Regional anaesthetic techniques

Premedication

Premedication before performance of regional anaesthesia is very rarely required. Patient anxiety regarding being awake for block performance or surgery itself, is often very amenable to simple explanation and reassurance. It is helpful to enquire about specific patient concerns, such as being able to see or hear the procedure, because these are often easily addressed. A small dose of i.v. benzodiazepine is useful in some patients to achieve anxiolysis when performing a block. Nerve blocks can themselves sometimes be considered a form a premedication, such as a femoral nerve block performed to alleviate the pain of proximal femoral fracture before definitive surgical fixation. Patients undergoing regional anaesthesia should be fasted preoperatively in the same manner as if they were undergoing general anaesthesia.

Timing

A well-organised operating list will not be delayed by delivery of regional anaesthesia, which when conducted appropriately can improve theatre suite efficiency. It is essential that sufficient time is allowed to perform the block without undue haste on the part of the anaesthetic team. The risk of inadvertent wrong-sided nerve block is increased if time pressure and other distractions are allowed to impinge, even subconsciously, on the concentration of the anaesthetic team.

Needles

The use of very fine spinal needles (25–27G) reduces the incidence of postdural puncture headache (PDPH), as does the use of pencil-point Whitacre and Sprotte needles ( Fig. 25.1 ). The 27G Whitacre needles appear to be associated with the lowest incidence of PDPH, but confident and successful use of these needles requires greater expertise than is needed for the use of larger needles.

For peripheral blocks, short-bevelled needles allow greater tactile appreciation of fascial planes and may reduce the likelihood of nerve damage because they displace nerves rather than penetrate them. A variety of insulated needles are available for plexus and peripheral nerve blockade using a nerve stimulator. Needle visibility during ultrasound guidance may be improved by using echogenic needles that have ‘corner stone’ reflectors positioned at the distal end of the cannula shaft. For peripheral nerve blocks, syringes are usually connected to the block needle via flexible tubing. This allows the anaesthetist to hold the needle steady while aspiration tests are performed and syringes are changed, as required. The system must be primed to prevent air embolism and to avoid ultrasound image artefacts.

Catheters

Continuous administration of local anaesthetic drugs has been made possible by the development of high-quality perineural catheters, which may be left in position for several days. Careful fixation is essential to maintain the position of the catheter in the postoperative period. Skin adhesive glues are useful in this regard because they provide excellent fixation and minimise leakage of local anaesthetic from the catheter puncture site. Catheters should be labelled clearly to prevent inadvertent injection of i.v. medication.

Non-Luer connection systems for regional anaesthesia

The accidental connection of an intravenous infusion line or syringe to an epidural, intrathecal or perineural needle or catheter (or vice versa) can result in fatal harm. The Luer connector has been in use for more than 100 years and has become the most common small-bore medical connection system. The ubiquity of the Luer system poses an intrinsic risk of inadvertent wrong-route administration of medication. International standard ISO 80369 applies a dedicated non-Luer connector (neuraxial connectors or NRFit) for epidural, intrathecal and perineural devices that cannot connect to intravenous Luer connectors, with the aim of reducing the risk of wrong-route drug administration. This represents a major change to the equipment of hospitals and is expected to take several years to implement fully.

Nerve stimulators

Few anaesthetists continue to use the historical technique of deliberately eliciting paraesthesia to confirm perineural needle tip position when performing a peripheral nerve block. Some anaesthetists use a peripheral nerve stimulator ( Fig. 25.2 ), but an increasing number now use ultrasound guidance to identify needle tip placement. If using peripheral nerve stimulation, it is important to explain to the patient the sensation elicited by electrical stimulation; this usually causes very little discomfort, unless the contracting muscle crosses a fracture site, when duration of stimulation should be kept to the absolute minimum necessary to confirm needle position.

