Equipment for regional anaesthesia

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Chapter 13 Equipment for regional anaesthesia

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).

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:

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).

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).

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).

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.

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.

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).

Needle visibility

The single greatest benefit of the use of ultrasound for peripheral nerve blocks is often said to be that it allows visualization of the needle throughout the performance of the block and adequate visualization of the needle tip is mandatory for safe and effective blocks. In essence larger (calibre) needles are more readily visible on ultrasound and the visibility of all needles becomes less as the distance from the probe increases or the orientation of the needle becomes less perpendicular to the ultrasound beam.

The ultrasound visibility of different needles has been investigated;3 however, no studies have investigated success rates with different needle types, so there is as yet no ‘ideal needle’, or any evidence that this will improve success rates or safety. There are, however, continuing developments of new needles aimed at facilitating block performance and needle visibility. Various surface coatings have been attempted to improve needle visibility, with little success. Piezoelectric vibrating needles is one example of an interesting new approach, but it has not gained currency.4 A more promising recent development is the SonoPlex needle (Pajunk, Geisingen, Germany), where a pattern deeply impressed into the surface of the needle shaft ensures that a strong reflective surface remains relatively perpendicular to the ultrasound beam at all needle angles (Fig. 13.6). Even more recently software-driven image enhancements (e.g. the ‘enhanced needle visualization’ software upgrade from Sonosite) can be used to allow better visualization of standard needles even when inserted in a steeper plane.

Needles and catheters

General considerations

Needles used to administer local anaesthetic drugs may be selected on the basis of one or more of the following features:

Some of these aspects are considered individually below.

Needle tip design

For the regional anaesthetist needle tip design is mainly concerned with reducing the risk of nerve injury. Although a variety of designs are available, none confer absolute safety: it is perfectly possible to place any needle tip inside a nerve and inject local anaethetic (LA) into it causing either temporary or long-lasting damage, or no damage at all.

Traditional hypodermic needles have a cutting tip designed for easy penetration of skin and tissues and are best avoided for peripheral nerve blocks. In contrast a short bevelled needle is more ‘blunt’ in use and is believed to give improved ‘feel’ of anatomical layers aiding identification of tissue planes by eliciting a ‘pop’ as the layers are penetrated (compare Fig. 13.8 elements A, F, H). Theoretically, a short bevelled (or facetted) needle can touch a nerve eliciting paraesthesia with less risk of nerve injury,1 but there is some controversy. In general, it appears that long, flat bevels are more likely to cause nerve trauma,8 but that the trauma will be more serious if a short bevel needle does make vigorous contact with a nerve.9

Pencil point needle tips (Fig. 13.8 elements C, D and G) are believed to separate tissues through which they pass rather than cutting them. It is not clear whether a short bevel or a pencil point tip is safer to use.

The Huber tip provides a blunt relatively non-cutting and side-facing advancing edge, which facilitates catheter placement and is hence commonly used on epidural needles and other needles for catheter-based peripheral nerve block techniques (Fig. 13.9).

Catheters

Insertion

Catheters are used to facilitate continuous nerve blockade. The different catheter sets available contain an introducer, a catheter and associated parts such as a filter and detachable connector. A popular introducer design is the 18 G insulated stimulating needle with a curved Huber tip (Fig. 13.10). It is believed to ease the placement of the associated 21 G catheter, particularly if the nerve is to be approached at right angles to its long axis. Another introducer design is the plastic over the needle cannula (usually 15–17 G) familiar as the traditional intravascular cannula, which after removal of the introducing needle is used to convey the catheter. This type of introducer for catheter placement tends to be more troublesome and less popular as once the needle has been removed, the cannula is prone to misplacement and kinking during insertion and passage of the final catheter.

Although a variety of different products are available and injection of LA or saline might open up the space to aid the insertion of the catheter, it can still be very difficult to position the catheter into the desired location.

