BLOCKADE OF THE ABDOMINAL MUSCLES MEASURED BY EMG DURING LUMBAR EPIDURAL ANALGESIA WITH ROPIVACAINE - A DOUBLE-BLIND-STUDY

A single shot of 20 ml of 1%, 0.75% or 0.5% ropivacaine was administer ed epidurally (at L2/3 level) to 30 volunteers, in a double-blind mann er. The blockade of the rectus abdominis muscle was measured quantitat ively by registration of the average rectified electromyographic signa l (AREMG) at the T7, T9 and T11 motor segmental levels and with a qual itative test for blockade of the rectus abdominis muscle (the so-calle d RAM test). The maximal cranial spread of analgesia, evaluated by the pin-prick method, was not significantly different for the three conce ntrations (T8-T10 dermatome; median value). The intensity of motor blo ckade, measured by the AREMG method, increased progressively from the T7 segment and caudally with all three concentrations. The blockade wa s partial (i.e. 85-25% of baseline AREMG activity was present at its m aximum) in all subjects. When the effect of the three concentrations o f ropivacaine was compared at the same segmental level, the intensity and duration of maximal motor blockade seemed to be dose-dependent, bu t the difference was not statistically significant. The total duration of motor blockade was shorter with the 0.5% solution than with the hi gher concentrations. The AREMG method gave a more exact and graded pic ture of blockade of the rectus abdominis muscle than the RAM test. The duration of sensory blockade did not outlast motor blockade at any le vel. In half of our subjects the maximal spread of sensory blockade wa s 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.