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Learning Ultrasound Guided Regional Anaesthesia

Chris Nixon
Department of Anaesthesia and Peri-operative Medicine
Auckland City Hospital, Auckland, New Zealand

 

This section is devoted to those who wish to introduce Ultrasound Guided Regional Anaesthesia (USGRA) into their clinical practice. As yet there is no absolute requirement to do so but there is a philosophical advantage to this. Seeing the needle, the target and the distribution of local anaesthetic is associated with better success and reduced doses of local anaesthetic. Safety would seem to be better but there is as yet no proof that this is so.

1. Scanning

As with any new technology, there is a desire to use it immediately. Unfortunately ultrasound imaging differs from other modalities and requires an understanding of how the image is generated and which factors may improve the image presented on the screen. The only way to achieve this is by understanding the physics and then spending lots of time learning how to scan. Practice at scanning does however come with the advantage that it does not harm the patient, is repeatable, and can be performed on any patient or volunteer. No needles are required for this!! It is only with considerable practice that the learner can develop good scanning ability and an appreciation of the sonoanatomy required for the blocks. This is a lesson in pattern recognition and an understanding of how to optimise the scanner settings. It will become apparent that the use of probe settings and position varies for individual blocks. Superficial blocks are usually best imaged with a linear high frequency probe whereas deeper blocks e.g sciatic, psoas plexus require a lower frequency curvilinear array. For any block it is important always to scan the area to determine the course of the nerve and confirm its identity, determine any local structure at risk (colour doppler for vessels) and to determine the best site for the block. With US you are no longer restricted to anatomical landmarks, and are free to explore the patients true anatomy.

2. Needle Control

Once scanning is mastered you are ready to learn the second required skill. This involves practicing needle technique so that you can reliably steer the needle along the ultrasound beam maintaining needle visibility and most importantly controlling the needle tip position. Remember that the technique involves placing the needle close to the nerve, rather than using a needle to find the nerve. In the latter approach used for paraesthesia and nerve stimulation techniques we choose needle with short bevels so that nerve damage is less likely. With US we choose a needle for its visibility and ease of use since the aim is to miss the nerve not hit it. To assist in this phase of learning there are a series of models which can be used —

  1. Simple home made phantom – Gelatine. This is a very cheap and easy phantom to make.[1] Its echo pattern resembles liver and olives can be used as targets. It has a short lifespan and is best keep refridgerated. Needle imaging is good but the tactile feel is not realistic.
  2. Tofu model - Readily available extra-firm tofu, wood dowel, and electrical wire are easily composed to create models for learning ultrasound-guided needle manipulation. Wood and wire targets embedded in tofu present hypo- and hyper-echoic targets that allow the learner to appreciate the relationship between the two-dimensional ultrasound screen image and three-dimensional target planes.[2]
  3. Pork phantom - Take a joint of pork and marinate in vodka. This eliminates the smell and provides a lifelike image and feel to the needle practice. Targets can be embedded in the meat if required. This model is easy to prepare, is more durable than the gelatine model but still has a lifespan of only a few days.
  4. Gel phantom - Commercially available phantoms which provide a needle practice. The images and feel are not as realistic as the pork model but the phantom can last for 5,000 needle passes. After repeated use old needle tracks are visible. Lifespan is best when using the smallest gauge needles.
  5. Cadaver and Animal labs - These provide the most lifelike experience away from a patient but are expensive and the preserve of university centres.[3,4] Some US courses throughout the world provide opportunities to use these.
  6. Spinal imaging model – placing a spine into a water bath provides a good opportunity to study the images obtained and comparing them with the model. Using a laser guide helps to interpret the anatomy.

3. Block Performance

It is sensible to start with the easiest blocks and graduate towards more difficult blocks as experience increases. A group, ZEURS, have put forward some guidelines for training but these are not standardised. The Regional Anaesthesia SIG (ANZCA) are debating standards for training and credentialing but no guidelines have been published to date.

Certain elements assist in the performance of US blocks —

  1. Depth of nerve – surface targets are easier to visualise and direct a needle towards
  2. Adjacent structures – it is easier to find a nerve if there is a recognisable structure beside it such as an artery.
  3. Positioning the patient – for optimum needle visibility the needle should be passing at right angles to the US beam. Most needles are difficult to see when the angle exceeds 45 degrees. Arranging the monitor, probe and the intended needle path along your visual axis assists in maintaining alignment of the needle and probe. These two requirements often mean that the patient positioning and/or the needle direction are different from those of standard nerve stimulator guided blocks.
  4. Injection technique -small injections of local anaesthetic (0.2-0.5ml) assist localisation of the needle tip and also are used to dissect tissues (hydrodissection and hydrolocalisation). We are interested in the pattern of spread of local anaesthetic around the nerve and along fascial planes. Additionally it is important to recognise intravascular injection (no LA visible at needle tip) and intraneural injection (nerve swelling). Our preference is not to inject in one site only but to manipulate the needle to produce a local anaesthetic pool around the nerve structures, the so called "doughnut sign." Finally, remember the aim is to miss nerves not hit them.