Design

All catheters have length markings to allow calculation of the depth of insertion distal to the needle. Catheters may have a single end hole or multiple side holes. The latter are perhaps unsuitable for continuous peripheral nerve block, because one or more of the holes may lie outside the fascial plane in which the nerve lies. Infused local anaesthetic will take the path of least resistance, which may be through an orifice that is either some distance from the nerve or outside its fascial surrounds. This leads to secondary block failure (the initial block which was administered through the needle works, but then wears off as the infusion fails to maintain anaesthesia).

Some very fine catheters have a removable wire stiffener inside which makes passage between the fascial planes easier. Unfortunately, this may also allow the catheter to pass out of the correct plane more readily, resulting in secondary failure of the block. Some catheters are radio-opaque, but if not, contrast may be injected to confirm accurate placement.

A stimulating catheter is made from insulating plastic material and usually contains a metallic wire, inside which the current is conducted to its exposed tip electrode (Fig. 13.11). Stimulating catheters are placed through a nerve block needle, which itself may be placed using nerve stimulation. The catheter can then be stimulated to reconfirm the catheter tip position in close proximity to the target nerve.

Needle diameter and length

Needle diameters are usually quoted in terms of standard wire gauge – usually shortened to gauge and represented as G. Table 13.2 shows the metric equivalents of some of the commonly used sizes. Standard notation is that the size is defined by the external diameter of the needle shaft, there being little or no standardization of internal diameters.

The diameter of needle required for a particular purpose will depend on two main factors:

Given that shorter needles are easier to control; most manufacturers produce each needle design in a choice of lengths so that the shortest adequate length for the application can be selected.

Spinal anaesthesia

Spinal needles are used to administer intrathecal drugs. The main consideration in the design is the need to minimize the risk of PDPH. The needle tip should produce minimal trauma and make the smallest possible hole into the dura. The tip may be either a bevelled cutting design or a non-cutting rounded pencil point.

The Quincke spinal needle has a cutting tip (similar to a hypodermic needle: compare Fig. 13.8 elements A–C). 27 G Quincke needles have an incidence of 2.5–3.5% of PDPH. It is often opined that to insert these such that the bevel is parallel to the largely longitudinal dural fibres, produces a smaller defect in the dura as less fibres are transected by the cutting edge of the needle.

For any given needle type, the smaller the bore the less likely the patient is to develop PDPH. Extremely fine 29 G needles are available, but are prone to bending at insertion and identification of back flow of CSF or blood is very slow and can be difficult. Currently needles of 24–27 G are most commonly used.

Atraumatic’ pencil point spinal needles like the Whitacre needle have a completely rounded non-cutting bevel with a solid tip, the opening being 2 mm proximal to the tip on the side. An adaptation of the Whitacre needle design is the Sprotte needle with a blunt ogival (bullet shaped) tip with an elongated lateral needle opening and a wider internal diameter for CSF flow (Fig. 13.8, elements D, E). These needle designs aim to push or stretch the dural fibres aside rather than cutting them, resulting in better dural closure after removal of the needle. They are associated with a significantly lower incidence of PDPH (0.8–1%).

Potentially the lateral opening of the needle may straddle the dura such that CSF can be aspirated, but injected local anaesthetic may be deposited into the epidural space.

There are concerns that pencil point needles have to be inserted further into the intrathecal space to ensure that the lateral hole (which is not at the tip of the needle) is within the CSF. The issue being that, if the needle is inserted above the level of L3/4, this may lead to the tip of the needle penetrating the conus medullaris of the spinal cord.

All spinal needles have a removable stylet to stiffen the needle and prevent possible coring of the skin, with resultant obstruction of the needle or contamination of the spinal space with epidermal tissue and skin bacteria. Introducer needles (18–20 G) are available to facilitate the insertion of smaller (25–29 G) and blunt spinal needles, and to avoid contact of the spinal needle with skin and subcutaneous tissue.

Spinal needles are available at various lengths to suit different sized patients. Currently in use are needles from 50 mm in length for paediatric patients, to 152 mm long needles for very obese patients, with 90 mm lengths most commonly used for the majority.

