Local Anaesthetic Techniques
Local anaesthetic techniques are used for both operative anaesthesia and for postoperative analgesia. They are becoming more popular as a result of advances in drugs, equipment and improved techniques of anatomical localization, including nerve stimulation and ultrasonic location. In addition, there is a greater appreciation of the need to improve postoperative pain control using techniques that not only reduce pain but have the ability to abolish it and potentially improve outcome. This chapter outlines the basic principles of patient management and the methods used in the performance of a variety of blocks which are commonly undertaken by the trainee anaesthetist. Regional techniques for obstetrics and dental surgery are described in other chapters.
FEATURES OF LOCAL ANAESTHESIA
Regional anaesthetic techniques may be used alone or in combination with sedation or general anaesthesia, depending on individual circumstances. Advantages of regional techniques include:
Avoidance of the adverse effects of general anaesthesia. These may range from relatively minor postoperative nausea and vomiting, sore throat or myalgia to major issues such as respiratory impairment, awareness, airway complications or aspiration pneumonitis. In addition, the management of many patients with significant medical co-morbidity such as diabetes, obesity, or chronic pulmonary disease, can be improved or simplified. In elderly patients, acute perioperative cognitive impairment may be limited by reducing or avoiding psychoactive drugs and maintaining contact with their surroundings.
Postoperative analgesia. Local anaesthetic techniques can be used to provide effective prolonged postoperative analgesia whilst avoiding the systemic effects of other analgesic drugs, especially opioids. This can be provided using long-acting agents or by utilizing continuous catheter techniques, either neuraxial or peripheral. Some patients may be distressed by the accompanying numbness and motor block, but adequate preoperative explanation should minimize this concern. In addition, it is important that both nursing staff and patient are aware of the risk of tissue damage to any blocked area whether from direct trauma or indirect pressure from poor positioning or prolonged immobility. Simple techniques such as supporting the arm in a sling after brachial plexus block may help prevent injury and encourage earlier mobilization.
Preservation of consciousness during surgery. The ability to assess neurological status continuously may be an advantage in patients with a head injury, diabetes or those undergoing carotid endarterectomy. Patient positioning may be safer, more comfortable and damage to pressure areas or joints avoided if the patient is awake. Airway and neck manipulation can be avoided; this may be especially important in a patient with severe rheumatoid arthritis or an unstable cervical spine. The awake patient undergoing caesarean section under regional anaesthesia is able to protect her own airway and experience the birth of the child.
Sympathetic blockade and attenuation of the stress response to surgery.
Improved gastrointestinal motility and reduced nausea and vomiting. This can allow earlier feeding and more rapid mobilization and discharge.
There are now several studies suggesting that the net effect of these features may lead to a reduction in the incidence of major postoperative respiratory complications, though claims of other pathophysiological benefits remain unproven.
However, some patients may be unhappy at the prospect of being awake during surgery. In this situation the combination of a regional block with target-controlled intravenous sedation or general anaesthesia may be valuable. Similarly, this combination works well for prolonged surgery, where patient positioning may be compromised by generalized discomfort or where operation at several sites is necessary.
COMPLICATIONS OF LOCAL ANAESTHESIA
The incidence of complications may be minimized by ensuring adequate supervision and training in local anaesthetic techniques and by exercising care in the performance of each block. Many anaesthetists recommend performing all blocks in the awake (or lightly sedated) patient. The advantages of this are:
It encourages careful, meticulous practice
It provides the anaesthetist with valuable information on block onset and efficacy
It alerts the anaesthetist to early complications such as inadvertent intravenous injection or intraneural injection.
Sufficient expertise and equipment must always be available to deal with potential complications. Complications common to many techniques are discussed in this section; more specific problems are considered later.
Local Anaesthetic Toxicity
LA toxicity usually results from accidental intravascular injection, an excessive dose of local anaesthetic or faulty technique, particularly during performance of Bier’s block.
Clinical Features and Treatment
The clinical features and treatment of LA toxicity are described in Chapter 4.
Prevention
The following precautions are useful to minimize the risk of LA toxicity:
Of these, the main safety measure is slow injection. This prevents rapid production of very high plasma concentrations even if the injection is intravascular. By this means, toxicity may be diagnosed early, the injection discontinued and a major reaction avoided. Rapid injection of local anaesthetic is not necessary for the performance of any block. The use of ultrasound-guidance to visualize local anaesthetic spread may be a useful addition to reduce intravascular injection with peripheral nerve block techniques.
