Effect of Local Anaesthetic Volume on Axillary Brachial Plexus Block Duration

Peripheral nerve blocks provide significant benefits to patients, including improved analgesia and decrease in general anaesthesia-related adverse effects. Ideally peripheral nerve block provides rapid onset and substantial perioperative analgesia. Use of ultrasound guidance in peripheral nerve blocks offers significant advantages compared to alternative techniques. It improves block success rate and shortens onset time.

To compare the effects of local anaesthetic volumes (10 vs 30 mL) of lidocaine 2% with epinephrine 1:200 000 on the duration of sensory and motor block following ultrasound guided axillary brachial plexus block.

Methods:

It is a prospective, randomized, observer blinded study. With institutional ethical approval and having obtained written informed consent from each, 30 patients will be studied.

Randomization and blinding:

Using a computer generated and sealed envelope technique, 30 patients will be randomly allocated in to one of two groups.

Group 1: patients will receive 10 ml of lidocaine 2% with epinephrine. Group 2: patients will receive 30 ml of lidocaine 2% with epinephrine.

Sample size and statistical analysis:

Sample size calculation will be based on duration of sensory block as the primary outcome parameter. Kaabachi et al found a sensory duration of axillary brachial plexus block (with 30 mL of lidocaine 1.5%) of 126 (SD=48) mins. The minimum sample size required to have a 90% probability of detecting a decrease in duration of 60 mins (level of significance 0.05) will be 13 patients per group using an unpaired student's t test. Total recruited 30.

Anaesthetic procedure:

Having established intravenous access, standard anaesthetic monitoring will be applied. Premedication with midazolam will be administered as clinically indicated (to a maximum of 3 mg). The operative arm will be abducted and externally rotated with elbow flexed at 90 degrees. Under strict aseptic condition, ultrasound guided axillary brachial plexus block will be performed. Having identified musculocutaneous, median, radial and ulnar nerves, a 50mm 22-gauge ultraplex short bevel insulated needle will be used with in-plane approach to block each nerve with either 2.5 ml (10 mL group) or 7.5 ml (30 mL group). All blocks will be performed by single operator experienced in the ultrasound peripheral nerve blocks.

Block assessment:

When the block procedure has been completed, a blinded observer will assess the onset of sensory and motor block15 in the innervation of each nerve every 5 mins, until surgical anesthesia is achieved or 30 mins have elapsed. Surgical anesthesia will be defined as a motor score ≤2, with absent sensation to cold and pinprick. Each nerve distribution will be individually assessed. Onset time will be measured from conclusion of the block (t=0) to attainment of surgical anesthesia. Block will be deemed failure if surgical anesthesia has not been achieved at 30 mins in one or more of the four nerve distribution. In case of failure, an additional rescue block or conversion to general anesthesia will be performed. Data from these patients will be analysed separately.

Intraoperative period:

All patients will receive 1 g of paracetamol and diclofenac 75 mg iv during surgery. In case of patients discomfort sedation and rescue analgesia in the form of fentanyl 25 micrograms aliquots IV will be administered to a maximum of 100 micrograms.

Postoperative period:

Upon arrival to recovery room, sensory and motor function will be assessed every 15 mins by a blinded observer.15 Assessment will be performed for each nerve separately. Block regression is defined as a return of sensation to cold and pinprick with motor power score ≥3 in any nerve region. Time to first request of postoperative analgesia and total opioid consumption for 24 hrs will be noted. Postoperative analgesia will be prescribed as paracetamol 1 g 6 hourly and diclofenac sodium 75 mg 12 hourly. Oxycodone 5-10 mg orally 4-6 hourly will be administered as rescue analgesia.

Primary Outcome:

The primary outcome will be the overall duration of sensory block.

Secondary Outcomes:

Duration of motor block Time to onset of the block. The quality of block intraoperatively Time to first request of postoperative opioid analgesia. Total opioid consumption at 24 hours. The incidence of adverse effects perioperatively: nausea, dizziness, tinnitus, vomiting, convulsions or arrhythmia.