Reliability and validity of a continuous pain registration procedure

Background Conventional pain rating scales [i.e. visual analogue scales (VAS) or numerical rating scales (NRS)] only provide a summary for different levels of pain felt, while the duration of these levels is not accounted for. If pain can be rated continuously, the area under the curve (AUC) of varying pain intensity over time can be calculated, which integrates varying pain intensity with duration. The present study examined the reproducibility and validity of a continuous pain rating procedure. Methods Twenty-eight healthy volunteers participated. Pain was induced using constant current delivered to the non-dominant forearm using bipolar electrodes. Pain was rated continuously on an electronic VAS monitored by a computer. For each participant, the level of current needed to achieve a weak, mild, slightly moderate and moderate level of pain was determined (part I). Next, participants were asked to rate the painfulness of six periods of electrical stimulation (part II). Unknown to the participants, they were presented with the four levels of current obtained in part I, where the level of current for mild and moderate pain was presented twice (in order to assess consistency). The order of presentation was randomized for all subjects. Results In general, participants produced reliable mean AUCs. In addition, the AUC of pain intensity over time could clearly discriminate between the four levels of pain used in the present study. Discussion A continuous pain registration procedure, using an AUC approach, may be a promising direction to explore. Results can be improved by allowing more training on the use of the electronic VAS.