CHAPTER 6 Nerve block regional anaesthesia
General principles
Nerve block regional anaesthesia is potentially of enormous value in the ED. It is useful both for providing an excellent quality of analgesia following traumatic injuries (e.g. a metacarpal block for a crushed finger; a femoral block for a fractured femur; a sciatic nerve block for a fractured ankle) as well as for providing anaesthesia for minor procedures (interscalene block for shoulder dislocation; ankle block for complex wound repair on the foot). There is good evidence to support the efficacy and safety of regional anaesthetic techniques in the ED both in adults and in children.
Some blocks have a high success rate with a blind technique without a nerve stimulator (e.g. femoral block, sciatic block, axillary block, supraclavicular block, interscalene block), but some blocks require a nerve stimulator (e.g. infraclavicular block), and most blocks are probably more successful with the use of nerve stimulators, although this is often debated. The use of ultrasound with or without nerve stimulators can further increase the ease and success rate of nerve plexus or peripheral nerve blockade. There is good clinical evidence to support this, and this is the technique that emergency medicine practitioners should learn and adopt. Studies have shown that ultrasound can decrease the failure rate of nerve blocks from a historical 20–30% to as little as 5% and that the speed of onset can be increased and the complications decreased.
Nerve blocks work on the principle of inducing analgesia or anaesthesia to a body region by injecting a local anaesthetic agent around the nerve or nerves innervating that region. Various techniques may be used to assist in positioning the needle tip adjacent to, but not penetrating, the nerve: the paraesthesia technique; the nerve stimulator; the ultrasound-guided block. Once the needle is located within the neural sheath, a large volume of local anaesthetic is injected (not under pressure) which spreads completely around the nerve (this can also be confirmed with ultrasound). Block success is increased if large volumes are used. Newer evidence with ultrasound-guided blocks shows that smaller volumes can be used because the local anaesthetic can be deposited circumferentially around the nerve under direct vision.
Adverse events following nerve block anaesthesia can include vasovagal reactions, systemic reactions to the local anaesthetic agent (including anaphylaxis, allergic reactions, toxic reactions and methaemoglobinaemia), local tissue pain and inflammation, as well as complications that can vary with the site being punctured. These complications can include nerve damage from intraneural injection, spinal cord damage or total spinal anaesthesia (interscalene block), phrenic or recurrent laryngeal nerve paralysis, pneumothorax or vascular puncture (interscalene or supraclavicular blocks). Fortunately these are rare and can mostly be avoided by meticulous attention to safety and the use of ultrasound techniques.
There are a few important factors to consider before embarking on a nerve block:
A detailed discussion of the pharmacokinetics and pharmacodynamics of local anaesthetic agents is beyond the scope of this book. In general, the choice of agents depends on the clinical setting, the required duration of anaesthesia or analgesia, the potential side effects, and, of course, availability. A sound knowledge of the pharmacology of local anaesthetics is not difficult, as there are only a handful of injectable agents readily available. The most commonly used agents for nerve block regional anaesthesia are lidocaine, bupivacaine, mepivacaine and ropivacaine, often with adrenaline (epinephrine) added in a concentration of 1:100 000 or lower. Levobupivacaine is a newer formulation, composed exclusively of the S-isomer of bupivacaine (regular bupivacaine is a mixture of the S- and D-isomers). The S-isomer has been shown to be more potent and longer acting, with a more favourable side-effect profile. This agent is much more expensive than the others, however.
The success of the block, the time of onset and the duration of anaesthesia depend on the volume of local anaesthetic agent injected, the agent used and the accuracy of injection. It is important to emphasise that the use of lidocaine in a nerve block does not necessarily accelerate the onset of the block significantly, as part of the lag time results from local tissue diffusion which remains much the same for any agent.
Ultrasound fundamentals for nerve blocks
Conventional peripheral nerve block techniques are performed ‘blind’, without visual guidance and are completely dependent on surface anatomical landmarks for localisation of nerves or nerve bundles. Some landmark techniques are better than others, since those that rely on bony landmarks have a higher success rate than those using soft-tissue landmarks. Even though the use of nerve stimulators increases the likelihood of placing the needle in proximity to the nerve, needle positioning is still dependent on external or palpable landmarks.
It is, therefore, not surprising that regional anaesthetic techniques have a reported failure rate of up to 20% because of incorrect needle and/or local anaesthetic placement. Multiple trial-and-error attempts at needle placement lead to frustration, unwarranted patient pain and time delay. Ultrasound is a practical option for assisting in nerve blocks as it is portable and non-invasive. ED physicians are also often familiar with ultrasound-guided procedures and can make the transition to ultrasound nerve blocks with ease.
The advantages of ultrasound-guided nerve blocks include the following:
In the real world of the ED, however, visualisation of nerves may not be as easy as described in training material, and the pictures you obtain may not look like those in the book. That does not necessarily reflect on your skill – images in books are always the best available. The images in this book are average images that you might expect to obtain yourself with average equipment. Here are some hints that might help you locate the nerves as easily as possible and how to optimise the imaging capability of your equipment:
