[Influence of femoral catheter stimulation intensity on post-surgical analgesia after total knee replacement].

INTRODUCTION

Stimulating catheters allow the catheter point to be positioned near the nerve, thus reducing the amount of local anaesthetic required for a successful block. There is currently a debate on what is the stimulation intensity required to provide adequate analgesia, although it does seem that if it is obtained with 1mAmp or less the block is more effective. The objective of the study was to demonstrate whether different neurostimulation intensities with the stimulating catheter at femoral nerve level, had an influence on the adequacy of post-surgical analgesia during the 48h after total knee arthroplasty.

MATERIAL AND METHODS

A comparative, prospective and randomised study was conducted on patients subjected to total knee replacement. After surgery with subarachnoid anaesthesia, a continuous femoral block was performed with a stimulating catheter at a neurostimulation intensity 0.2 and 0.5mAmp in Group 1, between 0.6 and 1mAmp in Group 2, equal or higher than 1.1mAmp in Group 3, and in Group 4 the catheter was introduced between 3-5cm without looking for a motor response. A dose of 0.2% ropivacaine 0.4mL/kg and an infusion at 5mL/h, with boluses of 10mL/30min, was administered via the catheter. Sciatic nerve block was also performed on all patients with 20ml of 0.5% ropivacaine. The patient demographics were recorded, as well as, post-surgical analgesia details, sensory and motor block in each area, boluses requested, rescue analgesia, and undesirable effects at 8, 16, 24, 36 and 48h.

RESULTS

A total of 124 patients were included, 32 in Group 1 (25.8%), 21 in Group 2 (16.9%), 31 in Group 3 (25%), and 40 in group 4 (32.3%). The 4 groups were homogeneous as regards age, height, weight and ASA. There were no statistically significant differences found in the post-operative pain, except during movement in the femoral area at 36 hours (p=.032). There were also no statistically significant differences found in the sensory block in the femoral area at 48 hours (p=.019) and in the femoral cutaneous nerve block at 8 hours (p=.049) or at 24 hours (p=.045). As regards motor block, differences were only found in the obturator nerve at 24 hours (p=.016). There were no differences in rescue analgesia, patient controlled analgesia (PCA) boluses requested or administered, except that the number of boluses requested at 16 hours was less in Group 3 (p=.049). There were also no significant differences in undesirable effects or in the level of satisfaction of the patients between the four groups.

CONCLUSIONS

In our study, no influence was found on the level of analgesia provided after knee replacement surgery with the neurostimulation intensity to which the neuromuscular system involved responded when a stimulating catheter is inserted at femoral level.