The Shamrock lumbar plexus block: A dose-finding study

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BACKGROUNDThe Shamrock technique is a new method for ultrasound-guided lumbar plexus blockade. Data on the optimal local anaesthetic dose are not available.OBJECTIVEThe objective of this study is to estimate the effective dose of ropivacaine 0.5% for a Shamrock lumbar plexus block.DESIGNA prospective dose-finding study using Dixon's up-and-down sequential method.SETTINGUniversity Hospital Orthopaedic Anaesthesia Unit.INTERVENTIONShamrock lumbar plexus block performance and block assessment were scheduled preoperatively. Ropivacaine 0.5% was titrated with the Dixon and Massey up-and-down method using a stepwise change of 5 ml in each consecutive patient. Combined blocks of the femoral, the lateral femoral cutaneous and the obturator nerve were prerequisite for a successful lumbar plexus block.PATIENTSThirty patients scheduled for lower limb orthopaedic surgery completed the study.MAIN OUTCOME MEASURESThe minimum effective anaesthetic volume of ropivacaine 0.5% (ED50) to achieve a successful Shamrock lumbar plexus block in 50% of the patients. Further analysis of the data was performed with a logistic regression model to calculate ED95 and to estimate the effective doses for a sensory lumbar plexus block not requiring a motor block of the femoral nerve.RESULTSThe Dixon and Massay estimate of the ED50 was 20.4 [95% confidence interval (95% CI) 13.9 to 30.0] ml ropivacaine 0.5%. The logistic regression estimate of the ED95 was 36.0 (95% CI 19.7 to 52.2) ml ropivacaine 0.5%. For a sensory lumbar plexus block, the ED50 was 17.1 (95% CI 12.3 to 21.9) ml and the ED95 was 25.8 (95% CI 18.6 to 33.1) ml.CONCLUSIONA volume of 20.4 ml ropivacaine 0.5% provided a successful Shamrock lumbar plexus block in 50% of the patients. A volume of 36.0 ml would be successful in 95% of the patients.TRIAL identifier NCT01956617.

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