A Note on Nerve Stimulation

Coulombs Law states there is a relationship between the charge density and the distance from a stimulating electrode. This has been shown in animal experiments and has lead to the notion that optimum position of the needle is achieved at a current of 0.5mA. If stimulation persists at less than 0.3mA the inference is that the needle may be intraneural and should be moved. Several recent studies have shown that nerve stimulation may not be as precise as previously thought.

  1. Nerve stimulation current thresholds differ between nerves.
  2. Diabetic nerves often do not stimulate.[5]
  3. Direct needle contact confirmed by US is not always associated with nerve stimulation.[6]
  4. Sensory nerves will never produce a motor response.
  5. Motor response not always necessary for block.[7]

Should Nerve Stimulation be Used with US?

The combination is attractive and studies have demonstrated the superiority of US in guiding the needle to the nerve over landmark techniques, minimising patient discomfort.[8] In this instance if you can see the nerve then nerve stimulation may merely interfere with the block as looking for a motor response is not necessary.[9] The US end-point for nerve blockade is the production of a hypoechoic ring of local anaesthetic around the target nerve. In fact, one of the problems with nerve stimulation is that whilst the nerve is identified the spread of local anaesthetic cannot be judged. Historically we have negated this disadvantage by using maximal volumes which we now can see greatly exceed requirements when needle positioning can be adjusted to produce circumferential spread around our nerve or plexus target. Nerve stimulation may still have a place where the nerve is deep and the needle angle makes tip visibility a problem, a situation which often applies more often in the inexperienced. As yet there is no hard and fast rule, but most US experts end up ditching the PNS and not the US.

Catheter Insertion with Ultrasound

US techniques usually adopt an approach where the target nerve(s) is imaged in short axis. The needle may be introduced either along the axis of the US beam (in-plane) or perpendicular to the beam (out of plane).[10] Inserting a catheter via an in-plane needle may be seen as a problem but in practice this appears not to be such a problem.

Two recent reports[11] have detailed the use of longitudinal imaging of both nerve and needle for catheter insertion. We believe these to be technically more difficult and have been described in only four cases to date.

References

  1. Gibson R, Gibson KI: Australasian Radiology 1995, 39:356-357.
  2. Pollard BA: New model for learning ultrasound-guided needle to target localization. Reg Anesth Pain Med 2008, 33(4):360-362.
  3. Tsui BC, Dillane D, Pillay J, Ramji AK, Walji AH: Cadaveric ultrasound imaging for training in ultrasound-guided peripheral nerve blocks: lower extremity. Can J Anaesth 2007, 54(6):475-480.
  4. Tsui B, Dillane D, Pillay J, Walji A: Ultrasound imaging in cadavers: training in imaging for regional blockade at the trunk. Can J Anaesth 2008, 55(2):105-111.
  5. Sites BD, Gallagher J, Sparks M: Ultrasound-guided popliteal block demonstrates an atypical motor response to nerve stimulation in 2 patients with diabetes mellitus. Reg Anesth Pain Med 2003, 28(5):479-482.
  6. Perlas A, Niazi A, McCartney C, Chan V, Xu D, Abbas S: The sensitivity of motor response to nerve stimulation and paresthesia for nerve localization as evaluated by ultrasound. Reg Anesth Pain Med 2006, 31(5):445-450.
  7. Sinha SK, Abrams JH, Weller RS: Ultrasound-guided interscalene needle placement produces successful anesthesia regardless of motor stimulation above or below 0.5 mA. Anesth Analg 2007, 105(3):848-852.
  8. Orebaugh SL, Williams BA, Kentor ML: Ultrasound guidance with nerve stimulation reduces the time necessary for resident peripheral nerve blockade. Reg Anesth Pain Med 2007, 32(5):448-454.
  9. Beach ML, Sites BD, Gallagher JD: Use of a nerve stimulator does not improve the efficacy of ultrasound-guided supraclavicular nerve blocks. J Clin Anesth 2006, 18(8):580-584.
  10. Fredrickson MJ: The sensitivity of motor response to needle nerve stimulation during ultrasound guided interscalene catheter placement. Reg Anesth Pain Med 2008, 33(4):291-296.
  11. Koscielniak-Nielsen ZJ, Rasmussen H, Hesselbjerg L: Long-axis ultrasound imaging of the nerves and advancement of perineural catheters under direct vision: a preliminary report of four cases. Reg Anesth Pain Med 2008, 33(5):477-482.