Association between subjective self-report of pain and the hemodynamic response to a prolonged noxious stimulus during cold water tolerance test: an fNIRS study

Four tolerance tests using cold water at different temperatures were employed to investigate the hemodynamic response to different intensities of a cold noxious stimulus. Twenty one healthy right-handed individuals (20 to 35 years old) with no history of neurological, psychological, or psychiatric disorders and analgesic-free were recruited. Each experiment started with a 30 s baseline and a 2 min immersion of the right hand in the tepid water (∼23C) for adaptation. This was immediately followed by the immersion of the same hand in a temperature-controlled cold water bath for as long as s/he can tolerate the stimulated pain but no longer than 5 min. The protocol was repeated for different bath temperatures of 15C, 10C, 5C, and 1C for all subjects. Oxyhemoglobin (HbO2) and deoxy-hemoglobin (Hb) concentration changes were continuously recorded using a functional Near Infrared Spectroscopy (fNIRS) device developed at Drexel University by means of three multi-distance probes located on the forearm and forehead. During each experiment, in addition to subjects’ pain threshold and tolerance, numerical pain rating scores on a 0 to 10 scale (NRS-11) were recorded every 15 s. Results from the forehead probes suggested that: 1) as the temperature of cold water decreased, subjects’ pain threshold and tolerance decreased; 2) the Hb and HbO2 data collected from ‘far’ (2.8 cm) and ‘near’ (1 cm) detectors followed a similar trend throughout the experiments; 3) during the initial arousal phase, the intensity of reported pain increased rapidly and so did the amplitude of HbO2 data; and 4) hemodynamic adaptation to a prolonged exposure to cold water was observed in all subjects; however, not all the subjects reported decreasing pain during the adaptation phase. This study suggests that fNIRS can be effectively used to monitor the generalized hemodynamic response to a cold water tolerance test.