Ultrasound in anaesthesia: practical recommendations

Alwin Chuan

Westmead Hospital, Sydney, Australia

Introduction

This presentation will focus on practical recommendations for ultrasound guided regional anaesthesia (UGRA), but the principles are broadly applicable for any ultrasound procedure.

Adopters of UGRA must master new knowledge topics of ultrasound (US) physics, equipment familiarity, and sonoanatomy. A major challenge is operator proficiency: manual dexterity skills are necessary to obtain good quality sonoanatomy images and performing real-time needle guidance.

Practical Recommendations for UGRA

Ergonomics
Avoid awkward body postures and minimise head movements by careful positioning of the patient, US machine and anaesthetist. Emphasise watching the screen and not one’s hands. Employing a comfortable grip on the transducer reduces hand fatigue and allows subtle movements. Along with inadequate equipment preparation (eg over-reaching for syringes), inefficient ergonomics leads to poor visualisation and increases needling difficulty.

US Machine Setup
Modern US machines have features that enhance nerve imaging. Ensure the preset is applicable for UGRA (usually ‘Nerve’), and enable compound imaging mode if available.

A common mistake is confusing the sidedness of the transducer with the sidedness of the image, causing a left/right or up/down disorientation error. Note the orientation marker (groove or raised button) on the transducer, and hold the probe such that patient movement is mirrored by image movement. Alternatively, on some machines, the image orientation indicator (a dot at the corner of the screen) is moved to the desired side via software control.

Enhancing the Sonoanatomy Images
For the majority of UGRA procedures, a high frequency (>7MHz) linear array transducer provides the best nerve short axis (SAX) views. High frequencies provide better resolution but at the expense of decreased penetration, and modern machines allow a range of frequencies (eg 6 to 13MHz) to be user selected. In practice, use the highest frequency available without sacrificing depth penetration.

A roadmap, or pre-scan, is useful to establish an overview of the regional anatomy. Use this opportunity to improve the image clarity by adjusting the gain controls and focal length to the target depth (if selectable). After completing the roadmap, improve the image frame rate by reducing the depth of view to just include the target nerve. This will minimise real-time motion artefacts during needling attempts.

Locating Nerves
Anisotropy is an US phenomenon describing a difference in reflected energy dependent on the plane of imaging. Best imaging occurs with the transducer perpendicular to the nerve, and progressively deteriorates away from 90 degrees. Practically, obtain true SAX views as angulation may cause nerves to be indistinguishable from surrounding non-neural tissue.
Vascular structures mimic nerves, but is differentiated by their pulsatility, compressibility (especially veins), and blood flow when interrogated with Colour Flow Doppler.
Traceback techniques are very powerful tools to distinguish nerves, particularly if anatomical variation exists. Starting from a location where there are prominent sonographic landmarks (eg vessels, bone, fascial planes) adjacent to the target nerve, the transducer is used to dynamically trace the nerve proximally or distally to map its course and divisions.

Real-time needle guidance
The most common error with novices performing UGRA is advancing the needle despite failure in confirming the needle tip position. Some techniques to identify the tip include ‘jiggling’ the needle to agitate the tissue; fascial plane tenting by the needle tip; hydro-location by using small (0.5ml) test boluses; and needle shaft twirling to increase specular reflections from the bevel surface.
Angle of insertion is the largest influence on the visibility of the needle tip. Steeper angles, irrespective of in-plane or out-of-plane needle approaches, increases the difficulty. Adjusting the needle trajectory to be perpendicular to the US imaging plane improves visibility, but may result in a non-traditional needle insertion point distant from the transducer.
Many peripheral nerves lie between fascial planes. Local anaesthetic injection through a needle tip accurately placed within the planes will hydro-dissect the fascia and naturally surround the nerve. Needle neurotrauma is thus reduced by aiming the needle at these layers, or adjacent to, rather than directly, at the target nerve.
By consensus, a circumferential spread of local anaesthesia around a nerve (‘donut sign’) should provide a fast onset, high quality block. Achieve this spread by using divided boluses and real-time assessment, repositioning the needle as necessary.
If the hypoechoic spread of local anaesthesia cannot be visualised, consider an intravascular injection. Alternatively, it suggests that the needle tip is not in the plane of imaging.

References and Further Reading

1. Chan V et al, Zuers Ultrasound Experts Regional Anaesthesia Statement, Eur Soc Anaesth Jan 2007.
2. Chuan A. Ultrasound Guided Lower Limb Regional Anaesthesia. University of Melbourne Perioperative Ultrasonography Course Material, 2007.
3. Sites BD et al, Artefacts and Pitfall Errors Associated with Ultrasound-Guided Regional Anesthesia: Part I. Reg Anesth Pain Med, 2007. 32(5):412 – 418.
4. Sites BD et al, Artefacts and Pitfall Errors Associated with Ultrasound-Guided Regional Anesthesia: Part II. Reg Anesth Pain Med, 2007. 32(5):419 – 433.
5. Sites BD et al. Characterising Novice Behaviour Associated with Learning Ultrasound-Guided Peripheral Regional Anaesthesia. Reg Anesth Pain Med, 2007. 32(2): 107 – 115.
6. Scott D, Chuan A. Regional Anaesthesia Pocket Guide. Adrenalin Strategics 2008.
7. Tsui B. The Importance of Ultrasound Landmarks: A “Traceback” Approach Using the Popliteal Blood
8. Vessels for Identification of the Sciatic Nerve. Reg Anesth Pain Med, 2006. 31:481-2.