Equipment for regional anaesthesia

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Chapter 13 Equipment for regional anaesthesia

Susanne Krone

Chapter contents

Nerve location devices 

Needles and catheters 

Ambulatory continuous infusions of local anaesthetic 

Non-Luer connectors 

The key to successful ‘conduction anaesthesia’ is deposition of the local anaesthetic drug accurately around the target nerve.

The introduction and widespread adoption of ultrasound imaging represents the largest change in regional anaesthesia in decades. Consequent to the use of ultrasound, for the first time the anaesthetist is able to view an image of the nerve directly, guide the needle under real-time observation, navigate away from sensitive anatomy, and observe the spread of the injected drug.

Previously, electrical stimulation, paraesthesia and the occasional unique anatomical feature such as the epidural loss of resistance to injection or detection of cerebrospinal fluid, were all that was available to confirm targeting. All such approaches to nerve targeting have relied on good anatomical knowledge and surface landmark identification. However, landmark techniques have limitations: variations in anatomy, the skill and experience of the anaesthetist, and equipment design all have an effect on success rates and complications. Many other regional anaesthetic (RA) blocks have relied on less precise information.

Equipment for RA is here broadly considered under two parts: devices concerned with target identification, and needle systems for drug delivery to the target area (including stimulating needles and their catheters).

Nerve location devices

Ultrasound

In 1978 La Grange et al1 used Doppler ultrasound to assist a series of supraclavicular brachial plexus blocks. Kapral et al2 described ultrasound-guided supraclavicular blockade in 1994. Since then there has been a year-on-year increase in related publications and a similar growth in clinical practice.

Over the past decade a large number of portable machines have become available. The very first machines had limited functionality and produced what is by comparison now such a poor image that one is tempted to wonder why practitioners and developers persisted with the technology. However, there are now any number of very good-quality portable or ‘laptop’ style machines available (manufacturers include SonoSite, Esaote and GE Healthcare). The machines may have Doppler, tissue harmonics and multibeam technology as standard, making them true alternatives to the traditional cart-based machines typically seen in radiology departments (Fig. 13.1).

image

Figure 13.1 M-Turbo portable ultrasound machine with linear broadband probe, SonoSite Inc. USA.

To optimize demonstration of nerves and surrounding structures, it is vitally important to understand the equipment and its limitations, and to have a good, sound anatomical knowledge of the structures being viewed.

Device specifics

Chapter 31 of this book is dedicated to the physics and technology of ultrasound imaging. The reader is encouraged to read the aforementioned chapter for further details of topics raised in this section on RA, which is concerned more with the application of ultrasound in RA and the small portable machines used for that purpose.

Most ultrasound machines have the following components in common:

• a signal generator producing an applied voltage (typically up to 100 volts) in brief bursts

• a piezoelectric transducer which converts electrical energy to acoustic pulses and vice versa

• a signal receiver which detects, amplifies and compresses signals returning to the transducer

• signal processing software for displaying the data in different modes

• a display together with the interface for controlling the machine

• a memory which stores still and video images and has connectivity ports for the output of images and other data.

Imaging modes used in ultrasound-guided regional anaesthesia

Modern scanners display ultrasound data in various forms. At present, brightness or B-mode is most commonly used for nerve imaging, producing a single two-dimensional image (hence also called 2D mode) from a slice of approximately 2 mm thickness and adjustable depth (Fig. 13.2).

image

Figure 13.2 Screen grab, supraclavicular approach to brachial plexus. B-mode image. 0.5 cm markers and 2.7 cm total depth can be seen on the right of the screen.

C-mode (colour) is useful to identify vessels within a region of interest (ROI) and flow is encoded as a colour image superimposed on the B-mode greyscale image. Colour flow ultrasound (CFU) measures directional flow and velocity whereas colour power Doppler (CPD) gives no directional information or estimation of velocity, but has greater sensitivity when the angle of the incident beam to the target surface is approaching the 90° angle – this being the angle that provides the best working image of target nerves.

These and other imaging modes, together with some further developments in image processing, are discussed more fully in Chapter 31. Software-driven post processing of the image is also now available to enhance needle visualization on some machines.

The recent advances in the development of three- and four-dimensional ultrasound imaging, although not yet routinely available, promise simultaneous multiple planes of view or a representation of the whole area of interest. This may give the operator an improved spatial awareness and understanding of anatomy and needle position.

Controls

On the ultrasound machine, the most important controls that are required to generate and optimize the image are those for frequency, depth adjustment and focal zone, time gain compensation (TGC) and imaging mode selection (2D, C-mode, etc.) (Fig. 13.3).

image

Figure 13.3 Control panel of M-Turbo, showing rotary controls for gain settings and hard keys also for mode selection and depth settings as well as track pad and QWERTY keyboard for other functions. Soft keys (controlling varied functions) are visible as unlabelled buttons below the screen area.

