Mapping the area of anaesthesia following an ultrasound guided arm nerve block in health volunteers'

The intervention being studied is the sensory loss produced by an ultrasound guided block of the posterior ante brachial cutaneous nerve of the forearm. The study will describe a novel ultrasound guided technique to identify the nerve in a fat filled tunnel as it exits the hiatus (opening) of the deep fascia of the arm. The right side will be used in all volunteers to maintain consistency. Volunteers will be supine with the arm adducted and the elbow flexed over the anterior chest. Scanning will be performed using a 15- to 6-MHz linear ultrasound probe and a transportable ultrasound machine (Sonosite, X-Porte). The ultrasound probe and its cable will be covered with a sterile disposable sleeve. Sterile ultrasound gel will be used for coupling. The probe will be placed transversely on the upper arm at the levelof the spiral groove to provide a short axis view of the radial nerve (RN) characterised by its typical ovoid shape and fascicular echo texture .The RN will then be followed distally to identify where it gives rise posteriorly to the PABCN just before penetrating the LIMS. The PABCN will then be followed as it transverses the fascial planes between the long head of triceps and brachialis muscles to exit via the hiatus in the deep fascia. Here, the PABCN enters the FFT and becomes recognisable as an oval hyperechoic structure. The position of the nerve at this site will be marked on the skin and the distance to the lateral epicondyle will be recorded. A still image of the nerve in the short axis at this site will be saved to the hard drive of the ultrasound machine. This will allow for the circumferential area of the nerve to be measured using proprietary software.The quality of the imaging of the PABCN will be graded as excellent (nerve visualised as a discrete hyperechoic oval structure within a hypoechoic FFT), good (nerve visualised as a discrete hyperechoic oval structure but hypoechoic FFT not seen) and poor (nerve not visualised). If the PABNC nerve is unable to be identified in the FFT (good or poor images) the block will not be performed. This will be reported.The nerve will then be followed distally in the FFT to identify branching of the nerve. This will be recorded on a 60 second video loop to permit later review. Typically, the PABCN has an anterior and posterior branch but variations in the number and origin of branching have been reported. The number and origin of the branches will be recorded.Following identification of the PABCN within the FFT an in-plane ultrasound guided block of the nerve will be performed. The site of injection will be disinfected with a wipe containing 70% isopropyl alcohol and 2% chlorhexidine digluconate (SoluPrep Antiseptic Wipe, 3M). An insulin syringe (BD-Ultra-fine) will be used to deliver a subcutaneous injection of 0.3-0.5ml 2% lignocaine prior to insertion of a 22G x 50mm echogenic needle (SonoPlex STIM,Pajunk). The needle will be advanced in a lateral to medial direction into the FFT. The PABCN will then be blocked by injecting 1ml 2% lignocaine (20mg) to achieve circumferential spread of the local anaesthetic around the nerve Sensation to light touch and sharpness will be assessed 15 minutes later using a commercially available tool widely used for testing peripheral neuropathy in diabetic patients (Neuropen, Owen Mumford). This tool assesses sharpness/pain in small nerve fibres via a sterile single-use semi-sharp monofilament that exerts a force of 40g when pressed into the skin. This type of sensory testing is well described in the literature The unblocked forearm will be tested first to ensure the volunteer understands what sensation to expect. Volunteers will be asked to state “Sharp” or “Not Sharp” when the monofilament is pressed onto the skin. Testing of the blocked forearm will start in an area of normal sensation and repeated in approximately 1cm incremental movements until loss of sensation (“Not Sharp”) is reported. This area will be outlined on the skin with a surgical marker pen and traced on to a paper transparency. The total CSBA (cm2) will then be calculated from photographs of the transparency using the software SketchAndCalc. If no area of sensory block can be elicited this will be recorded as a failed blockThe anterior surface of the forearm will be defined with a marker pen using the following borders. A line will be drawn joining the lateral epicondyle to the radial styloid and another from the medial epicondyle to the ulna styloid. This will delineate the lateral and medial borders. The anterior inter-epicondylar line at the elbow and anterior distal skin crease at the wrist will define the proximal and distal borders. Any sensory block in the anterior surface of the forearm will be deemed to have encroached into the territory of the LABCN and MABCN. This area will be calculated as a percentage (%) of the total CSBA using the SketchAndCalc software.Any sensory block on the dorsum of the hand will be deemed to have encroached into the territory of the median or radial nerves and calculated as a percentage (%) of the CSBA using the SketchAnd Calc software. As the block may miss the posterior branch of the PABCN the area of sensory loss over the lateral epicondyle region of the elbow will be reported. Photographs of the CSBA showing inter-individual variability will be used in any future publication.. All drugs and devices including 2% lignocaine and the Neuropen will be used according to their listed and approved indication on the Australian Register of Therapeutic Goods (ARTG). All interventions will be done by Dr Philip J Corke, Senior Staff Specialist Anaesthetist with 20 years experience in regional anaesthesia. All blocks will be performed in the anaesthetic bay of the operating theatres at Concord Repatriation General Hospital. Monitoring of the ECG (electrocardiography), non-invasive blood pressure, pulse oximetry and resuscitation equipment will be available in the anaesthetic bay if needed. All volunteers will receive verbal instructions regarding the need to protect the area of skin from inadvertent injury (hot surface, sharps) until normal sensation has returned (2-4 hours). All volunteers will be reviewed the following day. The skin over the area of local anaesthetic injection will be inspected for local signs of infection. Sensation and movement of the forearm will be assessed.