The role of electrical stimulation in ultrasound-guided subgluteal sciatic nerve block: a retrospective study on how response pattern and minimal evoked current affect the resultant blockade

Abstract

Nerve stimulation may be combined with ultrasound imaging for a block of deeply located nerves such as the sciatic nerve in the subgluteal region. At present, it is unknown how the use of nerve stimulation affects blockade after this nerve block. We retrospectively compared the effects of the two types of motor response and those of minimal evoked current above and below 0.5 mA on ultrasound-guided subgluteal sciatic nerve block using mepivacaine or ropivacaine, two local anesthetics with different onset time and duration. We reviewed records and video images of patients who, from April 2008 until October 2011, received ultrasound-guided subgluteal sciatic nerve block combined with nerve stimulation using 20 ml of either 1.5 % mepivacaine with 1:400,000 epinephrine or 0.5 % ropivacaine. Sensory and motor blockade data for 30 min after the block and for the duration of the blockade were gathered. Patients for whom any data were missing, the video image was poor, and/or intraneural injection was observed during the block were excluded from the study. The same data were compared in two ways: regarding the motor response pattern between the response of the tibial nerve and the common peroneal nerve, and regarding the minimal current between low current (< 0.5 mA) and high current (≥0.5 mA). The primary endpoints were the onset and duration of blockade of the sciatic nerve block. We analyzed the data of 170 and 99 patients who received mepivacaine and ropivacaine, respectively. The progress of sensory and motor blockade as well as block duration was similar between different motor response patterns after both anesthetics. The proportion of patients who developed sensory block of the tibial nerve and motor block at 30 min was higher in the low minimal current group than in the other group receiving mepivacaine. Patients in the former group also had longer block duration. With ropivacaine, complete motor blockade was present at 30 min in a higher proportion of patients after lower minimal evoked current than after higher minimal evoked current. When ultrasound-guided subgluteal sciatic nerve block was conducted with nerve stimulation, the motor response pattern did not markedly affect the progress of sensory or motor blockade or block duration. Lower minimal evoked current was associated with faster onset in sensory and motor block and longer block duration after mepivacaine and faster onset in motor block after ropivacaine. The clinical significance of this, however, has yet to be determined.