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 E1 which induced pricking pain on the volar surface of the left forearm by means of CO2 laser pulses. E1 was measured on a well-defined area, using laser pulses of different durations and constant power P. The dependence of E1 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 E1.