Effect of Local Anaesthetic Dilution on the Characteristics of Ultrasound Guided Axillary Brachial Plexus Block

To compare the effects of local anaesthetic 20 mL lidocaine 1% with epinephrine vs 40 mL lidocaine 1% with epinephrine on the onset 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, 20 patients will be studied.

Randomisation and blinding:

Using a computer generated and sealed envelope technique, 20 patients will be randomly allocated in to one of two groups.

Group 1: patients will receive 20 ml of lidocaine 2% with epinephrine. Group 2: patients will receive 40 ml of lidocaine 1% with epinephrine.

Sample size and statistical analysis:

In the absence of data from previous study using 20 ml of lidocaine with epinephrine for ultrasound guided axillary brachial plexus block. Sample size was calculated based on from our pilot study of 12 patients. We found mean onset of block of 11.25 (SD, 2.3) mins. The minimum sample size required to have an 80% probability of detecting a 30% decrease in onset time (level of significance 0.05) will be 7 patients per group using an unpaired student's t test. Recruited 10 per group

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 24-gauge stimuplex short bevel insulated needle will be used with in-plane approach to block each nerve with either 5 (Group 1) or 10 ml (Group 2). All blocks will be performed by 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 block in the innervation of each nerve every 2.5 mins, until surgical anaesthesia is achieved or 30 mins have elapsed (Table 1). Surgical anaesthesia 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 anaesthesia. Block will be deemed failure if surgical anaesthesia 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 anaesthesia 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 for 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: onset of block.

Secondary Outcomes: Duration of sensory and motor block and block performance data (Imaging time, Needling time, Performance time, No of needle pass, Vascular puncture and parasthesia)

Inclusion criteria

• ASA 1-3
• Patients aged >18 years undergoing elective upper limb (forearm or hand) surgical procedures.

Exclusion criteria

• Contraindications to regional anaesthesia
• Hypersensitivity to amide local anaesthetics.
• Chronic pain
• Language barrier
• Neuromuscular disorders or peripheral neuropathy
• Body mass index > 35
• History of hepatic and renal insufficiency
• Pregnancy
• Cognitive or psychiatric disorder
• Cardiac conduction abnormality.