The selection of ultrasound frequency is a balance between obtaining the best resolution and being able to achieve adequate penetration of tissues. The higher the frequency of the ultrasound, the shorter the wavelength of the sound waves produced and the better the resulting image resolution. High frequency provides high-resolution images of superficial structures (brachial plexus), but poorer quality images of deeper structures. Lower frequencies permit better penetration of tissues and are necessary for imaging and needling deeper structures such as the lumbosacral plexus and neuraxial components.

Frequency adjustment facilities (presented sometimes as a choice of penetration, general and resolution modes) can be integrated into the system if using broadband transducers or it may require a change in transducer (probe) depending on the type of system in use.

The choice of correct depth setting places the target in the centre of the image, allowing the target and surrounding structures to be viewed optimally. Some ultrasound machines (SonoSite) are fitted with an auto-focus function in the middle of the image area, others allow adjustment of the focal zone.

Time gain compensation (TGC) adjusts the brightness of the image, excessive gain can obscure important structures and insufficient gain can result in missing structures of low reflectivity. TGC control can be adjusted via sliders, near and far field controls or auto-gain control, depending on the machine.

Transducers

Transducer characteristics, such as operating frequency and probe shape, determine the image generated. The transducer frequencies used for peripheral nerve blocks range from 3 to 15 MHz. Modern transducers are broad bandwidth (broadband) transducers that are designed to generate more than one frequency. For example, a SonoSite HFL38 6–13 MHz transducer can generate ultrasound ranging in frequency from 6–13 MHz and is a 38 mm sized linear probe. With broad bandwidth transducers, the operator selects the examination frequency to match the target requirement. Linear and curvilinear (curved) transducers are most useful for nerve imaging to provide high-resolution images. Linear arrays produce images with a finely sampled, rectangular field of view, whereas curved arrays produce a diverging sector-shaped field of view that expands beyond the lateral extent of the transducer (Fig. 13.4).

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Figure 13.4 SonoSite transducers: from left to right: hockey stick (25 mm) high frequency linear array used in paediatric applications, small foot print sector probe (curvilinear array) as used for praecordial echocardiography, 38 mm high-frequency linear array used for peripheral nerve blocks.

The probe used should match the procedure being performed (Table 13.1). Choosing the wrong probe can make identification of anatomy difficult. It is important to use the highest frequency probe available for the depth of the target being scanned.

Table 13.1 Different types of probe and their applications in anaesthesia

image

Probes have different size footprints; small linear probes (3.8 cm footprint) are available which are more dextrous in paediatric practice compared to the standard sized adult probe of 5–6 cm.

Features

Image processing software

Additional image processing software is available and desirable to achieve better image quality, e.g. speckle reduction imaging and tissue harmonic imaging. Speckle is a very characteristic texture commonly seen in ultrasound images. It is an interference pattern superimposed on the ‘true’ image by scatterers too small and closely spaced to be resolved (see Chapter 31 for further description). The pattern depends on the beam angle and can be suppressed by combining images acquired with multiple steered ultrasound beams (spatial compounding, labelled variously on machines as compound mode, CT or multibeam). Multibeam and tissue harmonic imaging can offer some advantages over conventional (pulse-echo) imaging, including improved contrast resolution, reduced noise and clutter, improved lateral resolution, reduced slice thickness, reduced artefact and, improved signal-to-noise ratio. They are none the less user selectable modes reflecting the fact that they do also have downsides and do not always enhance the image obtained. Manufacturers have proprietary algorithms driving such features which in combination with the processing power of the device may determine the efficacy in any particular application of these imaging modes.

Image recording and remote viewing capability

A variety of integral still image and video capture functions are marketed to support training and data collection, but are often at extra cost. The use of proprietary formats for image capture by manufacturers is thankfully in abeyance, as these systems necessitate additional viewing and archiving software that is often clunky and integrates poorly with users’ own computers. USB export options and networking via Ethernet and wireless are available.

In the future, capture and storage of the complete patient episode may become routine and be considered a representative of the standard of care. Compatibility with local hospital PACS (Picture Archiving and Communications System) should, therefore, be considered when purchasing ultrasound machines.

The LCD screens of most devices have a very narrow viewing angle, and can only really be observed by one or two closely positioned observers. The facility to connect a second remote display is crucial both for teaching and for the benefit of patients who will be awake during ultrasound guided procedures.

On screen tutorials

User instructions presented as on-screen ‘help’ is an established feature in domestic consumer electronics and is increasingly prevalent in complex medical devices. Pattern recognition is an integral part of the use of ultrasound imaging to guide regional anaesthesia. In the Esaote MyLab One (Fig. 13.5A) the concept of on-screen help is further developed to include a library of images and explanatory notes: effectively a handbook of ultrasound guided regional anaesthesia, which can be shown alongside the real-time image to assist in the recognition of anatomy and the performance of blocks (Fig. 13.5B).

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Figure 13.5 A. MyLab One portable ultrasound, B. associated screen grab showing on screen tutorial for an interscalene block.

Images courtesy of Esaote SpA, Genoa, Italy.

Needle visibility

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