Stimulators that deliver a constant current and give a digital display of the current used are readily available. One lead is attached to an electrode on the patient's skin, and the other lead to the needle. After skin puncture, the stimulator is set to a frequency of 1–2 Hz and an initial current of 1–2 mA. Most stimulators have a visual display to confirm a complete circuit when the needle touches the patient. If this fails, connections should be checked or the electrode replaced. Failure to confirm a complete circuit could result in unwanted paraesthesia or nerve injury from repeated needling.

As the nerve is approached, motor nerve fibre stimulation causes muscle contraction in the appropriate distribution. The muscle contraction sought from nerve stimulation is often different from contractions resulting from direct contact of the needle tip with muscle tissue that overlies the target nerve. The current is reduced until visible muscle contraction is still present at a current of (optimally) 0.5 mA; reducing the current still further, the twitch should be seen to disappear when current is very low (e.g. 0.2 mA). A gentle aspiration test is then performed and 1–2 ml local anaesthetic solution slowly injected. Muscle contraction should cease immediately because of nerve displacement. If it does not, and an insulated needle is being used, the tip may have moved beyond the nerve or be placed intravascularly. In this circumstance gentle aspiration should be repeated, the needle withdrawn slightly and the procedure repeated. Severe pain on injection suggests intraneural injection, in which case the needle should be withdrawn and repositioned. When the needle tip has been correctly positioned, the remainder of the anaesthetic solution should be injected slowly with occasional test aspirations.

Ultrasound

A variety of high-quality ultrasound machines are now readily available and these have contributed greatly to advances in regional anaesthetic techniques. All ultrasound machines consist of a display, keyboard or touchscreen menu with transducer controls, computer processing unit and transducer. Many are also equipped with disk storage facilities or printers to allow a record to be made of procedures. The principles of ultrasound are discussed in Chapters 15 and 17 . Production of a clear target image and safe needle guidance requires sound cross-sectional anatomical knowledge along with excellent technical skills.

The most common transducers used for ultrasound-guided regional anaesthesia are the linear or curved array probes. Linear high-frequency probes (8–12 MHz) are used to produce superficial images of high resolution, such as would be required for interscalene or axillary brachial plexus block. Curved array low-frequency probes (4–7 MHz) provide improved penetration to visualise deeper structures but with reduced resolution. Using the curved array probe for deeper blocks will provide a broader field of view for appreciation of surrounding anatomical structures and landmarks, such as during performance of a subgluteal sciatic block. The view obtained by a chosen transducer can be optimised by altering the screen depth, the gain (screen brightness) and, on some machines, the adjustable focusing of the beam. Most ultrasound machines allow the operator to select anatomical structures of interest from a preset menu (e.g. ‘nerve’ or ‘vascular’), and this alters these variables automatically to improve image quality. The operator can manipulate the transducer position on the patient by a combination of pressure, alignment, rotation and tilt ( Fig. 25.3 ). The search for an optimal screen image is made easier if each of these movements are applied systematically, rather than changing them concurrently.

Most nerves exhibit a ‘honeycomb’ appearance on scanning – a combination of nerve fascicles and connective tissue, which varies in appearance depending on the individual nerve, its location and the angle of incidence of the probe. More proximal nerve roots, such as with interscalene imaging, tend to appear hypoechoic or dark as a result of reduced amounts of connective tissue compared with the axilla and peripherally.

There is increasing evidence that ultrasound offers several advantages over traditional landmark or nerve stimulator nerve localisation techniques ( Box 25.1 ).

Monitoring

Monitoring equipment should be appropriate to the anaesthetic technique and surgical procedure, with a minimum of three-lead ECG, NIBP and pulse oximetry for all blocks.

Asepsis

As a minimum for all peripheral nerve blocks, operators should wash their hands, wear gloves and draw up and store local anaesthetic syringes in a clean manner. Skin over the scanning and injection site should be prepared with a solution of chlorhexidine 0.5% in alcohol, which should be allowed to dry. Ultrasound probes should be covered with a sterile sheath. Sterile conductivity gel should be used to facilitate ultrasound wave penetration. All major blocks, such as neuraxial blocks or blocks siting a perineural catheter, should be performed under strict aseptic technique with sterile gloves, gown, hat, mask and drapes.

