Lumbar transversus abdominis plane block: the role of local anesthetic volume and concentration—a pilot, prospective, randomized, controlled trial

BACKGROUND The lumbar transversus abdominis plane (TAP) block has become an optional part of multimodal analgesia following several abdominal surgeries. There remains a lack of consensus regarding the extent of dermatomal blockade following lumber TAP block, as well as the optimal local anesthetic volumes and concentrations. The objectives of this pilot trial were to assess the feasibility of conducting a similar full-scale trial and gather information on relevant clinical outcomes, namely whether greater local anesthetic volumes would lead to more cephalad dermatomal blockade. METHODS The study was a prospective, double-blinded pilot randomized controlled trial (RCT) with three arms, each representing different local anesthetic volumes: 20 ml 0.5% ropivacaine, 30 ml 0.33% ropivacaine, and 40 ml 0.25% ropivacaine. We planned to recruit 30 females undergoing total abdominal hysterectomy for non-malignant pathology, who would then receive bilateral ultrasound-guided midaxillary TAP blocks at the completion of surgery. Randomized patients would be followed for 48 h post-block and would receive multimodal analgesia. The primary outcomes were measurements of patient recruitment and safety, to inform the feasibility of a larger trial. The main secondary outcome was the clinically pertinent endpoint of dermatomal blockade, which was assessed by loss of sensation to ice and pinprick. RESULTS Our target sample size was reached in 8 months, and the recruitment rate was 52% (31/60). A total of 58 TAP blocks were performed among 29 patients. All but one of the patients who received interventions were successfully followed and assessed up to 48 h. No patient safety-related adverse events were reported during the study period. The mean highest dermatome blocked in each group at any time point was T8. The 20 ml 0.5% ropivacaine group achieved a T9-L1 block that lasted for 48 h. The 30 ml 0.33% ropivacaine group had a sensory block from T9-L1 that regressed to T10-T12 between 24 and 48 h. The 40 ml 0.25% ropivacaine group reported an initial sensory block from T9-T12 that regressed by 24 h to include only the T12 dermatome. CONCLUSIONS This pilot study demonstrated that the study design is feasible and safe to be carried to a full-scale RCT. The preliminary clinical findings showed that increasing the volume, while maintaining a constant dose, of local anesthetic does not appear to extend the height of dermatomal blockade following midaxillary TAP block. This finding needs to be confirmed in future studies. TRIAL REGISTRATION ClinicalTrials.gov registration is: NCT01307215.