potential tool for monitoring postoperative pain. More recently, the surgical stress index has shown promising correlations with intra-operative painful stimuli. We compared both methods for their ability to assess postoperative pain, in 100 postoperative patients who were also asked to quantify their level of pain at different time points in the recovery room. The number of fluctuations per second and surgical stress index were significantly different between pain scoring <or= 5/10 and > 5/10 on a numeric rating scale (mean (SE) number of fluctuations per second 0.12 (0.02) vs 0.21 (0.03), respectively; p = 0.017, and surgical stress index 57 (1.4) vs 64 (1.9) points, respectively; p = 0.001). Both number of fluctuations in skin conductance per second and surgical stress index identified timepoints with moderate to severe pain with only moderate sensitivity and specificity.