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Performing regional anaesthesia is easier when one can quickly and accurately localize the target nerve or plexus.Two promising areas of accelerated investigations are notworthy. First are the studies seeking to understand the nature of needle to nerve proximity. Nerve localization was first based on paraesthesia- seeking techniques and more recently on peripheral nerve stimulation. Most anaesthesiologists used to elicit paraesthesia before observing a peripheral nerve stimulator- induced motor response. Many trials had done to improve nerve stimulator design to ensure proximity to the target nerve without impaliment.(80) Second and more exciting is the commercial development of high frequency ultrasonic probes that allow actuall visualization of the target nerve.Clinical studies documented the ability of this technology of ultrasonography to identify peripheral nerves and consequently improve block performance and safty. At the very least ultrasonography is likely to become a valuable tool in understanding needle to nerve proximity and its implied relationship to patient safty. (81)Indeed ultrasonography may allow more precise localization of peripheral nerves during some regional anaesthetic procedures(82), which could potentially correlate to improved efficiency, efficacy and patient comfort(83,84).Ultrasonography has been introduced into anaesthetic practice for over a decade. In recent years however, the interest for the use of this technology to aid in nerve localization has significantly increased.(85,86,87) Although ultrasound may be useful for nerve localization, one of its main benefits is to provide visualization of the dispersion of the local anaesthetic within the desired tissue plains.Ultrasound is a high frequency sound generated in specific frequency ranges and sent through tissues. Penetration into tissue is based in a large part on the range of the frequency produced. In other words lower frequencies (e.g- 2Mhz) penetrate deeper than high frequencies. (e.g.- 10mHz). As the sound passes through tissues it is either absorbed, reflected or allowed to pass through, depending on the density (”echo”-density) of the tissue. Meanwhile, all ultrasound dissipates in tissue producing heat. The ”listening” part of the probe ”listens” for reflections (echos) of the sound waves sent out and passes the information to the processing unit, so that time between sending and receiving equals distance. (88)The amount of energy reflected equals density. Substances containing a lot of water (e.g. cerebro-spinal fluid, blood) are very good conductors of sound and reflect very little, energy so they appear as dark areas, while substances which contain little water such as bone - which is a poor sound conductor- or a worse conductor (e.g.- air), reflect almost all the energy and appear very bright. On the other hand substances that conduct sound to a degree in between these two extremes, appear darker to lighter depending on the amount of energy they reflect. Most useful reflections (echos) occur at boundaries between tissues of differing densities. Most commonly seen are outlines of things which have different reflecting properties in ultrasound. (88)Tissue surfaces which may be echo-reflective but lie in a non-perpendicular plane to the ”listening” probe will have little if any energy returning from them and therefore won’t show up on ultrasound,the information contained in the ”return signal”, the echo, is heavily and cleverly processed to reduce the amount of artifact and produce a picture which corresponds to what we expect to see.(88)There are many applications of ultrasound in regional anaesthsia, in which the most popular one is ultrasound guided brachial plexus block which can be done by more than one approache (e.g. interscalene, supraclavicular, infraclavicular and axillary approach). Also peripheral nerves originating from brachial plexus can be blocked separately under ultrasound guidance.(89)In lower extremity ultrasound guidance is not commonly used due to great acceptance of spinal and epidural anaesthesia. However ultrasound may provide help in blocking lumbosacral plexus and nerves originating from it (e.g. femoral and sciatic nerve).(62)Morever use of ultrasound in neuroaxial block can help in detecting the direction of the spinal and epidural needle for an appropriate technique of injection of the local anaesthetic. (71)ConclusionIndeed, it seems that the introduction of ultrasonography in the field of anaesthesia provide great help in performing different techniques of regional anaesthesia (e.g. brachial plexus, lumboscral plexus and neuroaxial block). In addition it helped precisly in detection of the desired nerve or plexus to be blocked, meanwhile avioding many complications of the previous conventional blind regional anaesthesia techniques (e.g. injuery of the surrounding tissues or blood vessels), and also saving the amount of local anaesthesia used, hence decreasing its toxic effects.
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