Test Dose
This may be used before administration of the main dose of local anaesthetic drug. It is indicated particularly for epidural block, where it should be capable of demonstrating inadvertent intravenous (i.v.) or subarachnoid injection. A test dose of 4 mL plain lidocaine 2% is sufficient to cause mild symptoms in most patients after accidental i.v. injection, and any features of local anaesthetic blockade 2 min after injection are good evidence of accidental subarachnoid block. No test dose is infallible; the most important factor in avoiding local anaesthetic toxicity is slow administration of the main dose.
Hypotension
There are several possible mechanisms by which a local anaesthetic technique may cause hypotension. The anaesthetist must always remember that surgical factors may be responsible.
Sympathetic Blockade
A limited sympathetic block may be produced by peripheral nerve anaesthesia, but only central blocks are likely to produce hypotension by this mechanism.
Total Spinal Blockade
This occurs occasionally during subarachnoid block if excessive spread of local anaesthetic solution occurs, and is a recognized complication of epidural block if the dura has been penetrated (see below). Apnoea may occur if local anaesthetic solution reaches the cerebrospinal fluid (CSF) from perforation of a dural cuff during interscalene brachial plexus block, or the ventricular system during retrobulbar nerve block.
Vasovagal Attack
This is more likely to occur in an anxious patient with a rapidly ascending spinal block. Symptoms are pallor, nausea, and bradycardia associated with hypotension. It can occur with the patient in the supine position and usually resolves rapidy if i.v. atropine 0.3–0.6 mg or ephedrine 5–6 mg is administered. Cautious i.v. sedation (e.g. midazolam 1–2 mg) may be helpful.
Motor Blockade
To avoid unnecessary distress, patients must be warned of the possibility of limb weakness or paralysis which may persist for some time after operation.
Pneumothorax
This is a potential hazard of supraclavicular brachial plexus, intercostal and paravertebral blocks. Hence these techniques should not be performed as bilateral blocks or in outpatients.
Urinary Retention
This may follow the use of central neuraxial blocks. It is important to avoid overhydration, as bladder distension may require catheterization. The use of large volumes of crystalloid in the treatment of hypotension often has a very transient effect and predisposes patients to urinary retention, or worse, pulmonary oedema, when the block regresses.
Neurological Complications
Carefully performed blocks rarely result in neurological complications. Risk factors include obesity, diabetes and the perioperative use of potent anticoagulants. The incidence of neurological complications resulting from central neuraxial blocks is likely to be less than 4 per 10 000 or 0.04%.
Neuritis with persisting sensory changes and/or weakness may result from trauma to the nerve, intra-neural injection or bacterial, chemical or particulate contamination of the injected solution. Injection of the incorrect solution has caused some of the most severe neurological complications. To avoid this, all drugs must be checked personally and labelled by the anaesthetist immediately before injection.
Anterior spinal artery syndrome may follow an episode of prolonged, severe hypotension and results in painless permanent paraplegia. Adhesive arachnoiditis has been described after subarachnoid and epidural blockade and may lead to permanent pain, weakness and bladder or bowel dysfunction. It is suspected that this complication results from injection of the incorrect solution. Haematoma or abscess formation in the spinal canal after subarachnoid or epidural anaesthesia results in weakness and sensory loss below the level of spinal cord compression. It is associated with intense back pain and is a neurosurgical emergency which demands immediate decompression to avoid permanent disability.
GENERAL MANAGEMENT
Patient Assessment and Selection
Careful preoperative evaluation is as important before a local anaesthetic technique as it is before general anaesthesia, and the same principles of preoperative management apply. Therapy to improve the patient’s condition before surgery should be commenced if appropriate. It is inappropriate to proceed with surgery under local anaesthesia for the sake of convenience in the poorly prepared patient. A decision should be made on the need for immediate surgical intervention before the anaesthetic technique is chosen.
The preoperative visit should be used to establish rapport with the patient. A clear description of the proposed anaesthetic technique should be given in simple terms, but there is rarely a need for excessive detail. Patients require an explanation of the reasons for selecting a regional technique along with its advantages and potential disadvantages, but there should be no attempt at coercion to accept a particular technique.
