The effect of pre- versus postinjury infiltration with lidocaine on thermal and mechanical hyperalgesia after heat injury to the skin.

The aim of the study was to evaluate the effects of pre- and postinjury infiltration with lidocaine on alterations in mechanical and thermal sensitivity after heat injury to the skin. In the first part of the study, 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 left and right calves at least 24 h apart in 8 healthy, unmedicated male volunteers, in order to investigate the effects of the injury on sensitivity in untreated skin. In the second part of the study, burn injuries (15 x 25 mm rectangular thermode, 50 degrees C, 6 min) were produced twice in each subject on the medial side of the left and right calves 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) on one day, and the same block was performed 35 min after injury (postinjury block) on the other day. Warm detection thresholds (WDT) and heat pain detection thresholds (HPDT) were determined within and outside the injury before and at regular intervals after injury. Areas of hyperalgesia to pinprick and brush were determined at regular intervals after 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 areas of hyperalgesia to pinprick and brush were found in- and outside the injury in all subjects. These findings were relatively constant throughout the study period and reproducible between the 2 days of examination. In the second part of the study, it was observed that pre-injury infiltration with lidocaine reduced hyperalgesia to pinprick and brush 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 treatment at the end of the study period. It is concluded, that a shortlasting 'preemptive' infiltration with lidocaine may postpone but not prevent the occurrence of hyperalgesia outside a thermal injury.