Anlage von kontinuierlichen Nervus femoralis Kathetern zur Analgesie bei komplexen Knieoperationen. Ist ein Stimulationskatheter der konventionellen Methode überlegen ? Eine randomisierte, kontrollierte Studie

The procedure of placing a catheter for continuous regional anaesthesia is often associated with fear and pain in the patient. Thus, we evaluated the use of midazolam and fentanyl to improve patient's comfort and cooperation. METHODS: After an oral dose of 20 mg clorazepate, 174 patients receiving peripheral nerve catheters for regional anaesthesia where randomized into 3 groups to receive either intravenous placebo, 3 mg midazolam or 0.1 mg fentanyl immediately before catheter placement in a double-blind manner. Stepwise regression analysis was used to identify factors associated with patient's assessment of subjective discomfort (measured using a VAS 0-10) during the procedure. Amnesia was evaluated 24 h later. The anaesthetist rated patient's cooperation during catheter placement. RESULTS: Female sex and longer duration of catheter placement had significant negative impact on patient's comfort, whereas fentanyl showed an improvement. Age, body mass index, midazolam and the type of catheter had no influence. The following day 27% of the midazolam group, 6% of the placebo group and 9% of the fentanyl patients did not remember catheter placement. Patient's cooperation was poor in 26% of the midazolam patients but only in 9% of the placebo and 3% of the patients receiving fentanyl. Of the placebo patients 18.4% had to be supplemented with fentanyl because they found the procedure of catheter placement unbearable. No side effects occurred in either group. CONCLUSION: As patient's comfort and cooperation were significantly improved by fentanyl, we recommend fentanyl to facilitate catheter placement for regional anaesthesia. The local anaesthetic prilocaine has a low systemic toxicity mainly because of a high absorption in the lung and a large volume of distribution and thus is associated with a lower risk of neurological or cardiac side effects. However, the major disadvantage is the formation of methaemoglobin by its metabolite o-toluidine. Especially patients with severe cardiopulmonary disease, pronounced anaemia and congenital glucose 6-phosphate dehydrogenase deficiency are more susceptible to suffer from clinical symptoms of methaemoglobinaemia. The formation of methaemoglobin is highly variable and the underlying causes for the high inter-individual variations are poorly understood. Thus, this prospective observational study was performed to identify factors that are associated with increased prilocaine-induced methaemoglobinaemia. Patients and methods: 162 Patients undergoing major knee surgery under general anaesthesia combined with peripheral nerve blocks (femoral nerve block, combined femoral/sciatic nerve block, or lumbar plexus block) received a single bolus injection of 300 mg or 400 mg prilocaine about 30 minutes before surgery via a catheter. The proper placement was verified using nerve stimulation via a stimulating catheter. Three hours after prilocaine injection, venous blood samples were drawn and methaemoglobin levels were measured by standard photometric technique. Various variables were recorded and subjected to a stepwise multiple regression analysis. Results: The mean met-Hb level for all patients was 2.7% (range: 0.9 – 15.4%). A higher dose of prilocaine and younger age were the most important predictive factors for higher methaemoglobin formation. Female sex and to a lesser extend the use of high-concentration / low-volume prilocaine also increased methaemoglobin levels. These four factors of the model explain 36% of the total variance. Other investigated factors, including the patient’s height, weight, body mass index, the site of catheter insertion, the anaesthetist’s judgement concerning the difficulty of catheter placement, duration of catheter placement or an inadvertent puncture of a venous or arterial vessel, had no significant impact on the concentration of methaemoglobin. Conclusion: In the light of our data the use of prilocaine for regional block is safe since the older patients who might be more susceptible to suffer from clinical symptoms of methaemoglobinaemia usually form less methaemoglobin. However, since prediction of high methaemoglobin levels is far from being perfect, anaesthesiologists performing regional blocks in patients who might be jeopardised by a decrased oxygen transport capacity should continue to use lower doses of prilocaine or switch to another local anaesthetic.