Potential problems related to the intended block should be anticipated and sought. Anatomical deformities or pain affecting patient positioning may render some blocks impractical. A history of allergy to amide local anaesthetics is rare, but is an absolute contraindication, as is infection at the site of needle insertion. For most blocks, recent anticoagulant therapy and bleeding diatheses are also absolute contraindications, and the use of major blocks in patients with distant infection or receiving low molecular weight heparin, rivaroxaban or potent anti-platelet drugs such as clopidogrel, requires careful consideration. The use of non-steroidal anti-inflammatory drugs is not generally considered to be a contraindication to neuraxial block unless combined with other anticoagulant agents. The decision to perform spinal or epidural anaesthesia and the timing of catheter removal in a patient receiving antithrombotic therapy should be made on an individual basis, weighing the small, but definite risk of spinal haematoma against the benefits of regional anesthesia for a specific patient. The patient’s coagulation status should be optimized at the time of spinal or epidural needle/catheter placement and indwelling catheters should not be removed in the presence of therapeutic anticoagulation because this seems to significantly increase the risk of spinal haematoma. Close monitoring is vital to allow early evaluation of neurological dysfunction and allow prompt intervention where necessary.
Sympathetic blockade with consequent vasodilatation may lead to profound hypotension in patients with significant aortic or mitral stenosis because of the relatively fixed cardiac output. Hypovolaemia must be corrected before contemplating subarachnoid or epidural anaesthesia.
There is no evidence that neuromuscular disorders or multiple sclerosis are adversely affected by local anaesthetic techniques, but most anaesthetists use regional anaesthesia in such patients only if there are obvious benefits to be gained; any perioperative deterioration in the neurological condition may be associated by the patient with the local anaesthetic procedure. Raised intracranial pressure is a contraindication to central neuraxial blockade but peripheral techniques may be considered.
Selection of Technique
Local anaesthetic drugs may be administered by:
If regional anaesthesia has been selected primarily to provide analgesia during and after surgery under general anaesthesia, a more peripheral technique may be more appropriate to provide a more selective motor and sensory blockade with less functional impairment.
Because a local anaesthetic technique renders only part of the body insensible, it is essential that the method employed is tailored to, and sufficient for, the planned surgery. Account must be taken of the duration of surgery, its site (which may be multiple, e.g. the need to obtain bone grafting material from the iliac crest) and the likelihood of a change of procedure in mid-operation. The problem of multiple sites of surgery may be met by one block which covers both sites, or by more than one regional anaesthetic procedure where indicated. The duration of anaesthesia may be tailored to the anticipated duration of surgery by selection of an appropriate local anaesthetic agent, or may require the use of a technique which allows further administration of drug.
Premedication
Manipulation of fractures and other short emergency procedures are often carried out using a local anaesthetic technique in the unpremedicated patient, as rapid recovery is desirable. However, premedication is helpful before inpatient elective or emergency surgery. An oral benzodiazepine allays anxiety, but an opioid (e.g. morphine) alleviates the discomfort of prolonged immobility which may be required during a long procedure. Preoperative analgesia may be required before definitive surgical fixation. A nerve block may be useful in these circumstances, e.g. a femoral block may be performed in the Emergency Department to alleviate the pain from a fractured femoral shaft. Patients should be fasted for all but the most minor peripheral nerve blocks.
Timing
It is essential that sufficient time is allowed to perform the block without undue haste on the part of the anaesthetist. This is largely a matter of organization and the experienced practitioner seldom causes delay to an operating list. Any preoperative delay is compensated for by the ability to return the patient to bed immediately after completion of surgery.
Resuscitation Equipment
A full range of resuscitation equipment must be immediately available and in working order whenever a local anaesthetic technique is used. This includes:
an anaesthetic breathing system through which oxygen may be administered under pressure via a face mask or tracheal tube
a laryngoscope with two sizes of blade, a range of tracheal tubes and an introducer
a table which may be rapidly tilted head-down
intravenous cannulae and fluids
thiopental or propofol to control convulsions
drugs to treat bradycardia or hypotension, especially atropine, ephedrine and metaraminol or phenylephrine
lipid emulsion 20% for treating serious systemic toxicity (see Ch 4).
Regional Block Equipment
Regional anaesthesia may be used with basic equipment, but some special items increase the success rate and reduce the risk of complications.