The design of the hub should facilitate CSF visualization; some smaller gauge needle hubs are designed to have a magnifying effect to assist early recognition of CSF backflow.

Microspinal catheters

Although epidural catheter systems may be used for continuous spinal anaesthesia, concerns about PDPH have led to the development of microspinal catheters. Intrathecal catheters offer great flexibility in allowing both the extent and duration of a spinal anaesthetic to be titrated precisely.

Various designs of needle and catheter system are available ranging from 20 G catheter through 18 G Tuohy needle, through to a 24 G catheter placed over a 29 G Quincke spinal needle which is then withdrawn on the end of a flexible stylet. Fig. 13.12 shows a typical 25 G microcatheter through 21G Sprotte needle system. Catheters as fine as 28–32 G are available.

Dural puncture with an 18 G Tuohy needle makes this procedure most suitable for use in the elderly who are less prone to PDPH. A stiff 20 G catheter introduced in the subarachnoid space may induce transient radicular irritation.

The utility of microcatheter systems is limited by the difficulties in ensuring that such a fine catheter is correctly placed. Combined spinal/epidural techniques provide many of the benefits without the same concerns regarding catheter position, leaving spinal catheter systems to a select, but loyal following. In the past, a small number of patients have developed the cauda equina syndrome following continuous spinal anaesthesia. Although initially ascribed to the use of the indwelling microspinal catheters themselves, it now seems likely that the true cause was the repeated localized exposure of nerve roots to very high concentrations of hyperbaric 5% lidocaine.10

Epidural anaesthesia

Epidural access is most commonly performed using the Tuohy needle with a Huber tip to facilitate insertion of the epidural catheter (Fig. 13.13). The Huber tip is relatively blunt and the shape ensures that the catheter emerges at an angle of 20° to the shaft. The disadvantage is that a catheter that has been passed beyond this needle tip cannot be withdrawn without the risk of being sheared off.

The Crawford epidural needle, with a short conventional bevel, is sometimes used for a paramedian approach, as it slides off the lamina easily and a catheter can thread directly upwards when a 45–60° angle is used to enter the epidural space.

The standard Tuohy needle is approximately 11 cm in length with an 8 cm needle shaft and 3 cm hub assembly, although a longer version (15 cm) is available. The diameter is most commonly 16 or 18 G for adult patients.

Most needles have 1 cm markings on the shaft to show the depth of insertion from the skin surface (Fig. 13.14). A 19 G needle of 5 cm length for paediatric use has 0.5 cm surface markings. All epidural needles are supplied with close fitting stylets to prevent tissue entering the lumen on insertion. Catheters for epidural placement are offered with a choice of single end hole or multiple side holes, have appropriate centimetre markings and, once in place, are attached to a 0.2 µm mesh hydrophilic filter through which the anaesthetic agent is injected.

A low-friction syringe is most commonly used to aid identification of the epidural space using the technique of loss of resistance to injection. Such syringes can be made of plastic, glass or combined glass and metal. Spring-loaded and other novel loss-of-resistance devices are designed to automatically discharge when the epidural space is reached, but are not in common usage.

Equipment for combined spinal/epidural (CSE) techniques

CSE combines the benefits of spinal anaesthesia – namely the certainty of a definitive endpoint (appearance of CSF) and rapid onset – with the flexibility of the catheter-based technique seen in continuous epidural anaesthesia.

The two approaches to CSE currently in practice are the needle-through-needle technique (NTN) and the double-space technique. When two separate spaces are used, the epidural component is completed before the intrathecal injection is attempted.

With the NTN, the intrathecal injection follows epidural insertion of the Tuohy needle, which then serves as an introducer for the very fine (25–27 G) intrathecal needle. After the spinal needle is removed the epidural catheter is inserted through the Tuohy needle. Usually, a pencil point type spinal needle of sufficient length (120 mm) to allow 13–15 mm projection beyond the end of a standard Tuohy needle is used (Fig. 13.15). Although sufficient projection is important longer spinal needles can puncture the anterior aspect of the dura and cause a greater CSF leak.