Inadvertent wrong-sided block

Inadvertent wrong-sided peripheral nerve blockade is an avoidable anaesthetic complication. Such events are uncommon but can have serious consequences. These include complications from the unnecessary block such as nerve injury and local anaesthetic toxicity, delayed hospital discharge, patient anxiety and distress and the risk of wrong-sided surgery if the error is not identified promptly ).

A ‘stop before you block’ moment, undertaken as part of every peripheral nerve block, is a strong preventative measure to avert wrong-sided blocks. Many hospitals have adopted the World Health Organization (WHO) surgical safety checklist, including a ‘sign in’ before anaesthesia is commenced, where the surgical site marking is confirmed against the written consent form. This is not a substitute for a correctly performed ‘stop before you block’ moment, which should occur before every peripheral nerve block immediately before needle insertion and requires the anaesthetist and anaesthetic practitioner to reconfirm:

• patient identity and consent form;

• the side of the block; and

• the surgical site marking.

Circumstances that require particular vigilance include the following:

• Peripheral nerve blocks performed as a sole procedure rather than in conjunction with a surgical procedure, such as a femoral nerve block the day before the operative management of a proximal femoral fracture. In these circumstances an operative marking may not have been made and a full WHO surgical safety checklist process is recommended, including relevant written procedural consent for the block and block site marking.

• Moving or turning patients between ‘sign in’ and performance of the nerve block. This can result in the block site ‘moving’ in relation to the anaesthetist and equipment in the room. Certain blocks often necessitate repositioning of patients (e.g. popliteal blocks), and additional care is required to use the ‘stop before you block’ appropriately.

• Delays and distractions before and during performance of the peripheral nerve block. Certain time-critical and important distractions cannot be avoided by the anaesthetist and anaesthetic assistant, and these will occasionally occur at key moments in the performance of a block. Vigilance is required by the anaesthetist to be aware of these distractors and if any disruption to the usual ‘stop before you block’ procedure occurs, the entire procedure should be discontinued and re-started at a more appropriate time.

• Covering of preoperative surgical marking with blankets, dressings or drapes.

• Presence of an ultrasound machine in the anaesthetic room for purposes other than performance of the block (e.g. for venous access). The use of an ultrasound machine in the anaesthetic room for this indication should alert the anaesthetist to the high risk of the wrong-sided block because of the potential unconscious selection of block side based on the presence of an ultrasound machine on one side or other of the patient.

• Presence of negative external and internal influences on anaesthetist's performance. Time pressure, hunger, thirst, fatigue, background noise and emotional distraction can all increase the risk of wrong-sided block. If these are present and may affect task performance, the anaesthetist should always respond and address these demands before undertaking a peripheral nerve block.

Supplementary techniques

A local anaesthetic may be the only drug administered to the patient, or it may form part of a balanced anaesthetic technique. During surgery, patients may be awake, or sedated by i.v. or inhalational means. Intermittent boluses of midazolam or target-controlled infusions of propofol are commonly used to provide intraoperative sedation. General anaesthesia may be used as a planned part of the procedure. A combination of regional and general anaesthesia may be useful to obtain advantages from both, particularly for prolonged procedures or where positioning is difficult because of trauma or arthritis.

When a surgical tourniquet is used, the chosen block must extend to the tourniquet site unless the procedure is brief. Discomfort from prolonged immobility on a hard table may be relieved by the administration of analgesia during surgery. This type of discomfort is usually not relieved by sedative drugs, which may result in the patient becoming agitated, confused and uncooperative.