Needles
The use of very fine spinal needles (26G) has significantly reduced the incidence of post-spinal headache as has the use of pencil-point 25G Whitacre and 24G Sprotte needles (Fig. 24.1A). The 27G Whitacre needles appear to be associated with the lowest incidence of post-spinal headache 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 appear to reduce the likelihood of nerve damage. A variety of insulated needles are available for plexus and peripheral nerve blockade using a nerve stimulator (Fig. 24.1B). Ultrasound needle visibility may be improved by using echogenic needles which have ‘corner stone’ reflectors positioned at the distal end of the cannula shaft (Fig. 24.11C).
FIGURE 24.1 (A) Left to right: Quincke, Whitacre, Sprotte and Spinocath needles. (B) Left to right: standard-bevelled, short-bevelled, insulated short-bevelled and insulated Tuohy needles.
A recent patient safety initiative aimed at reducing drug administration errors, has recommended the development and evaluation of spinal needles and catheter infusion systems with non-Luer connectors that cannot therefore attach to intravenous equipment or standard syringes. This should help prevent wrong route intrathecal injection and stop the accidental intravenous administration of drugs intended for epidural or regional block.
Immobile Needle Technique
For plexus and major nerve blocks, local anaesthetic drug is drawn into labelled syringes and connected to the block needle by a short length of tubing (Fig. 24.2). This allows the anaesthetist to hold the needle steady while aspiration tests are performed and syringes changed. The system must be primed to prevent air embolism and also to avoid image artefact when using ultrasound-guidance.
Catheters
Continuous administration of local anaesthetic drugs has been made possible by the development of high-quality catheters, which are introduced through a needle (or occasionally over a needle; Fig 24.1A) and may be left in position for hours or even days. Careful fixation is essential to maintain the position of the catheter in the postoperative period. Catheters, in particular spinal (subarachnoid) catheters, should be labelled clearly to prevent accidental overdosage.
Nerve Stimulators
Few anaesthetists now aim to deliberately elicit paraesthesiae when performing a major nerve block; many still use the nerve stimulator (Fig. 24.2) but an increasing number now use ultrasound-guidance. It is important to explain to the patient the sensation elicited by nerve stimulation. It causes 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. The incidence of paraesthesia with short-bevelled insulated needles is very low because of their ability to stimulate without direct neural contact. They are also more likely to displace nerves rather than penetrate them.
As the nerve is approached, motor fibre stimulation causes muscle contraction in the appropriate distribution. The current is reduced until maximal contraction is still present at a current of, optimally, around 0.5 mA. At this point, a gentle aspiration test is performed and 2 mL of local anaesthetic solution slowly injected. Muscle contraction should cease immediately due to nerve displacement. If it does not, and an insulated needle is being used, the tip may have moved, be beyond the nerve or placed intravascularly; 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 repositioned. When the correct position has been found, the remainder of the anaesthetic solution should be injected slowly with repeated aspiration tests. Performance of the block in the awake patient allows better assessment of early intravascular toxicity and intraneural injection in addition to encouraging gentle and careful technique.
Ultrasound
The most significant recent change in the practice of regional anaesthesia has been the introduction of ultrasound guidance. A variety of high quality scanners and probes are now available and vast improvements in image quality have contributed greatly to advances with these techniques. The ultrasound transducer functions as both a transmitter and receiver with the beam reflected, refracted and scattered after it encounters structures of different acoustic impedance, returning to the transducer to produce the target image. Production of a clear target image as well as location and safe needle guidance in real time, requires sound cross-sectional anatomical knowledge along with excellent technical skills, which develop only following adequate training and repetitive hands-on practice. Transducers can be either linear or curved array, with higher frequency probes (8–12 MHz) generally used to produce superficial images of high resolution, such as would be required for interscalene or axillary block. Lower frequency probes (4–7 MHz) provide improved penetration to visualize 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, for example during performance of a subgluteal sciatic block or an infraclavicular brachial plexus block. Most nerves exhibit a distinctive ‘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, due to reduced amounts of connective tissue compared with the axilla and peripherally. As well as visualizing the target nerve structures, ultrasound guidance is useful to identify other important structures such as blood vessels and pleura in order to avoid complications and also allows visualization of local anaesthetic spread. Needle advancement can be tracked in real time, allowing subtle adjustment of needle position to ensure optimal local anaesthetic distribution.