Commercially available kits have been produced for CSE to overcome various concerns (Fig. 13.16). Using matched needles to avoid damage to the spinal needle as it is passed through the Tuohy epidural needle prevents introduction of very fine metal particles into the CSF. Minimal drag between the two needles is essential to generate the dural ‘click’ when the spinal needle punctures the dura.

A conventional spinal needle, which does not lock within the much larger epidural needle, is minimally held by the dura alone and is difficult to handle and stabilize during injection of spinal medication. The displacement of the spinal needle during aspiration of the CSF and injection may result in failed anaesthesia or may push the spinal needle deeper, leading to nerve damage or anterior dural perforation. Several adjustable locking devices have been developed, but some designs can interfere with the essentially tactile task of identifying dural puncture. Fig. 13.17 shows the collar of Portex CSEcure system which permits easy needle advancement, but can lock it 0.5 mm intervals whilst still allowing the spinal needle to be rotated.

To avoid unintentional passage of the epidural catheter through the dural hole caused by the spinal needle, a special epidural needle has been developed. It contains an orifice in the leading edge, formed by the back wall of the Huber tip, for separate spinal needle passage. This and other designs for NTN CSE, such as dual lumen needles, have so far not made any significant impact in this field.

Ambulatory continuous infusions of local anaesthetic

Elastomeric infusion pumps are discussed in Chapter 19. The devices used for ambulatory regional analgesia, patient-controlled analgesia and wound irrigation are fundamentally the same.

References

1 La Grange P, Foster PA, Pretorius LK. Application of the Doppler ultrasound blood flow detector in supraclavicular brachial plexus block. Br J Anaesth. 1978;50:965–967.

2 Kapral S, Krafft P, EibenbergerK, Fitzgerald R, Gosch M, Weinstabl C. Ultrasound-guided supraclavicular approach for regional anaesthesia of the brachial plexus. Anesth Analg. 1994;78:507–513.

3 Maecken T, Zenz M, Grau T. Ultrasound characteristics of needles for regional anesthesia. Reg Anesth Pain Med. 2007;32:440–447.

4 Klein SM, Fronheiser MP, Reach J, Nielsen KC, Smith SW. Piezoelectric vibrating needle and catheter for enhancing ultrasound-guided peripheral nerve blocks. Anesth Analg. 2007;105:1858–1860.

5 Board of the Faculty of Clinical Radiology, Royal College of Radiologists. Ultrasound training recommendations for medical and surgical specialties. London: Royal College of Radiologists; 2004.

6 Sites BD, Chan VW, Neal J, Weller R, Grau T, Koscielniak-Nielsen ZJ, et al. The American Society of Regional Anesthesia and Pain Medicine and the European Society of Regional Anaesthesia and Pain Therapy. Joint Committee recommendations for education and training in ultrasound-guided regional anesthesia. Reg Anesth Pain Med. 2009;34:40–46.

7 Kaiser H, Niesel HC, Hans V. Fundamentals and requirements of peripheral electric nerve stimulation. A contribution to the improvement of safety standards in regional anesthesia. Reg Anesth. 1990;13:143–147.

8 Selander D, Dhuner KG, Lundborg G. Peripheral nerve injury due to injection needles used for regional anesthesia. Acta Anaesthsiol Scand. 1977;21:182.

9 Rice ASM, McMahon SB. Peripheral nerve injury caused by injection needles used in regional anaesthesia: influence of bevel configuration, studied in a rat model. Br J Anaesth. 1992;69:433.

10 Faccenda KA, Finucane BT. Complications of regional anaesthesia Incidence and prevention. Drug Safety: An International Journal of Medical Toxicology and Drug Experience. 2001;24(6):413–442.

11 National Patient Safety Agency. Safer spinal (intrathecal) epidural and regional devices – Part A. http://www.nrls.npsa.nhs.uk/alerts/?entryid45=94529&q=0%C2%ACwrong+route%C2%ACaccessed28/7/11, 24 November 2009. NPSA/2009/PSA004A (and updated 31 January 2011: available at