Aftercare

At the end of surgery, clear postoperative instructions should be given to both patients and healthcare professionals taking over their care. This may include general advice about analgesia management when a block wears off and care of the insensate limb. It may also include a reiteration of the preoperative advice about block-specific adverse effects, such as Horner's syndrome after interscalene brachial plexus blockade.

Block failure

When confronted with the apparent failure of a neuraxial or peripheral nerve block to produce the desired analgesic or anaesthetic effect, the anaesthetist must take a calm, step-wise approach to identify the precise problem and remedy this accordingly. In the circumstances of a block sited preoperatively this is always best done before the patient is prepared and positioned for surgery. Clear communication with the patient regarding sensory expectations is mandatory, and this should have been undertaken as part of the consent process. To determine whether particular myotomes or dermatomes are inadequately anaesthetised, a focused examination of the motor and sensory systems is required ( Fig. 25.4 ). This requires excellent communication between clinician and patient to avoid ambiguity, and a senior anaesthetist may be required at this point. Inadequate sensory blockade in some dermatomes may sometimes be remedied by repeat nerve block, but this should be performed by an experienced practitioner and with the total dose of local anaesthetic in mind. Inadequacy of regional blockade to achieve anaesthesia to the site of surgery should not be managed by increasing doses of sedation, although i.v. analgesics such as fentanyl can be a useful adjunct in awake surgery. Occasionally, conversion to general anaesthesia is required.

Indications

Intravenous regional anaesthesia is suitable for short procedures when postoperative pain is not marked, such as manipulation of Colles’ fracture or carpal tunnel decompression. Recovery is rapid, and the technique is appropriate for day-case surgery.

Method

Intravenous regional anaesthesia involves isolating an exsanguinated limb from the general circulation by means of an arterial tourniquet and then injecting local anaesthetic solution intravenously. Analgesia and weakness occur rapidly and result predominantly from local anaesthetic action on peripheral nerve endings.

An orthopaedic tourniquet of the correct size is applied over padding on the upper arm. All connections must lock, and the pressure gauge should be calibrated regularly. An i.v. cannula is sited in the contralateral arm in case administration of emergency drugs is required. An i.v. cannula is inserted into a vein of the limb to be anaesthetised. A vein on the dorsum of the hand is preferred because injection into more proximal veins reduces the quality of the block and increases the risk of toxicity. Exsanguination by means of an Esmarch bandage improves the quality of the block and increases the safety of the technique by reducing the venous pressure developed during injection. In patients with a painful lesion (e.g. Colles’ fracture), elevation combined with brachial artery compression is adequate. The tourniquet should be inflated to a pressure 100 mmHg above systolic arterial pressure.

In an adult, 40 ml prilocaine 0.5% is injected over 2 min ensuring that the tourniquet remains inflated. Analgesia should be achieved within 10 min, but it is important to inform the patient that the feeling of touch is often retained. The anaesthetist must be ready to deal with toxicity or tourniquet pain throughout the surgical procedure. The tourniquet should not be released until at least 20 min after injection, even if surgery is completed. This delay allows for diffusion of drug into the tissues so that plasma concentrations do not reach toxic levels after release of the tourniquet. The technique of repeated reinflation and deflation of the cuff during release has little effect on plasma concentrations and is not advised.

Tourniquet pain

Tourniquet pain may be troublesome if the cuff remains inflated for longer than 30–40 min. It is sometimes alleviated by inflating a separate tourniquet below the first on an area already rendered analgesic by the block; the first cuff is then deflated. Failing this, general anaesthesia is preferable to administration of large and often ineffective doses of opioids and sedatives.

Choice of drug

The agent of choice for this procedure is isobaric prilocaine 0.5%. It has an impressive safety record with no major reactions reported after its use, although minor adverse effects such as transient light-headedness after release of the tourniquet are not uncommon. Prilocaine has distinct pharmacokinetic advantages for IVRA and does not cause methaemoglobinaemia in the doses used.