Asepsis
A ‘no-touch’ technique is essential. Drapes should be used for all major blocks and gloves and gown should be worn by the novice. Gown, gloves, hat and mask are recommended for all central blocks even with a ‘no-touch’ technique, especially when a catheter is inserted either centrally or peripherally. Taking precautions seriously fosters good practice. For all ultrasound-guided blocks a sterile field should always be prepared using antiseptic solution and the probe covered with a sterile sheath or adhesive dressing before commencing the block. It is vitally important to cover the skin with sterile conductivity gel to remove the air-skin interfaces and allow good ultrasound wave penetration.
Monitoring
It is essential that the anaesthetist remains with the patient throughout the operative procedure. Monitoring equipment should be appropriate to the anaesthetic technique and surgical procedure, with a minimum of ECG, non-invasive blood pressure and pulse oximetry.
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 infusion of propofol are commonly used. 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 additional trauma or significant 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 is relieved by the administration of an opioid either as a premedicant or i.v. during surgery. This type of discomfort is not relieved by sedative drugs, which often result in the patient becoming agitated, confused and uncooperative. An i.v. infusion of remifentanil is being used increasingly for this purpose although this technique is not for the beginner and requires careful respiratory monitoring, preferably by continuous nasal capnography in addition to pulse oximetry.
After-Care
Clear instructions should be given to the nurses caring for the patient.
After day-case surgery, the patient must be in a safe condition at the time of discharge. Plexus blockade with a long-acting agent is inappropriate because of the risk of the patient injuring the anaesthetized limb, but is suitable for postoperative pain relief in supervised inpatients following major surgery, particularly when the limb is immobilized or conversely when continuous passive mobilization is required. Patients who have received central nerve blockade should have routine nursing observations at least until the block has worn off.
Continuous infusion techniques are suitable for use only by experienced anaesthetists. When used correctly, administration by infusion is safer than repeated bolus injection of drug, but regular observations are essential and the nursing staff must have an adequate level of knowledge to appreciate possible complications. An anaesthetist must be available within the hospital at all times.
INTRAVENOUS REGIONAL ANAESTHESIA
Ideally, intravenous regional anaesthesia (IVRA) (Bier’s block) should be the first local anaesthetic technique learnt by a trainee, because its technical simplicity allows the trainee to concentrate on acquiring the skills of patient management. In practice, however, this technique is being used increasingly by Emergency Department staff and less frequently by anaesthetists, who often prefer to block the brachial plexus. Bier’s block is simple, safe and effective when performed correctly using an appropriate drug in correct dosage. Deaths from IVRA have resulted from incorrect selection of drug and dosage, incorrect technique and the performance of the block by personnel unable to treat toxic reactions. The drug involved in these deaths, bupivacaine, was not the most suitable agent and is now contraindicated. The lessons to be learned from these deaths are applicable to all local anaesthetic techniques, and emphasize that expert guidance is essential even when learning the most basic blocks.
Indications
Intravenous regional anaesthesia is suitable for short procedures when postoperative pain is not marked, e.g. manipulation of Colles’ fracture or carpal tunnel decompression. Recovery is rapid, and the technique is appropriate for outpatient surgery. Premedication may delay patient discharge and a reassuring visit pre-operatively from the anaesthetist is usually sufficient in these circumstances.
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 intravenous cannula is sited in the contralateral arm in case administration of emergency drugs is required. An indwelling cannula is inserted into a vein of the limb to be anaesthetized. A vein on the dorsum of the hand is preferred; injection into 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 with careful observation that the tourniquet remains inflated. Analgesia is complete within 10 min, but it is important to inform the patient that the feeling of touch is often retained at this time. 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 necessary.
Tourniquet Pain
This 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 prilocaine 0.5% plain. It has an impressive safety record with no major reactions reported after its use, although minor side-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 for IVRA.
CENTRAL NERVE BLOCKS
Spinal anaesthesia is a term that may be used to denote all forms of central nerve blockade, although it usually refers to intrathecal administration of LA. The term subarachnoid block (SAB) avoids ambiguity. The technique of SAB is basically that of lumbar puncture, but knowledge of factors which affect the extent and duration of anaesthesia, and experience in patient management are essential. Epidural nerve block may be performed in the sacral (caudal block), lumbar, thoracic or cervical regions, although lumbar block is used most commonly. Local anaesthetic solution is injected through a needle after the tip has been introduced into the epidural space, or may be injected through a catheter placed in the space.
