Factors influencing pricking pain threshold using a CO₂ laser

The purpose of this study was the evaluation of the best experimental conditions in healthy subjects for the measurement of the minimal thermal energy density E_t which induced pricking pain on the volar surface of the left forearm by means of CO₂ laser pulses. E_t was measured on a well-defined area, using laser pulses of different durations and constant power P. The dependence of E_t on the stimulus power P, the size A of the radiated area and the surface temperature T_e were explored. In the first part of the study, these relations were obtained using a computer program, from the calculated spatio-temporal distribution of the skin temperature during, and following, a laser pulse which caused pricking pain. The second part studied a set of subsequent measurements carried out on a group of five healthy trained subjects and agreed only in part with the calculated data. We found that the measurement error on E_t was less than 10% with P between 1.5 and 3 W, and A between 0.15 and 0.25 cm², respectively. The influence of sensitization and adaptation phenomena on the measured data was also explored. We also show a rhythmic annual change of T_e and E_t.