THE EFFECT OF PREINJURY VERSUS POSTINJURY INFILTRATION WITH LIDOCAINEON THERMAL AND MECHANICAL HYPERALGESIA AFTER HEAT INJURY TO THE SKIN

The aim of the study was to evaluate the effects of pre- and postinjur y infiltration with lidocaine on alterations in mechanical and thermal sensitivity after heat injury to the skin. In the first part of the s tudy, burn injuries (15 X 25 mm rectangular thermode, 50-degrees-C, 7 min) were produced twice in each subject on the medial side of the lef t and right calves at least 24 h apart in 8 healthy, unmedicated male volunteers, in order to investigate the effects of the injury on sensi tivity in untreated skin. In the second part of the study, burn injuri es (15 X 25 mm rectangular thermode, 50-degrees-C, 6 min) were produce d twice in each subject on the medial side of the left and right calve s at least 24 h apart (n = 10). This was preceded by subcutaneous (s.c .) infiltration with 5-6 ml of 1% plain lidocaine (pre-injury block) o n one day, and the same block was performed 35 min after injury (posti njury block) on the other day. Warm detection thresholds (WDT) and hea t pain detection thresholds (HPDT) were determined within and outside the injury before and at regular intervals after injury. Areas of hype ralgesia to pinprick and brush were determined at regular intervals af ter injury. In the first part of the study, it was observed that both WDT and HPDT were decreased within but not outside the injury, and are as of hyperalgesia to pinprick and brush were found in- and outside th e injury in all subjects. These findings were relatively constant thro ughout the study period and reproducible between the 2 days of examina tion. In the second part of the study, it was observed that pre-injury infiltration with lidocaine reduced hyperalgesia to pinprick and brus h outside the injury more effectively than postinjury block, but only for the first 70 min after injury, while no significant difference was observed 100-190 min after injury. Likewise, there was no difference in thermal thresholds inside the injury between pre- and postinjury tr eatment at the end of the study period. It is concluded, that a shortl asting 'preemptive' infiltration with lidocaine may postpone but not p revent the occurrence of hyperalgesia outside a thermal injury.