Physiological Effects of Subarachnoid Block
Local anaesthetic solution injected into the CSF spreads away from the site of injection and the concentration of the solution decreases as mixing occurs. A differential blockade of fibres occurs because small fibres are blocked by weaker concentrations of local anaesthetic solution. Sympathetic B fibres are blocked to a level approximately two segments higher than the upper segmental level of sensory blockade. Motor blockade may be several segments caudal to the upper level of sensory block. A sensory level to T3 with SAB may be associated with total blockade of the T1–L2 sympathetic outflow.
Respiratory System
Low SAB has no effect on the respiratory system and the technique is an important part of the anaesthetist’s armamentarium for patients with severe respiratory disease. However, motor blockade extending to the roots of the phrenic nerves (C3–5) causes apnoea and blocks which reach the thoracic level cause loss of intercostal muscle activity. This has little effect on tidal volume (because of diaphragmatic compensation), but there is a marked decrease in vital capacity resulting from a significant decrease in expiratory reserve volume. The patient may experience dyspnoea, and difficulty in taking a maximal inspiration or in coughing effectively. A thoracic block may lead to a reduction in cardiac output and increased ventilation/perfusion imbalance, resulting in a decrease in arterial oxygen tension (PaO2). Awake patients with a high spinal block should always be given oxygen-enriched air to breathe.
Cardiovascular System
The cardiovascular effects are proportional to the height of the block and result from denervation of the sympathetic outflow tracts (T1–L2). This produces dilatation of resistance and capacitance vessels and results in hypotension. In awake patients, vasoconstriction above the height of the block may compensate almost completely for these changes, thereby maintaining arterial pressure, but general anaesthetic agents may reduce this compensatory response, with consequent profound hypotension. Hypotension is exacerbated by:
Prevention of Hypotension
Both the incidence and the degree of hypotension are reduced by limiting the height of the block and, in particular, by keeping it below the sympathetic supply to the heart (T1–4).
It is common practice to attempt to minimize hypotension during SAB or epidural anaesthesia by preloading the patient with 500–1000 mL of crystalloid solution i.v. before or during the performance of the block. These volumes are usually ineffective even in the short term, may risk causing pulmonary oedema in susceptible individuals either during the procedure or when the block wears off, and may lead to postoperative urinary retention. Appropriate fluid should be given to replace blood and fluid losses and prevent dehydration.
Bradycardia may occur because of:
neurogenic factors, particularly in awake patients, i.e. vasovagal syndrome
paradoxical Bezold-Jarisch reflex; decreased venous return and heightened sympathetic tone leads to forceful contraction of a near empty left ventricle, with consequent parasympathetically mediated arterial vasodilatation and bradycardia
Careful patient positioning, maintenance of a normal circulating volume and the use of pharmacological agents (see later), if required, should minimize the incidence of hypotension.
SAB has no direct effect on the liver or kidneys, but reductions in hepatic and renal blood flow occur in the presence of hypotension and reduced cardiac output associated with high spinal blocks.
Gastrointestinal System
The vagus nerve supplies parasympathetic fibres to the whole of the gut as far as the transverse colon. Spinal blockade causes sympathetic denervation (proportional to height of block), and unopposed parasympathetic action leads to a constricted gut with increased peristaltic activity. This is regarded by some as advantageous for surgery.
Nausea, retching or vomiting may occur in the awake patient and are often the first symptoms of impending or established hypotension. If nausea or retching occurs, the anaesthetist must assess arterial pressure and heart rate immediately and take appropriate measures.
Physiological Effects of Epidural Block
The physiological effects of epidural blockade are similar to those following SAB, but may develop more slowly. Additional effects may occur from the much larger volumes of anaesthetic solutions used, as there may be appreciable systemic absorption of local anaesthetic and adrenaline if an adrenaline-containing solution is used.
Indications for Subarachnoid Block
Blockade is produced more consistently and with a lower dose of drug by the subarachnoid route than by epidural injection. Duration of analgesia is usually limited to 2–4 h depending on surgical site and may be prolonged by catheter techniques. Catheter techniques may also be used to establish block height more carefully in more compromised patients. SAB is most suited to surgery below the umbilicus and in this situation the patient may remain awake. Surgery above the umbilicus using SAB is less appropriate and would necessitate addition of a general anaesthetic, in order to abolish the unpleasant sensations from visceral manipulation resulting from afferent impulses transmitted by the vagus nerves.
Types of Surgery
Urology: Subarachnoid 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.
Full access? Get Clinical Tree