Lower limb

Intravenous regional anaesthesia of the foot may be produced using the same dose of prilocaine and a calf tourniquet positioned carefully at least 10 cm below the tibial tuberosity to avoid compression of the common peroneal nerve on the fibular neck.

Contraindications to central nerve blocks

Most contraindications are relative, but the following are best generally regarded as absolute contraindications to neuraxial blockade:

• uncorrected abnormality of coagulation;

• significant hypovolaemia;

• infection at the injection site;

• systemic sepsis manifested by pyrexia or rising inflammatory markers despite resuscitation and antibiotic therapy;

• severe stenotic valvular heart disease (particularly aortic stenosis) or obstructive cardiomyopathy;

• raised intracranial pressure;

• patient refusal; and

• allergy to local anaesthetic medication

Anatomy of the epidural and subarachnoid space

The epidural space is the space between the periosteal lining of the vertebral canal and spinal dura mater. It contains spinal nerve roots, lymphatics, blood vessels and a variable amount of fat ( Figs 25.5 and 25.6 ). Its boundaries are as follows:

• Superiorly – foramen magnum, where the dural layers fuse with the periosteum of the cranium; hence, local anaesthetic solution placed in the epidural space cannot extend higher than this

• Inferiorly – sacrococcygeal membrane

• Anteriorly – posterior longitudinal ligament

• Posteriorly – ligamentum flavum and vertebral laminae

• Laterally – pedicles of the vertebrae and intervertebral foramina

In the normal adult the spinal cord begins at the foramen magnum and ends at the level of L1 or L2 (though it may end lower); here it becomes the cauda equina. The subarachnoid space extends further than the cord, to the level of S2. Below this level, the dura blends with the periosteum of the coccyx. Between the dura and arachnoid is the subdural space, within which the local anaesthetic solution may spread extensively.

The volume of the vertebral canal is finite. An increase in volume of contents of one compartment reduces the compliance of the other compartments and increases the pressures throughout.

Indications

Blockade is produced more consistently and with a lower dose of drug by the spinal route than by epidural injection. Duration of analgesia is usually limited to 2–4 h depending on surgical site and may be prolonged by use of intrathecal opioids such as diamorphine, fentanyl or morphine. These drugs carry a minimal risk of serious adverse effects such as respiratory depression, but nausea, pruritus or urinary retention are not uncommon.

Spinal anaesthesia is most suited to surgery below the umbilicus and in this situation the patient may remain awake. Surgery above the umbilicus using spinal block is less appropriate and would usually necessitate addition of a general anaesthetic to abolish the unpleasant sensations from visceral manipulation resulting from afferent impulses transmitted by the vagus nerve.

Types of surgery

• Urology. Spinal block is commonly employed for urological procedures such as transurethral prostatectomy, but it should be remembered that a block to T10 is required for surgery involving bladder distension. Perineal and penile operations may also be carried out using a low ‘saddle block’, peripheral blockade or caudal anaesthesia.

• Gynaecology. Minor procedures may be performed reliably with a block to T10. Pelvic floor surgery and vaginal hysterectomy may also be carried out readily with spinal anaesthesia extending to T6, but for procedures requiring laparoscopic assistance, general anaesthesia is necessary.

• Obstetrics. Spinal anaesthesia is considered the technique of choice for the clear majority of elective caesarean sections and a large proportion of emergency ones. The technique is discussed in detail in Chapter 43 .

• Orthopaedics. Spinal anaesthesia is suitable for virtually every type of lower limb surgery and these are discussed in more detail in Chapter 36 .

Performance of spinal anaesthesia

Preparation

Full monitoring must be applied and wide-bore i.v. access secured. A full sterile technique must be used (mask, gown, hat, gloves, sterile drapes, antiseptic skin preparation). The skin should be cleaned with 0.5% solution chlorhexidine in alcohol, which should be allowed to evaporate fully (to prevent wicking of the potentially neurotoxic solution into the needle when it touches the skin).

