Risk Factors For Prilocaine Induced Methaemoglobinemia Following Peripheral Regional Anaesthesia

Abstract

Background: The major disadvantage of the local anaesthetic prilocaine is the formation of methaemoglobin with high interindividual 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.