ISPUB.COM The Internet Journal of Anesthesiology Volume 10 Number 2 1 of 8 Risk Factors For Prilocaine Induced Methaemoglobinemia Following Peripheral Regional Anaesthesia F Vasters, L Eberhart, T Koch, P Kranke, H Wulf, A Morin Citation F Vasters, L Eberhart, T Koch, P Kranke, H Wulf, A Morin. Risk Factors For Prilocaine Induced Methaemoglobinemia Following Peripheral Regional Anaesthesia. The Internet Journal of Anesthesiology. 2005 Volume 10 Number 2. Abstract Background: The major disadvantage of the local anaesthetic prilocaine is the formation of methaemoglobin with high inter- individual variation. Its underlying causes 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 received a single bolus injection of 300 mg or 400 mg prilocaine about 30 minutes before surgery via a catheter. Three hours after prilocaine injection, venous blood samples were drawn and methaemoglobin levels were measured. 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 lower 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: 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. INTRODUCTION For surgical procedures of the lower limb, plexus blocks and peripheral nerve blocks constitute a widely established analgesic procedure. After placing the nerve catheter it is important to provide fast onset of the block to improve the workflow within the operating theatre. This aim can be reached by using a drug with fast onset. Prilocaine is a medium-long-acting local anaesthetic with a fast onset of action that is comparable with lidocaine and mepivacaine but has a significantly lower systemic toxicity [ 1 , 2 ]. This is mainly because of a large virtual volume of distribution and a high absorption of the drug in the lung that protects to a great extent against systemic toxic reactions even in case of accidental intravasal injection [ 3 , 4 ]. Various toxicologic studies showed about 50 % lower blood levels of the drug compared to other fast-onset and medium-long-acting local anaesthetic agents [ 5 , 6 , 7 ]. However, prilocaine bears the disadvantage of the formation of methaemoglobin (met-Hb) induced by its metabolites o-toluidine and nitrosotoluidine [ 6 , 7 , 8 , 9 , 10 ]. Maximum levels of met-Hb are usually reached 2-4 hours after prilocaine injection, and return to normal range again after 8-24 hours in most cases [ 7 , 11 ]. A positive correlation has been shown between the prilocaine dose used for a nerve block, and the time until the maximum met-Hb peak was reached [ 11 , 12 ]. Following a single dose of 300-600 mg prilocaine, met-Hb concentrations in the range of 15% can be detected. This is normally considered harmless, but in certain circumstances, for example in anaemic patients or in