Hemodynamic Response to Repeated Noxious Cold Pressor Tests Measured by Functional Near Infrared Spectroscopy on Forehead

The objective of this research was to assess the utility of a simple near infrared spectroscopy (NIRS) technology for objective assessment of the hemodynamic response to acute pain. For this exploration, we used functional near infrared spectroscopy (fNIRS) to measure the hemodynamic response on the forehead during three trials of a cold pressor test (CPT) in 20 adults. To measure hemodynamic changes at the superficial tissues as well as the intracranial tissues, two configurations of ‘far’ and ‘near’ source-detector separations were used. We identified two features that were found to be fairly consistent across all subjects. The first feature was the change of total hemoglobin (THb) concentration in a given condition divided by the duration of that condition \( {\text{THb}}^{\prime} \). Statistical analyses revealed that during the first CPT trial \( {\text{THb}}^{\prime} \) significantly changed from its baseline value in all channels. Also, adaptation to repeated CPTs was observed in both \( {\text{THb}}^{\prime} \) parameter and the reported post-stimulus pain rating scores. The second feature was the difference between the maximum and the minimum of the evoked changes in the THb concentration (ΔTHb). A significant correlation was observed between the post-stimulus pain rating score and ΔTHb at all channels. An asymmetrical activity was observed only at the ‘far’ channels. These results suggest that fNIRS can potentially be used as a reliable technique for the assessment of the hemodynamic response to tonic pain induced by the CPT.