Blockade of the abdominal muscles measured by EMG during lumbar epidural analgesia with ropivacaine – a double‐blind study

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Abstract
A single shot of 20 ml of 1%, 0.75% or 0.5% ropivacaine was administered epidurdlly (at L2/3 level) to 30 volunteers, in a double‐blind manner. The blockade of the rectus abdominis muscle was measured quantitatively by registration of the average rectified electromyographic signal (AREMG) at the T7, T9 and T11 motor segmental levels and with a qualitative test for blockade of the rectus abdominis muscle (the so‐called RAM test). The maximal cranial spread ofanalgesia, evaluated by the pin‐prick method, was not significantly different for the three concentrations (T8‐T10 dermatome; median value). The intensity of motor blockade, measured by the AREMG method, increased progressively from the T7 segment and caudally with all three concentrations. The blockade was partial (Le. 85–25% of baseline AREMG activity was present at its maximum) in all subjects. When the effect of the three concentrations of ropivacaine was compared at the same segmental level, the intensity and duration of maximal motor blockade seemed to be dose‐dependent, but the difference was not statistically significant. The total duration of motor blockade was shorter with the 0.5% solution than with the higher concentrations. The AREMG method gave a more exact and graded picture of blockade of the rectus abdominis muscle than the RAM test. The duration of sensory blockade did not outlast motor blockade at any level. In half of our subjects the maximal spread of sensory blockade was either equal to or higher than the spread of partial motor block. In the other half, this relationship was reversed ‐ the maximal cranial level of partial motor block was 1–4 segments higher than the maximal level of analgesia.