Spinal blockade may be performed with the patient sitting or in the lateral decubitus position ( Table 25.2 , Fig. 25.7 ). If it is anticipated that the procedure may be technically difficult, the midline is usually more discernible with the patient in the sitting position, but the risk of hypotension in the sedated patient or after development of the block may be increased.

Table 25.2

Techniques of spinal anaesthesia

Type of block	Upper level of analgesia	Position during lumbar puncture	Volume of solution
Saddle block	S1	Sitting 5 min	1 ml hyperbaric solution
Low thoracic	T10–12	Sitting/lateral decubitus	2–3 ml hyperbaric solution
Mid thoracic	T4–6	Lateral decubitus/sitting (immediately supine)	3–4 ml hyperbaric solution
Unilateral: A unilateral block, or at least a differential block between limbs, may be achieved by the slow injection of small volumes (1–1.5 ml) of hyperbaric solution in the lateral position. This position then must be maintained for at least 15 min to minimise spread. On return to the supine position there may still be some contralateral spread, and the necessity for smaller volumes and dosage may increase block failure rate.

Technique of spinal blockade in the lateral position

For the right-handed anaesthetist, the patient is positioned on the operating table in the left lateral position. The patient's back should lie along the edge of the table. A curled position, with knees drawn to the body and chin on chest, opens the spaces between the lumbar spinous processes. An assistant stands in front of the patient to assist with positioning and to provide reassurance.

Landmark techniques, an example of which is described next, are most commonly used to achieve spinal blockade; however, ultrasound can be used to assist in the placement of spinal (and epidural) needles and is of particular utility in obese patients or those with abnormal vertebral anatomy.

A line (Tuffier's line or the intercristal line) between the iliac crests approximates to the fourth lumbar vertebral body, and spinal injection should be performed at the L3/4 or L4/5 space (see Fig. 25.7 ). All drugs should be drawn into non-Luer connection syringes directly from sterile ampoules using a filter needle to prevent the injection of glass particles into the intrathecal space. A selection of non-Luer connection spinal needles (22–27G) should be available.

The skin and subcutaneous tissues are infiltrated with local anaesthetic using a narrow-gauge needle. Most fine-gauge spinal needles come with a 19G introducer needle to assist in the guidance of the flexible spinal needle to the intrathecal space. This introducer needle, or the spinal needle itself, is inserted in the midline, midway between two spinous processes. In the well-positioned patient, the needle is directed at right angles to the skin, toward the naval. Passage through the interspinous ligament and ligamentum flavum into the spinal canal is appreciated easily with a 22G needle ( Fig. 25.8A ), but these needles are now rarely used because of the high incidence of PDPH. With some practice, these structures can be discerned with a 25G or 27G pencil-point needle, and a click may be felt immediately after the needle tip passes beyond the ligamentum flavum and punctures the arachnoid mater. When the needle tip has breached the ligamentum flavum and epidural space to enter the intrathecal space, the stilette is withdrawn from the needle and the hub observed for CSF flow. A gentle aspiration test can be performed if free flow of CSF is not observed or the needle carefully rotated through 90 degrees.

The most common reasons for difficulty accessing the intrathecal space are incorrect patient positioning, failure to insert the needle in the midline and directing the needle laterally ( Fig. 25.8B ). This last fault is seen most easily from one side and is usually apparent to onlookers but not to the anaesthetist, who looks only along the line of the needle.

When CSF is obtained, the non-Luer connection syringe containing the injectate should be carefully attached to the spinal needle, taking care not to displace the tip. Gentle aspiration, with eddies of CSF seen in the local anaesthetic solution, confirms the needle tip position and the solution is injected slowly (although some have recommended rapid injection to obtain effective blocks using small volumes of injectate). Further aspiration towards the end of injection confirms that the needle tip has remained in the correct place. Needle and introducer are withdrawn together and the patient then repositioned to facilitate the desired spread of the block.