The Minimal Effective Volume (MEV90) of Ropivacaine 0.75% for Ultrasound-guided Transmuscular Quadratus Lumborum Block for Unilateral Percutaneous Nephrolitotomy: a Study Protocol for a Dose Finding Trial

This double-blind dose-finding trial is based on a biased coin up-and-down sequential design, where the volume of local anesthetic administered to each patient depends on the response from the previous one. The TQL block is performed preoperatively, and the first patient recruited receives 20 mL of ropivacaine 0.75%. In case of block failure, the next patient will receive a higher volume (defined as the previous volume with an increment of 2 mL). Given a successful block for the first patient, the next patient will be randomized to either a lower volume (defined as the previous volume with a reduction of 2 mL) or the same volume as the previous patient. The respective probabilities being b=0.11 for a reduced volume and 1-b=0.89 for the same volume. Block success is defined as patient reported numeric rated scale (NRS) pain (NRS value ≤ 3 (0-10/10)), 30 minutes after arrival in the post anesthesia care unit (PACU). The NRS pain value is our primary and only outcome in the evaluation of the block. A minimum of 25 eligible patients are needed to achieve precise estimation of MEV90 with narrow 95% confidence intervals derived by bootstrapping. Following inclusion of 25 patients early termination is considered when interim analysis shows sufficiently stabilization of MEV90 estimate. The final sample size is not known a priori, but a maximum of 40 patients will be enrolled in the trial.