Background and Objectives: Skin-temperature increase is a reliable but late indicator of success during regional-anesthesia techniques. The goal of this study is to determine the distribution of skin-temperature changes during different regional techniques. Does skin temperature increase in the whole area innervated by the blocked neural structures or only in certain regions within this area with the capability to react preferentially to sympathetic block (i.e., vessel-rich skin)? Although onset time may vary between different regional-anesthetic techniques, we hypothesized that the distribution of skin warming is equal.

Methods: Skin temperature was assessed continuously by infrared thermography in 24 patients who received either combined femoral-nerve and sciatic-nerve block, epidural anesthesia, or spinal anesthesia.

Results: Apart from differences in time of onset, no differential spatial distribution of skin-temperature changes could be detected. The earliest and greatest rise of skin temperature occurred at the great toe (10.6°C ± 0.4°C), became smaller proximally, and was negligible above the ankles, irrespective of the type and extent of block. Videothermography revealed that cold blood flows through subcutaneous veins immediately after onset of sympathetic block and initially decreases skin temperature (0.6°C ± 0.3°C) during onset of spinal anesthesia.

Conclusion: Irrespective of the applied regional-anesthetic technique, skin-temperature changes are more pronounced distally. Thermography prevents false measurements of skin temperature above subcutaneous veins and displays flow of cold blood as the mechanism of initial skin-temperature drop after regional anesthesia. Measurements of skin-temperature increase cannot be used to evaluate the extent of analgesia or sympathetic block.