In 1978 La Grange et al ¹ 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).

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.

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

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).

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

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.

Figure 13.6 Pajunk SonoPlex needle 22 G with ‘Cornerstone reflectors’.

Adjuncts

Peripheral nerve block procedures should be carried out under aseptic conditions. Sterile plastic covers for the entire ultrasound probe as well as sterile ultrasound gel are available, although for ‘single shot techniques’, many practitioners use only a sterile adhesive plastic occlusive dressing over the probe. Acoustic couplants (such as water-based ultrasound gel) are necessary to exclude air and bridge the probe surface/skin interface and promote ultrasound transmission.

Needle guides designed to keep the needle within the plane of the ultrasound beam are available, but have not been found useful enough to gain any popularity.

Introduction of ultrasound imaging into clinical practice

The successful use of ultrasound is highly operator dependent and as such has a distinct learning curve. Practitioners using ultrasound without training have been shown to have more complications and lower success rates. Guidelines for ultrasound-guided regional anaesthesia have recently been published.^5,⁶ However, a universal agreement on how to teach ultrasonography for regional blocks is still lacking. A combination of basic and advanced workshops, and ongoing supervised practice, seems a logical approach to the safe and effective performance of ultrasound-guided blocks.

Commercial phantoms are available to practice needling techniques and enhance hand–eye coordination, or they can be homemade (using cooking gelatine and fruit!), but they suffer poor realism and are clearly no substitute to scanning live models for practising ultrasound image generation and interpretation.

Nerve stimulators

Nerve stimulators in general are discussed more fully in Chapter 17. A peripheral nerve stimulator (Fig. 13.7) allows localization of peripheral nerves without the need for direct contact between the needle and the target nerve to elicit paraesthesia. The technique was first described in 1912 and is still in common use, although in the developed world now somewhat relegated to second place in favour of ultrasound.

Figure 13.7 A Braun Stimulplex peripheral nerve stimulator.

Characteristics of a nerve stimulator for localization of peripheral nerves include:⁷

1. adjustable constant current output from 0 to 5 mA. It should be easy for the operator or an assistant to adjust the delivered current

2. a short stimulation pulse. The shorter the stimulation pulse, the greater the ratio of the current required to stimulate when the needle tip is 1 cm away from the nerve compared to when the needle is immediately adjacent to the nerve. Most commercially available nerve stimulators produce monophasic square wave impulses of between 0.1 and 1.0 ms

3. clearly marked polarity. The needle should be connected to the cathode (−) and the anode (+) to the surface of the patient’s skin

4. display showing the current being delivered.

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:

• needle tip design

• facility for electrical stimulation

• suitability for catheter placement

• needle diameter

• needle length.

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

Figure 13.8 Needle tips for comparison: A. 21 G hypodermic needle. Spinal needles: B. 22 G Quincke, C. 25 G Quincke, D. 25 G Whitacre, E. 25 G Sprotte. Peripheral nerve block needles: F. 21 G Locoplex (Vygon, France), Teflon coated, G. 22 G pencil point Polymedic, (te me na SAS, France), Teflon coated, H. 22 G SonoPlex (Pajunk, Germany), proprietary NanoLine coating over embossed pattern.

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).

Figure 13.9 Tuohy type nerve block needle with Huber point. Note the white insulation material on the shaft.

Stimulating needles

The shaft of a stimulating needle is coated with an insulator (e.g. Teflon) so that any electrical current passed through the needle spreads into the tissues from the tip only (Fig. 13.8 elements F–H). This coating also decreases friction allowing easier advancement of the needle. They now always have an electrical lead attached for connection to a nerve stimulator (in the past needles were used with a crocodile clip onto the non-insulated needle shaft).

Peripheral nerve block needles are usually supplied with a short length of clear flexible tubing between the needle hub and the Luer syringe connector in order to assist detection of aspirated blood and allow an ‘immobile needle’ before and during drug injection.

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.

Figure 13.10 A. An insulated Tuohy type needle with an integrated electrical connection (E) and a side arm injection port (S). B. A catheter (C) can be seen entering the hub assembly ready to be passed through the needle.

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.

Figure 13.11 A. A Pandin stimulating catheter and selection of Huber tipped insertion needles. B. Magnified view of the catheter tip to show conducting parts.

Photos courtesy of HDC Corporation, USA.

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.

Table 13.2 Standard wire gauge sizes and the metric equivalent diameter

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

• The viscosity of fluid that is to be injected. Local anaesthetics are usually prepared in aqueous solutions, which will pass through small diameter needles, but needles which are very small will significantly limit the speed at which other fluids (e.g. CSF or blood) can be aspirated. The latter is required for confirmation of correct placement or for the early recognition of a potential complication. Oil-based agents (for neurolytic blocks) will only pass easily through the larger diameter needles (at least 18 G).

• The rigidity required for its insertion. The longer the needle the more flexible it becomes, making it more prone to deflection, bending, buckling or even breakage. Thus, epidural needles are usually of 16 G or 18 G size to minimize these risks. Spinal needles need to be even longer, but large diameter needles make post-dural puncture headache (PDPH) more likely and so finer needles are used and inserted through an introducer.

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.

Figure 13.12 Intralong spinal catheter system, 21 G Sprotte needle with 25 G spinal catheter and internal Teflon coated stylet. Pajunk Medizintechnologie (Geisingen, Germany).

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.

Figure 13.13 A Tuohy needle with a Huber tip and an epidural catheter emerging at an angle of 20° to the shaft.

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.

Figure 13.14 16 G Tuohy needle and associated catheter showing the centimetre markings.

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.

Figure 13.15 27 G pencil point needle exiting a 16 G Tuohy needle, and the associated epidural catheter for comparison.

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.

Figure 13.16 Combined spinal/epidural kit with locking needle, Portex, Smiths Medical, UK.

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.

Figure 13.17 Portex CSEcure locking collar, see text.

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.