This paper proposed a novel objective pain intensity assessment technique using the change of peripheral arterial stiffness β, which reacts to a peripheral sympathetic nervous activity. The stiffness β is calculated from continuous arterial pressures and photo-plethysmograms beat-by-beat based on the log-linearized peripheral arterial model. In the experiment, the validity of the proposed method was examined by comparing estimated normalized arterial stiffness βn with simultaneously-measured numeric rating scale (NRS) as a general index of pain intensity when electrocutaneous stimuli were applied to the healthy subjects. The results showed the maximum amplitude of electrocutaneous current Imax has linear correlation with the normalized stiffness βn (R2 =0.77, p < 0.05). In addition, both the maximum amplitude of electrocutaneous current Imax and the normalized stiffness βn strongly correlated with NRS values by sigmoidal curves (Imax v.s. NRS: R2 =0.91, p < 0.01; βn v.s. NRS: R2 =0.92, p < 0.01). It was therefore concluded that the proposed method can assess pain intensity objectively.
